[time-nuts] Re: DIY Low offset Phase Noise Analyzer

[Lux, Jim via time-nuts]

On 7/13/22 5:24 AM, Dan Kemppainen via time-nuts wrote:

Erik,

Just a thought. The 7805 in your schematic appears to be a big part of 
the signal chain. I've run into issues with 78xx series regulators 
being noisy. Spent a few weeks chasing down noise issues in some 
equipment and the 78xx regulator was a part of the problem. Replaced 
it with LT3042 as part of the solution.


Also note, not all 7805's are created equal. Different vendors may be 
better/worse than others.


Good luck!

Dan



+1 for the 200 mA LT3042 (and the higher current 0.5A LT3045). There are 
now negative voltage versions, too, LT3094, also 0.5A.


Noise from 10Hz to 100kHz is around 0.8 microvolts.  At 10 kHz 2 
nV/sqrt(Hz).  But the real thing these things are good at is PSRR at 
higher frequencies.  >70dB at 1 MHz


They have enable inputs, can be paralleled willy-nilly, etc






On 7/12/2022 9:45 PM, time-nuts-requ...@lists.febo.com wrote:


I'm struggling with the noise floor.
First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with 
shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with 
1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz 
was still -140dBc/Hz
The setup was simplified to this schematic: 
http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG
The REF_buffer creates a virtual ground, the Audio_LNA amplifies into 
the differential audio output .

Why did the lower noise opamp not make a difference?
Also the setup is acting like a nice microphone. Tapping the housing 
is clearly audible. Which component may be causing the microphony?

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[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[Lux, Jim via time-nuts]

On 7/12/22 3:51 PM, ed breya via time-nuts wrote:
I forgot to mention that you should also consider possible effects 
from the RF present, on the LNA. This can be more significant than 
SMPS frequencies getting where they don't belong, especially since the 
RF is intentionally right at the opamp's input. Your LPF only reduces, 
and does not eliminate, the 2F and harmonics, so there can be 
significant RF present on the LNA circuit.


A simplistic view is that the RF is far beyond the opamp's GBW or 
closed loop gain and should have no response, but it's not at all 
beyond upsetting or altering the operation. This can result in extra 
DC offsets and noise due to RF rectification in the input circuits, 
which only remain "linear" at frequencies where the output and 
feedback can keep up with the input.


This can be fixed if necessary, by adding extra RF filtering, 
particularly some built to low-pass at a higher cutoff frequency well 
above the analysis frequency, and well below the expected f and 2f.


For instance, in your circuit it looks like L1 is 1 mH, with 100 nF 
caps, which ideally cuts off quite low. However, 1 mH is a pretty big 
choke, and will tend to have a lot of inter-winding capacitance (and 
high resistance - don't forget to include it in noise), making it less 
effective at the higher frequencies. Adding an LC section in front of 
it, but set up for something in the MHz region, will give much greater 
rejection of the f and 2f, due to having more appropriate smaller L 
and C.


Anyway, if it works fine as is, then no problem, but it's something to 
be aware of if you get strange effects down the road.


Ed



and a single LC is only a single pole, so the roll off isn't all that 
great in a dB/decade sense.

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[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik, Kaashoek)

[Lux, Jim via time-nuts]

On 7/12/22 8:53 AM, Erik Kaashoek via time-nuts wrote:

I'm struggling with the noise floor.
First tests where done with a 5nV/sqrt(Hz) opamp. Noise floor with 
shorted mixer output at 10kHz was -140dBc/Hz. Then I tried with 
1nV/sqrt(Hz) opamp, but that made no difference, noise floor at 10kHz 
was still -140dBc/Hz
The setup was simplified to this schematic: 
http://athome.kaashoek.com/time-nuts/PNA/SSPNA.JPG


What's the noise contribution of the resistors? V = sqrt(4*k*T*R*B)

51 ohms  is sqrt (4 * 1.38E-23 * 300 * 51) = 0.9 nV/sqrt(Hz)  100 ohms 
is ~1.4 nV/sqrt(Hz)


What kind of op amp? what's the current noise vs the voltage noise?  - 
you might low voltage noise, but high current noise, and that current 
noise across the input impedance can turn into surprisingly high voltage 
noise at the output.



The REF_buffer creates a virtual ground, the Audio_LNA amplifies into 
the differential audio output .

Why did the lower noise opamp not make a difference?
Also the setup is acting like a nice microphone. Tapping the housing 
is clearly audible. Which component may be causing the microphony?


My guess would be a parasitic capacitance between circuit and housing. 
changing the distance changes the C.





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[time-nuts] Re: GPS failed

[Lux, Jim via time-nuts]

On 7/10/22 4:19 PM, skipp Isaham via time-nuts wrote:

Hello to the Group,

I'd like to get some opinions and war stories regarding GPS reliability at
high RF level and elevation locations.

Background:  Three different hill-top GPS receivers, all different types, using
different antennas mounted on an outside fixiture, plain view of the open sky,
all stopped working.

Test antennas were brought in and placed on a fixture well away from the
original antennas, the recevers went back in to capture and lock.

 From what I understand, the original antennas are what I would call straight
preamp with no pre-selection / filtering.

The ordered and now inbound replacements are said to contain a SAW filter
system. It is the intent of the client to just place these "improved antennas" 
in
to service and get on with life.

I would suspect a GPS antenna (and receiver) could be subject to RF overload
or blocking, however, we're assuming nothing major has changed at the site, nor
any nearby location.  One might think there are more GPS receivers being pushed
out of reliable operation by the world around them, I'm just not hearing those 
stories
from a lot of people using them (GPS receivers).


yes, this happens.  We used to have a Pendulum timing receiver with a 
typical "small white cone" type amplified antenna - if someone was on 
the roof with a cellphone, it lost lock, presumably from the (way out of 
band) emissions.



As to where the interfering source is - it doesn't take much, and it 
could be some distance away.  After all, this was the big deal with 
LightSquared - it was moderately high powered terrestrial broadcast 
transmitters in the satellite downlink band next to GNSS.





Any new install GPS receiver antenna ordered will/should contain some 
pre-selection
to potentially avoid a problem, even some years down the road? Seems like that's
where things are going... no more off the shelf, wide band, (hot) preamplified 
GPS antennas
in busy locations?

Thank you in advance for any related comments and/or opions ...

cheers,

skipp

skipp025 at jah who dot calm
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[time-nuts] dual supplies Re: Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Lux, Jim via time-nuts]

On 7/10/22 9:07 AM, Bob kb8tq via time-nuts wrote:

Hi

Yes it is a pain to implement dual supplies. I ponder that issue every time
I build one of these setups. I’ve built a lot of them …. If you are going to
do a single supply, setting up a “virtual ground” is probably the best way
to go. Do it with a drive circuit to provide very clean 15V off of a 30V supply
then tack everything ( including *all* the mixer grounds to that 15V supply.



Yeah, but that virtual ground brings with it it's own set of problems. 
For instance, it has to both sink and source current, so you can't just 
use a 3 terminal regulator to create the midpoint, although I've seen 
schemes with a resistor from virtual ground to negative supply, but 
that's not very power efficient - the resistor needs to see, say, 10x 
the maximum sink current.


Now that I think about it, some sort of op amp driven appropriately (or 
a complementary PNP/NPN pair?) might work - but then you're concerned 
about the output Z of the op amp, and how it varies over frequency. And, 
of course, the bias current.


We face this in designing low noise instruments for space. You're 
running off a DC bus of some sort, and DC/DC inverters tend to be 
noisier than straight out buck converters or linear regulators.


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[time-nuts] Re: DIY Low offset Phase Noise Analyzer (Erik Kaashoek)

[Lux, Jim via time-nuts]

On 7/7/22 8:55 AM, Bob kb8tq via time-nuts wrote:

Hi

Yes, you do need to know the system gain. Since we are talking about
gain at audio, measuring the gain directly is not a crazy thing to do. One
of the things that makes audio spectrum analyzers a nice tool for this that
they eliminate the “variable gain to the sound card” issue.

Some sound card setups are a lot easier to work with than others. If you
are restricted to the sound input on your motherboard things can get a bit
crazy. It is not unusual for folks to dig up a “pro” (whatever that means
on a sound card ) card that has better drivers and more access to this and
that.

Given how fast the PC world changes, the board that was a wonderful thing
last time somebody dove in, likely is long out of production by now. The drivers
that made it work so well may have been “improved” and it no longer gives
you the control it once did. This makes for a bit of trial and error to get it 
all
going.

Bob



Rather than a sound card, it might be better to pick a small singleboard 
like a Teensy that has a decent ADC, and make a "sampling engine" with a 
USB interface.


Or, in general, going to a USB interface sound interface might be good.  
You can get them with a lot of channels (at least 8) and they sample 
simultaneously, so the uncertainty in USB latency won't bite you.  
Google for things like the Focusrite Scarlett



I've not tried it for this kind of application, but it is likely to have 
better noise properties than a "inside the PC" card. Typically 24 bit 
converters and 192kHz sample rates.



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[time-nuts] Re: Fixing PN degradation via ADEV measurement

[Lux, Jim via time-nuts]

On 6/20/22 2:39 AM, Magnus Danielson via time-nuts wrote:



So, a counter is really like an ADC for phase, with wide bandwidth 
input and a sub-sampling mechanism (trigger/time-base). Through 
processing frequency estimates can be provided. Aliasing occurrs in 
the sub-sampling. Modern counters can provided estimation filters than 
goes from a higher sub-sampling rate to a lower, which to some degree 
removes aliasing, but not fully. These frequency estimation methods 
form a form of decimation filter.


Cheers,
Magnus 


An intruiging thought as I drink my first cup of coffee (meaning it's 
not well thought out)..


jumping off from "counter is similar to an ADC for phase" - is there a 
time domain equivalent for Nyquist criterion?   Certainly there's the 
cycle ambiguity.. you know when the zerocrossing occurred, but not how 
many are in between (although a counter usually does). For everything 
else there is a frequency/time duality, so I suspect there is.  The 
criterion is usually explained in terms of information - so there should 
be an equivalent "has all the information" statement for counters/gate 
widths/precisions.


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[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Lux, Jim via time-nuts]

On 6/12/22 6:30 PM, Bob kb8tq via time-nuts wrote:

Hi

Tear into some of your SC cut based OCXO’s. Take a look at the crystal package. 
For
bonus points, open up the crystal package. If you have the gear to test it, 
take a look
at what the gas *is* inside the package. ( Good luck with that :) :) :) )

If you had the gear and the willingness to scrap out OCXO’s you would find that 
a number
of fast warmup OCXO’s have a *tiny* amount of He in the package. Measuring this 
would
be tough ( it’s that small). Go through the thermal modeling and it’s *way* 
more conductive
(thermal wise) than a *perfect* vacuum ……

Bob



And the worst thing is that if your vacuum sealed widget is in an 
atmosphere with more He around (like waiting for a launch, in a place 
that does more He leak tests, etc.), the He will diffuse into your 
package and it doesn't work like expected.


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[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Lux, Jim via time-nuts]

On 6/10/22 1:57 PM, Dr. David Kirkby wrote:
On Fri, 10 Jun 2022 at 17:39, Lux, Jim via time-nuts 
 wrote:


On the subject of rapid warm up. I suppose if you had a need, one
could
dump as much power as you need into the heater. Turn on oscillator,
lights in room dim for a few moments.


Is that not likely to damage a crystal? Different parts of the crystal 
and likely to be at significantly different temperatures at the same 
time, putting a lot of stress on the crystal due to a thermal 
gradient. It's probably a bit academic, as nobody is going to make an 
oven that heats up in fractions of a second, but if one did, I suspect 
it might not do the crystal a lot of good. This is only an educated 
guess - I don't have anything to back it up.
Oh, it would be disastrous, although quartz is pretty strong, all the 
rest of the mounting components might not be.


At the other extreme,  would there be any advantage in actually 
heating the crystal very slowly, over the course of an hour/day/week, 
so the temperature gradient across the crystal is very small? Of 
course, if an oven took ages to reach the correct temperature, it 
would be inconvenient for most applications, but for some 
applications, the advantages might outweigh the disadvantages. Of 
course, if one does this, I suspect one would have to cool the crystal 
slowly too to prevent a significant thermal gradient across the crystal.


I know it's a bit different, but I have a 600 mm f4 Nikon camera lens. 
I was told that Nikon cools the front element over a period of 6 
months to reduce stresses in the glass.


Big glass mirrors for telescopes do the same.





Dave


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[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Lux, Jim via time-nuts]

On 6/10/22 12:13 PM, Richard (Rick) Karlquist wrote:

I think there is a severe misunderstanding of this issue.
First of all, "rapid warmup" is a red herring.  The real
issue is "rapid frequency stabilization".


Indeed. But to a certain extent, that's why oscillators don't have 5 
second warm up times - it wouldn't help. So the "few minutes" is a good 
compromise between design simplicity and waiting for the internals to 
equilibrate.


I suspect that those fancy USOs in vacuum bottles take a long, long time 
to come to equilibrium.


And that is one of the claims to fame of the CSAC - from power on to "on 
frequency" is quite short.





The time it takes for the oven to cut back (typically only
a minute or two) is a very minor part of the time budget
to get to frequency stabilization.  You could have an AT
cut oscillator that reached "oven warmup" in 1 second, but
then you would have something like a 1 hour wait to get
frequency stability, due to the thermal stresses. 

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[time-nuts] Re: Is SC the most stable cut for lowest phase noise?

[Lux, Jim via time-nuts]

On 6/9/22 8:53 PM, Bob kb8tq via time-nuts wrote:

Hi

There happen to be *some* AT cut based OCXO’s that beat the typical
SC cut on warmup … just saying …. :)

Bob

On the subject of rapid warm up. I suppose if you had a need, one could 
dump as much power as you need into the heater. Turn on oscillator, 
lights in room dim for a few moments.


But everything is a tradeoff, and I suspect that over time "standard 
designs" sort of migrate to particular ratios of things like peak vs 
average heater current, etc.  Especially in applications driven by 
design rules for things like "maximum current per connector pin" or 
"component derating" - I suspect that drives things more than the 
fundamental physics, in most cases.  Why are diodes rated at 1 or 3 
Amps?  And not 2?


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[time-nuts] Re: Realtime comparing PPS of 3 GPS

[Lux, Jim via time-nuts]

On 5/31/22 3:23 AM, Carsten Andrich via time-nuts wrote:

On 31.05.22 01:10, glen english LIST via time-nuts wrote:
Be aware not to confuse the antenna ground plane  (the patch will 
always have its own plane because the top metalization must be fed 
against a plane or counterpoise -  and a ground plane behind the 
antenna.


I can see the usefulness of the larger ground plane for any purchased 
patch antenna to reduce the likelihood of interference underneath (if 
the feed coax has a good RF contact with the plane), and if the plane 
is coupled well, it may improve the low angle response .


The supplementary ground plane doesnt have to have a galvanic 
connection if the gap between the underside of the patch is low- IE 
use purely a capacitive coupling to tie the patch antenna ground to 
the large ground sheet-

[...]

That means reducing the gap to about 0.05mm  OR increasing the area- 
probably means using a bigger patch.


Hi Glen,

thank you for the insight. I was referring to a ground plane behind 
the antenna.


Gaps below 1~2 mm between a magnetic "puck"-type patch antenna with 
IP67 housing and an external ground plane seem practically challenging 
to me. When it comes to stacked patch multi-band antennas like u-blox' 
ANN-MB [1], the gap between the top patch and the external ground 
plane is probably significantly higher. Yet, u-blox generally 
recommends the use of a symmetric ground plane for the RTK 
applications [1,2]. From my experience, the M8P and F9P RTK fix barely 
works without a ground plane under the u-blox antennas.
While it's just an empirically educated guess, I'd assume that what is 
required for RTK will not hurt for timing.


Could you share your expert opinion on this? My antenna expertise is 
admittedly limited to reading data sheets and picking the right one 
for the particular RF measurement requirements.


Thanks and best regards,
Carsten


I would think that the large grounded sheet below the antenna helps more 
for making the pattern uniform, and, to a certain extent, suppressing 
some multipath coming from "below" the plane of the sheet.  - not as 
good as a choke ring(s), but not bad.


That is, the sheet is not intended to couple to the antenna's ground 
plane, but is there as a predictable surface (and, probably, to provide 
a magnetic material for a puck to stick to).


As such, the distance from the antenna's ground plane is not 
particularly critical.

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[time-nuts] Re: measuring tiny devices

[Lux, Jim via time-nuts]

On 5/26/22 8:18 AM, Bob kb8tq via time-nuts wrote:

Hi

The real answer to the problem is to dig into the bowels of 1940’s electronic 
craft.
There are various methods for setting up an L/C filter. You short this / open 
that sweep
to find a dip or a peak. You move it to the “right” place. Just what you do 
depends
very much on the filter design. Many L/C’s got done this way or that way simply
because they would fit a known alignment method.

While it all sounds very cumbersome and obscure it actually isn’t. Long ago I 
stumbled
upon a gal setting up very complex L/C IF filters this way. The display gyrated 
this way
and that way as she did this or that. I don’t think it took her more than a 
minute to get
the whole thing set up….. to this day, I’m amazed by how fast she was.

Do I have any useful links to actually read up on  this magic? … sorry about 
that.

Bob


There are actually computer driven screwdrivers to do tuning on cavity 
filters. The operator puts the screwdriver to each cavity in turn. The 
filter is hooked up to a VNA with a computer that runs the scripts..



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[time-nuts] Re: measuring tiny devices

[Lux, Jim via time-nuts]

On 5/26/22 8:24 AM, Richard (Rick) Karlquist wrote:

The tweezers are really good *only* for single components - even if they 
come with test leads, that's for measuring something like a motor start 
capacitor.
I tried using tweezers (cheap ones to be sure) to measure a moderately 
complex assembly (trying to figure out stray C).  It was a gruesome failure.




VNA's of any kind (no matter how small their size) don't work well on 
components that are too far away from 50 ohms, at least if

you make a simple minded s11 smith chart measurement.  There
are complicated work-arounds for these measurements, but they
require different configurations depending on what you are
measuring, so there is no turn key or universal solution.



https://www.mwrf.com/technologies/test-measurement/article/21849791/copper-mountain-technologies-make-accurate-impedance-measurements-using-a-vna

describes the various approaches



With the low price of available VNA's, anyone can afford to
buy one, but that doesn't mean they know how to use it correctly.


Oh man, is that ever true.


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[time-nuts] measuring tiny devices

[Lux, Jim via time-nuts]

On 5/25/22 3:16 PM, ed breya via time-nuts wrote:
Thanks Mike, for info on LCR alternatives. It's good to know of others 
out there, if needed. I have an HP4276A and HP4271A. The 4276A is the 
main workhorse for all part checking, since it has a wide range of 
LCZ, although limited frequency coverage (100 Hz - 20 kHz). The 4271A 
is 1 MHz only, and good for smaller and RF parts, but very limited 
upper LCR ranges. I think it works, so I can use it if needed, but 
would have to check it out and build an official lead set for it. I 
recall working on it a few years ago to fix some flakiness in the 
controls, so not 100% sure of its present condition.


The main difficulty I've found in measuring small chokes is more of 
probing/connection problem rather than instrument limitation. For most 
things, I use a ground reference converter that I built for the 4276A 
many years ago. It allows ground-referenced measurements, so the DUT 
doesn't have to float inside the measuring bridge. The four-wire 
arrangement is extended (in modified form) all the way to a small 
alligator clip ground, and a probe tip, for DUT connection, so there 
is some residual L in the clip and the probe tip, which causes some 
variable error, especially in attaching to very small parts and leads. 
When you add in the variable contact resistance too, it gets worse. 
Imagine holding a small RF can (about a 1/2 inch cube) between your 
fingers, with a little clip sort of hanging from one lead, and 
pressing the end of the probe tip against the other lead. All the 
while, there's the variable contact forces, and effects from the 
relative positions of all the pieces and fingers, and the stray C from 
the coil to the can to the fingers. I have pretty good dexterity, and 
have managed to make these measurements holding all this stuff in one 
hand, while tweaking the tuning slug with the other.


I had planned on making other accessories like another clip lead to go 
in place of the probe tip, but not yet built. I also have the official 
Kelvin-style lead set that came with the unit, so that's an option 
that would provide much better accuracy and consistency, but the clips 
are fairly large and hard to fit in tight situations, and the DUT must 
float. Anyway, I can make all sorts of improvements in holding parts 
and hookup, but usually I just clip and poke and try to get close 
enough - especially when I have to check a lot of parts, quickly.


The other problem is that the 4276A is near its limit for getting 
measurements below 1 uH, with only two digits left for nH. The 4271A 
would be much better for this, with 1 nH vs 10 nH resolution.


If I get in a situation where I need to do a lot of this (if I should 
get filter madness, for instance), then I'll have to improve the tools 
and methods, but I'm OK for now, having slogged through it this time. 



You might check out the NanoVNA - people have made a variety of novel 
fixtures for measuring small parts (i.e. 0604 SMTs)


It certainly has the measurement frequency range you need. The trick is 
figuring out whether you want to do a series or shunt measurement, and 
that sort of depends on the reactance of your device at the frequency of 
interest.

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[time-nuts] Re: Noise down-converter project

[Lux, Jim]

On 5/16/22 8:11 AM, g...@hoffmann-hochfrequenz.de wrote:

Am 2022-05-16 15:16, schrieb Robert LaJeunesse:

FYI there are some rather flat video filter ICs that have been made in
the past. The 6th order HMC1023LP5E is tunable, at its 28MHz setting
its flat then down 0.1dB in the teens, down 0.35dB at 20 MHz. That
same setting is 60dB down at about 90 MHz. It is also a dual part,
designed for matched I-Q filtering.


Declared dead at DigiKey.



Digikey is EOLing them - last time buy is July 31 2022  (in the US)


It's not entirely dead yet.  Mouser has them - they're marked EOL - but 
you can buy them for ~$40 each


This is one of those parts from Hittite (HMC partnumber) and they tend 
to do small runs, but on the other hand, if demand seems to pop up, they 
may make them again.


On the other hand, watch out for "custom parts" that just happen to have 
a Hittite part number.  At JPL, we had a vector modulator built by 
Hittite, it got a standard part number, and I assume you could buy them 
until they ran out. But I get emails every once in a while asking where 
to get that part we referenced.








Sent: Sunday, May 15, 2022 at 5:29 PM
From: "ed breya" 



The actual filter I've been using does a good job on the higher
frequencies, but is poor on flatness. It has about 2-3 dB p-p passband
ripple, with periodicity around 5-7 MHz. I've tried various padding
arrangements at both ends, all of which tend to flatten it only a 
little

bit at best. Looking at it with the TG/SA setup, the character is
intrinsic to filter, and not due to just its reaction to the mixer and
cabling and such.

I hate building filters. Designing them in principle is easy, with all
sorts of available tools online, but actually rounding up the real 
parts

(and their parasitics) and physical implementation is a PITA. But, I
suppose I'll have to do it eventually for this project. I know how nice
it can be, with the right filter, but for now, I'll have to go with 
what

I have.


Did you choose a Chebyscheff design to start with? These accept some 
ripple

in the pass band, maybe some dB, to buy a steep rise of attenuation
above f-3dB.



I agree with Ed here, easy in the tool, not necessarily easy in real 
life. One aspect of more "aggressive" designs - Chebyshev, Cauer, etc. 
is that they tend to be more sensitive to component variations - 
especially Cauer (Elliptical) because they depend on that carefully 
placed zero to get the rejection close to cutoff.

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[time-nuts] Re: Time-nuts at WSTS conference

[Lux, Jim]

On 5/12/22 2:48 PM, Gary Woods wrote:

On Thu, 12 May 2022 12:01:48 -0600, you wrote:


I've found that corridor discussions have included redefinition of
SI-second, quantum computers, optical clocks, security on PTP clocks,
time-scale algorithms, uncertainty of different measures. Oh, and I just
won a bottle of scotsh whiskey.

Excellent, Magnus!  I remember a tech seminar years ago, where the
instructor avowed that 80% of the education took place during the
coffee breaks!

The best international standards start as a discussion the hallway or 
bar, with scribbles on a paper napkin.


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[time-nuts] Re: Effect of temperature on cheap puck style GNSS antennas?

[Lux, Jim]

On 5/11/22 11:50 PM, Matthias Welwarsky wrote:

Dear list members,

My DIY GPSDO has a rather well defined dependence to the environmental
temperature, which correlates almost linearly with a frequency shift of the
OCXO. However, at times I see the error against the GNSS reference increasing
with its case temperature not warranting such effect.

My antenna is one of those cheap, magnetic, active antennas you'd put on a car
roof. It's facing south and has full exposure to the sun, obviously.

During sunrise I see the TIC error increasing 20ns-30ns over lets say 2000
seconds. The GPSDO case temperature rises, too, during that time as the room
temperature increases, but it is only by 0.3°C.

I'm wondering if the temperature of the antenna, which of course rises much
faster than the room temperature, can have an effect of this magnitude?



Very possible. I've seen fairly large changes (nanoseconds over a 0-40C 
temp range) in delay in the LNA and bandpass filter for GNSS receivers 
with temperature. If they're using any sort of ceramic filter or ceramic 
antenna, then that can have a fairly large tempco in the time delay.



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[time-nuts] Greenhall paper on FFT

[Lux, Jim]

https://www.ion.org/publications/abstract.cfm?articleID=13963

Also here: https://ntrs.nasa.gov/citations/20060030427

Download it from JPL here:
https://trs.jpl.nasa.gov/handle/2014/11024
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[time-nuts] Re: Simple simulation model for an OCXO?

[Lux, Jim]

On 5/3/22 1:57 AM, Matthias Welwarsky wrote:

Magnus, Jim - thanks a lot. Your post encouraged me to look especially into
flicker noise an how to generate it in the time domain. I now use randn() and
a low-pass filter. Also, I think I understood now how to create phase vs
frequency noise.



There's some papers out there (mentioned on the list in the past) about 
synthesizing colored noise. Taking "White" noise and running it through 
a filter is one approach. Another is doing an inverse FFT, but that has 
the issue of needing to know how many samples you need. Although I 
suppose one could do some sort of continuous overlapping window scheme 
(which, when it comes right down to it is just another way of filtering)).


https://dl.acm.org/doi/abs/10.1145/368273.368574

or

C. A. Greenhall. Models for Flicker Noise in DSN Oscillators. The Deep 
Space Network Progress Report, Volume XIII, pp. 183-193, February 15, 1973.

https://ipnpr.jpl.nasa.gov/progress_report/XIII/XIIIY.PDF

W. J. Hurd. Efficient Generation of Statistically Good Pseudonoise by 
Linearly Interconnected Shift Registers. The Deep Space Network Progress 
Report, Volume XI, pp. 1-1, October 15, 1972.

https://ipnpr.jpl.nasa.gov/progress_report/XI/XIQ.PDF


A Wideband Digital Pseudo-Gaussian Noise Generator
W. J. Hurd
https://ipnpr.jpl.nasa.gov/progress_report/III/IIIP.PDF
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[time-nuts] Re: Simple simulation model for an OCXO?

[Lux, Jim]

On 5/2/22 7:03 PM, Magnus Danielson via time-nuts wrote:

Hi Jim,

Thanks for the corrections. Was way to tired to get the uniform and 
normal distributions right.


rand() is then by classical UNIX tradition is generated as a unsigned 
integer divided by the suitable (32th) power of two, so the maximum 
value will not be there, and this is why a small bias is introduced, 
since 0 can be reached but not 1.


I'll bet it's "pre-Unix" -

System/360 Scientific Subroutine Package Programmer's Manual, Version 
III, DOI: 10.3247/SL2Soft08.001 says 1968 version, but I'm pretty sure 
the SSP is older (it is Version 3 after all)


I used it on an IBM 1130 as a mere youth in 1969

http://media.ibm1130.org/1130-106-ocr.pdf


    SUBROUTINE RANDU(X,IY,YFL)
    IY = IX*899
    if (IY) 5,6,6
5   IY=IY+32767+1
6   YFL=IY
    YFL=YFL/32767.
    RETURN
    END


GAUSS does normal distribution, with this comment:

Y approaches a true normal distribution asympototically as K approaches 
infinity. For this subroutine, K was chosen as 12 to reduce execution time.


It also helps that the variance of a uniform distribution is 1/12, so 
summing 12 numbers produces a distribution with a variance of 1.



But it's older than that.. I found a reference to it in some 
documentation for 7090 from 1961.  Since Unix wasn't even a name until 
1970...





In practice the bias is small, but care is taken never the less.


Yes, that's a clever technique.

And the less said about the actual "randomness" of generators from that 
era, the better.





Cheers,
Magnus

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[time-nuts] Re: Simple simulation model for an OCXO?

[Lux, Jim]

On 5/2/22 6:09 PM, Magnus Danielson via time-nuts wrote:

Matthias,

On 2022-05-02 17:12, Matthias Welwarsky wrote:

Dear all,

I'm trying to come up with a reasonably simple model for an OCXO that 
I can
parametrize to experiment with a GPSDO simulator. For now I have the 
following
matlab function that "somewhat" does what I think is reasonable, but 
I would

like a reality check.

This is the matlab code:

function [phase] = synth_osc(samples,da,wn,fn)
# aging
phase = (((1:samples)/86400).^2)*da;
# white noise
phase += (rand(1,samples)-0.5)*wn;
# flicker noise
phase += cumsum(rand(1,samples)-0.5)*fn;
end

There are three components in the model, aging, white noise and 
flicker noise,

with everything expressed in fractions of seconds.

The first term basically creates a base vector that has a quadratic 
aging
function. It can be parametrized e.g. from an OCXO datasheet, daily 
aging

given in s/s per day.

The second term models white noise. It's just a random number scaled 
to the

desired 1-second uncertainty.

The third term is supposed to model flicker noise. It's basically a 
random

walk scaled to the desired magnitude.






Another thing. I think the rand function you use will give you a 
normal distribution rather than one being Gaussian or at least 
pseudo-Gaussian.


rand() gives uniform distribution from [0,1). (Matlab's doc says (0,1), 
but I've seen zero, but never seen 1.) What you want is randn(), which 
gives a zero mean, unity variance Gaussian distribution.


https://www.mathworks.com/help/matlab/ref/randn.html


A very quick-and-dirty trick to get pseudo-Gaussian noise is to take 
12 normal distribution random numbers, subtract them pair-wise and 
then add the six pairs. 


That would be for uniform distribution. A time-honored approach from the 
IBM Scientific Subroutine Package.



The subtraction removes any bias. The 12 samples will create a 
normalized deviation of 1.0, but the peak-to-peak limit is limited to 
be within +/- 12, so it may not be relevant for all noise 
simultations. Another approach is that of Box-Jenkins that creates 
much better shape, but comes at some cost in basic processing. 

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[time-nuts] Re: GPS Control Loop

[Lux, Jim]

On 4/26/22 1:31 PM, ASSI wrote:

On Montag, 25. April 2022 18:27:01 CEST André Balsa wrote:

A PDF of Shera's article can be found here (many thanks to whomever is
hosting this file):

https://www.qsl.net/n9zia/wireless/QST_GPS.pdf

To me, there is no doubt Shera's original design inspired all the following
DIY GPSDO designs in one way or another. Also this remark:

"Figure 6 also suggests that two major causes of frequency
instability—temperature shift and aging—could be predicted and largely
eliminated by tracking the performance of the VCXO for a while to estimate
the aging parameters and by measuring the ambient temperature. The
predicted corrections could be applied to the VCXO independently of the
PLL, which might allow much longer loop filtering time constants to be
used, further reducing GPS jitter. Although this scheme would be ultimately
limited by sources of crystal frequency instability that are random and
inherently unpredictable, it might be interesting to explore."

Establishing aging parameters for a modern non-ovenized crystal is a fools
errand in my experience, at least if you keep the system operational for a
long enough period of time.  If you don't, then you'll need to learn the aging
parameters anew or you'll at least have to wait out the retrace before re-
using data from the previous run.  When the initial retrace / aging transient
has subsided, a linear model is good enough for short timescales (out to
several days), but the actual logarithmic aging behaviour ensures that the
slope gets very small.  I have some systems that are going into their fifth
year of mostly uninterrupted, self-ovenized operation and aging induced
frequency drift is swamped by other influences at the timescales of a
reasonably imaginable control loop, although it is still visible on (much)
longer timescales of course (currently drifting at about 100…200ppb/year).
Feed-forward compensation of temperature fluctuation does work reasonably
well, but you can expect only about one order of magnitude performance
improvement from doing that, maybe two if you manage to get a really close
coupling of the sensor to the actual crystal temperature. IIRC, some TCXO used
to have a second quartz platelet with a special cut to act as a temperature
sensor.  It's also possible to interrogate the crystal temperature by exciting
multiple harmonics and looking at their frequency difference, but I don't know
if any commercial applications employ that effect.



That's the MCXO  - uses third harmonic and fundamental to measure the 
temperature.  Q-Tech sells them. Or, more properly, has them in their 
catalog and may be happy to quote a price and delivery. The datasheet 
revisions are >5 years ago, except for the Space version which was 
updated a couple years ago.  They're fairly good temp stability (a few 
ppb over  -40 to +90) and lower power than a OCXO of comparable 
performance.  The fancy temperature compensation doesn't say anything 
about aging, of course.  It's unclear whether cycling the temperature up 
and down at room temp makes an oscillator age faster than one held at a 
constant higher oven temp.


https://q-tech.com/products/mcxo-products/
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[time-nuts] Re: 100 MHz Low Phase Noise Mobile GPSDO/GNSSDO?

[Lux, Jim]

On 4/19/22 2:53 AM, Markus Kleinhenz via time-nuts wrote:

Hello Carsten,

thats quite the set of requirements you got there.

I believe you will have a hard time finding a COTS solution that fits
most of your requirements.

You will likely have to split your signal chain into multiple devices:

1. UBlox ZED-F9T based GPS-Receiver with 1PPS out
2. Something like a JacksonLabs RCM (a ULN Oscillator that locks to a
externally provided 1PPS)
3. Distribution Unit

So you can spread your requirements.

Another big challenge will be the 1ns relative accuracy between multiple
(moving!) devices. Without communication between devices this should not
be possible.



I think that 1 ns might be achievable, if the receivers are in the same 
general area, so they see the same propagation, but perhaps not as an 
"off the shelf" device.


We are getting <1ns with post processing using GIPSYx in a weak signal 
environment with generally poor geometry from GNSS standpoint (we're 
going to be in GEO, above the constellation, so the signals we see are 
on the other side of Earth).


Yes, that's post processed so you can "fit" a kinematic model over a 
larger span of time that's on both sides of your observation.


Can you do that in real time, only looking backwards?  I suspect it 
depends on the motion dynamics and their predictability (whether by a 
priori knowledge, or constrained by laws of physics, and estimated by 
some filter).



This is sort of the boundary between knowledge (post process) and 
control (real-time).



I would venture that there are people doing precision UAV work that are 
either doing what you need, or working towards it.





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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/14/22 3:00 PM, ed breya wrote:
It depends on context and your definition of low noise. I was 
picturing ones commensurate with the 1/f discussion, trying to get 
down toward DC. Many are spec'd for so many nV or uV p-p over 0.1 to 
10 Hz. The OP-07 is a good example of this class.


A lot of opamps are spec'd ok for their intended use, but you have to 
look fully at the charts and numbers for your actual use. For 
instance, the ADA4817 looks great as a HF/RF amp, with 4 nV/rt-Hz, but 
that's at 100 kHz. Similar for the OPA656. At 10 Hz, it looks like 
it's well over 100 nV/rt-Hz.


The JFET types are fine for high-Z amplifiers and such, but can't 
reach the near-DC region in the few nV/rt-Hz (at 10 Hz, say).


The CFAs may be OK too - they're built for low-Z, so may tend to be 
operating the + inputs at fairly high bias currents, like the low 
noise opamps.


The good part overall, is that to get near the nV/rt-Hz range, the 
source resistance has to be very low anyway, so at least the bias 
currents are manageable, although zero would be nicer. 



of course, if your low noise frequencies of interest start at 100kHz, 
then whether the 1/f crossover is at 100Hz or 10kHz is less important :)


My recreational complaining gripe is that there's a lot of interesting 
low noise parts intended for the microwave market, but they don't even 
bother giving data below 50 MHz.  So it becomes a matter of tribal lore 
- and you have the risk of "we got a really good batch" or "we don't 
make those anymore".


Vishay Siliconix *used* to make JFETs (e.g. JANTXV2N4858) with good 
noise properties (3 nV/sqrt(Hz) at 1 kHz) - but they're not in the JFET 
business any more.  They even put the noise on the datasheet, although 
with a "not guaranteed JEDEC spec", footnote. And they still sell them, 
maybe (It says "contact factory for JFET products" - factory stock, I 
suppose). Or you can scrounge around and find 4 or 8 from some dealer.  
And you can buy plenty of 2n4858s from other people, but *they* don't 
say anything about the noise.  And how do you find parts like this? Word 
of mouth from someone who spent a bunch of time looking at data sheets, 
bought a bunch of different kinds, and tested them.


I suppose that's why this is a recreational complaint - I (or anyone) 
who needed low noise FETs basically has to suck it up and go through 
datasheets, but parts, and test them.  There's no "easy fix"




JANTXV2N4858 Vishay 70244.pdf
Description: Adobe PDF document
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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/14/22 9:54 AM, ed breya wrote:
The low noise opamps are indeed designed for the feature, and they 
tell you about it, and how and why. The trade-offs are usually input 
bias current being quite high (due to bigger or paralleled transistors 
running at fairly high currents), and only good for a low resistance 
environment. 



Actually not exactly - Parts like the OPA656 have about 6 nV/sqrt(Hz) 
voltage noise and tiny fA/sqrt(Hz) current noise, and are intended for 
use in high Z circuits.


The ADA4817 is about 4 nV/sqrt(Hz) and the OPA2810 (dual) and OPA810 
(single) are buffers with  6nV/Sqrt(Hz) and 5 fA/sqrt(hz). The AD8001 is 
a current feedback amp (normally for low Z) and is 2nV/sqrt(Hz), but 2 
pA/sqrt(Hz) on the non-inverting input, so a 1M input source will turn 
that into 2mV, which is pretty large. There's some folks who have 
combined a FET source follower with the AD8001.


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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/10/22 2:50 AM, Leon Pavlovic wrote:

What would be a (very) good lab setup for the 1/f noise evaluation? I'm
talking about BJTs and JFETs.

What instrumentation and power supply are needed? How not to measure in a
wrong way - like 1/f noise in the instrumentation or the power supply
circuitry? Active collector/drain current sources vs pure resistive...

I'm quite familiar with the low NF measurements, so I guess the same
practice would apply: insert your DUT, short its input side, have a
low-noise post amp and display the noise on FFT SA?
___



Except that just like for microwave amplifiers, the noise depends on the 
impedances.


Shorting the input of an opamp will give you the noise voltage, but not 
the noise current. Putting a known resistor gives you the noise current 
(as IR plus the noise voltage plus the noise of the resistor)


For devices, I would think you set up a standardized circuit (every 
application is different, so that's a challenge).


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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/9/22 10:03 AM, use...@teply.info wrote:

On 09.04.22 15:31, Richard (Rick) Karlquist wrote:


I am seeing a lot of unsupported "theories" about what should be done 
to make devices with low 1/f noise.  It might be instructive for 
everyone

to read Marv Keshner's PhD dissertation (Stanford) discussing 1/f noise.
He looks at all kinds of theories and shows that there is no valid 
cookbook for how to make low 1/f noise devices.  It's the classic

non reproducible process.  I remember an FCS
talk many years ago that NIST guru Fred Walls gave with some theory
on how to get low 1/f noise.  Unlike his other papers which were
well received (and rightly so), this one was rapidly debunked.
I felt bad for Fred, getting out too far over his skills.

Thanks for the hint towards the thesis, I'll ask our library to fetch 
a copy.


Recently I was discussing some measurement results with my colleagues 
as we're trying to come up with a low noise JFET which can 
successfully be integrated into a SiGe BiCMOS process, and quite often 
we're also struggling to identify why exactly variant A has 
significantly lower noise than variant B, or why a new approach does 
not improve noise the way it was expected.
So from a manufacturing process design point of view, achieving low 
1/f noise indeed is closer to sheer dumb luck than the proverbial 
"more art than science" suggest. 



This is very, very true. Some manufacturers get very low noise or very 
low leakage (or both), essentially by being "lucky".  From what I've 
been told, there's no good models, nor predictions - so people share 
"lore" of "if you get these 2N FETs from the mfr in England, they're 
really good" until they aren't.   There isn't enough market for these, 
so I suspect research money to "solve the problem" isn't available.


Like all those microwave MMICs with low noise, they worry about 100 MHz 
and up (if not 1GHz), they certainly don't worry (or control) for noise 
at 5 MHz, or where the 1/f knee is. So just because you got good results 
with a batch of them, the next batch might not.  It's not even clear you 
could come up with a standardized test method, because the noise depends 
on a lot of other factors (drain current, for instance).


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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/9/22 6:31 AM, Richard (Rick) Karlquist wrote:


I am seeing a lot of unsupported "theories" about what should be done 
to make devices with low 1/f noise.  It might be instructive for everyone

to read Marv Keshner's PhD dissertation (Stanford) discussing 1/f noise.



https://dspace.mit.edu/handle/1721.1/16024 is his MIT thesis.. Is that 
what you're thinking of.


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[time-nuts] Re: Low Phase Noise 10 MHz bench signal source sought

[Lux, Jim]

On 4/7/22 3:09 PM, Joseph Gwinn wrote:


This BTULN OCXO looks very good.  While they do mention that it
contains its own regulator, no numbers are given, so it's hard to
know what to make of that claim.

  

I wouldn't assume battery power - but this is where a phone call helps -
they'll be happy to tell you.

I must say that I've run into the battery-power (or *really* good lab
power supply) approach, neither which is applicable in non-lab
applications.  I suppose if Wenzel does the packaging, they will
ensure that full OCXO performance is achieved when powered from the
usual lab AC power.  The box would also shield the power wiring and
frequency-control input (if any) and associated wiring from passing
EMI.

Joe Gwinn





That's a "call Wenzel and ask"  - They can answer your questions, and 
all we can do is speculate.  Even if I had a Wenzel box here in front of 
me with test data, that doesn't mean YOUR Wenzel box would be the 
same.   And when it comes to "in the catalog, but really we have a 
subset of stuff on the shelf" that's where calling is best.  Who knows 
what work they've been doing.  They might have had a job to build half a 
dozen sources, and they built 8, so that they'd have 6 guaranteed to 
meet the requirements, and now they happen to have a spare, or a spare 
that didn't quite hit -180, but only got -179.  And you'd be happy with 
that.


Wenzel is not some giant WalMart of oscillators, with forklifts going 
hither and yon full of palletized ULNs - there are probably 50-60 people 
there total, so odds are, you'll be talking to someone who actually has 
touched (with gloves) your oscillator.

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[time-nuts] Re: Commercial solution - 122.88 Mhz low noise source

[Lux, Jim]

On 4/3/22 5:09 PM, Bob kb8tq wrote:

Hi

Does the Abricon have an EFC input? If so, rigging up a PLL to lock
122.88 to 10 MHz is probably the best option.

Bob



And what about moving off the board - so you don't have to chase the 
temperature swings.






On Apr 3, 2022, at 7:57 PM, Jeff Blaine  wrote:

I've got a couple of Red Pitaya 122-16 SDR and want to discipline them to an 
external low noise GPSDO source.  Wanted to see if there were some easy 
solutions that fell into this category.

For the prior generation (QS1R), I had built a homebrew OCXO based on a custom 
low noise crystal from ICM and that worked great for the 125 Mhz application.  
Unfortunately this Pitaya is a bit removed in Fc and ICM has been out of biz 
for several years now.

The RP's on-board oscillator is the respectable Abracon ABLNO 122.88 which runs 
about -115 dBc/hz at 100 Hz spacing and that's probably not a limitation for 
the unit.  Unfortunately the OSC is a bit too sensitive to external temp and 
wanders around quite a lot (+/- 5 PPM) compared to the homebrew OCXO - over the 
annual ambient temp range of (10-35C) (all datasheet referenced values). The 
Abracon unit is mounted on the board with the FPGA and sees additional temp 
range depending on other heat generation factors (FPGA loading).

I have a couple of commercial GPSDO on the bench now, so a pretty clean 10 Mhz GPSDO 
reference is available now.  And I've seen enough board discussion to know that a 
homebrew GPSDO solution is one of those "it's harder than it looks" things.  
Hence the desire just to buy whatever is the silver bullet, as long as it does not 
require too much silver.  ha ha

Appreciate any suggestions.  TKs!

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

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[time-nuts] Re: Low Phase Noise 10 MHz bench signal source sought

[Lux, Jim]

On 4/2/22 4:47 PM, Joseph Gwinn wrote:

On Sat, 02 Apr 2022 03:27:06 -0400, time-nuts-requ...@lists.febo.com
wrote:
time-nuts Digest, Vol 216, Issue 3


   11. Re: Low Phase Noise70 10 MHz bench signal source sought
   (Richard (Rick) Karlquist)

Date: Fri, 1 Apr 2022 19:12:07 -0700
From: "Richard (Rick) Karlquist" 
Subject: [time-nuts] Re: Low Phase Noise70 10 MHz bench signal source
sought
To: Discussion of precise time and frequency measurement
,Bob kb8tq 
Message-ID: <0f524fb8-2220-635e-3f62-28f7f0816...@karlquist.com>
Content-Type: text/plain; charset=UTF-8; format=flowed

He [Joe] should be looking at Wenzel Associates and NEL.
Wenzel specs -170 dBc at 100 Hz offset.

I know of Wenzel, but they don't make bench-top lab instruments.
Rack-mount is available, but as a custom part.


Wenzel will be happy (for a price) to put it in any size or shape box 
you want. Most people want rack mounts, but it's mostly a matter of 
sheet metal work and they can certainly do that.






I did find NEL, and they do look very good.  The NEL 2030A and 2030B,
which are rack-mount, are plausible.

Both are likely expensive, with long lead times.


My experience with Wenzel is that the cost isn't huge - the dominant 
thing will be the stuff that goes in the box (i.e. the oscillator).  
Essentially, you're paying for a few week's engineering and tech time to 
lay out and fab the thing. At 5-10k/work week, yes, you're looking at 
$10-20k for "the box and assembly" for a one-off.


I wouldn't have them put a $200 streamline OCXO in a box 

If you're comfortable assembling the parts a linear supply from Acopian, 
a box, using frontpanelexpress.com to do the machining of the front and 
back panels, and the baseplate, and your oscillator. (that's what we've 
done at JPL on multiple occasions - although considering technician time 
it's probably cheaper to have Wenzel do it)


Wenzel also has a sort of intermediate level where you get a plate with 
a bunch of parts on it to do some function, and you put *that* in a box.



Depending on your frequency and performance, you *might* find a "in 
stock"  oscillator.  A 10 MHz Onyx or something like that might be 
basically stock.




If he is going to measure phase noise of another source with it,
he may need a VCOCXO to make a PLL.

Yes, voltage control is also needed, so a Rubidium can be used to
stabilize the OCXO.  Or the OCXO already has that built in.


Most of them have an electronic tuning input. (which can be left out as 
a special order).


My $200 (10 years ago) speedlines have a tuning input. Of course, it's 
not hitting your -170 dBc requirement, but the ULN and other ones do (or 
come close).




What is also needed is a very quiet source of DC power for the Rb,
OCXO, et al.  None of the vendors seem to specify their PSRR (power
supply rejection ratio), and I assume that all published curves are
obtained using a battery-powered unit under test.


No, that's a "call them and ask", but most have pretty good PSRR 
(although what are you looking for?) - the guy or gal on the phone will 
tell you what it is in a "not guaranteed on the data sheet" sort of way. 
Unless you want them to hit a spec, but you'll pay for it.


https://wenzel.com/model/btuln/ mentions that they have an internal low 
noise regulator - it's about 5dB shy of your -170 at 100Hz requirement.


I wouldn't assume battery power - but this is where a phone call helps - 
they'll be happy to tell you.




501-04609.pdf
Description: Adobe PDF document
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[time-nuts] RAGA - was Re: Re: Low Phase Noise70 10 MHz bench signal source sought

[Lux, Jim]

On 4/2/22 9:42 AM, Tom Van Baak wrote:
Yes, I believe hp did what you propose in their Santa Clara cesium 
test lab. It was an ensemble of selected 10811 oscillators tightly 
phase locked to improve short-term stability and phase noise. Rick 
might know the details.


During a tour I saw the instrument rack but was unable to peek inside. 
Performance details were not given and I didn't get to count the 
oscillators but I think it was on the order of half a dozen. I figured 
that using 2 or 3 probably isn't worth all the effort. And using ten 
or more is diminishing returns or maybe beyond requirements.


You can imagine if you were allowed to select the best of the best 
10811's as they came off the manufacturing test line and then combined 
a bunch of them as they did, the results would be quite impressive.


If you develop a board for this I think it would be of interest to a 
number time-nuts. It merely takes time to measure each oscillator in 
the pile and pick the best ones. But combining them takes the tricky 
circuit design that you are planning. Please keep us informed of your 
progress.


The concept of replicated simple electronics reminds me of:

http://phk.freebsd.dk/raga/

/tvb 


Separate email, because it's about RAGA..


I encountered a weird problem when I hooked up 4 OEM GPS modules to 4 
Beaglebones.. Side by side, they wouldn't all get GPS lock. It turns out 
the (unshielded) receivers radiate enough stuff to foul up adjacent 
receivers.   Move them several meters apart and no problems.  I didn't 
get around to seeing if there is some effect on the timing performance, 
even if they "get lock", but I'll bet there is.  At JPL we build 
multichannel (multiple RF channels) precision receivers, and this is 
something that is a problem (at least to the effect that we need to 
impose a crosstalk spec on the electronics chains).


I leave it as an exercise for the dedicated time-nut to decide how far 
to separate them before things like propagation non-isoplanarity, solid 
earth tides, and seismic effects, not to mention that since they're in 
places that are nanoseconds different, with the differences being SV 
related,  create a bigger problem than the "fail to lock"


When you get to "sub-meter" there's all kinds of interesting things that 
you have to worry about.  On some receivers, variations in delay through 
the LNA and bandpass filter and other electronics might vary nanoseconds 
over a 20C temperature variation.


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[time-nuts] Re: Low Phase Noise70 10 MHz bench signal source sought

[Lux, Jim]

On 4/2/22 9:42 AM, Tom Van Baak wrote:
Yes, I believe hp did what you propose in their Santa Clara cesium 
test lab. It was an ensemble of selected 10811 oscillators tightly 
phase locked to improve short-term stability and phase noise. Rick 
might know the details.


During a tour I saw the instrument rack but was unable to peek inside. 
Performance details were not given and I didn't get to count the 
oscillators but I think it was on the order of half a dozen. I figured 
that using 2 or 3 probably isn't worth all the effort. And using ten 
or more is diminishing returns or maybe beyond requirements.


You can imagine if you were allowed to select the best of the best 
10811's as they came off the manufacturing test line and then combined 
a bunch of them as they did, the results would be quite impressive.


If you develop a board for this I think it would be of interest to a 
number time-nuts. It merely takes time to measure each oscillator in 
the pile and pick the best ones. But combining them takes the tricky 
circuit design that you are planning. Please keep us informed of your 
progress.



I wonder if there would be a clever way to do the measuring in parallel. 
(other than using N(N-1)/2 TICCs to do an N cornered hat of some sort)


(or N TICCs, if you're just looking to get rid of really bad ones, and 
you're comparing against your "house" standard)






The concept of replicated simple electronics reminds me of:

http://phk.freebsd.dk/raga/



Allan also had a scheme: "This is accomplished by using an ensemble of 
inexpensive oscillators, such as TCXOs or MCXOs."


D. W. Allan, J. A. Kusters and C. E. Wheatley, "CTXO, clever time 
crystal oscillator (clock),"/Proceedings of the 1999 Joint Meeting of 
the European Frequency and Time Forum and the IEEE International 
Frequency Control Symposium (Cat. No.99CH36313)/, 1999, pp. 354-357 
vol.1, doi: 10.1109/FREQ.1999.840780.



https://ieeexplore.ieee.org/document/840780



/tvb


On 4/2/2022 12:21 AM, g...@hoffmann-hochfrequenz.de wrote:

I still have this idea to lock a flock of MTI-260 (which I have) to a
common Lucent-GPS, slowly to keep them as independent as possible,
but in-phase.

Say 8 or 16 pcs. The outputs would be Wilkinson-ed together, so 2**n
oscillators are preferred. The phase noise should average away.
That's what my VTOCXO ctl board was originally for. It can do much more,
like local 1pps that is not needed here.

Seems costly, but the last few dB are always costly. Should fit a 3H 
19" box,

and the parts are available NOW and not next year.

And, a 9 or even 17-cornered hat, has that been done?

Gerhard,

who loves replicated simple electronics like my 220nV/rtHz amplifier 
with 20 * ADA4898.


Lockaflock (TM)

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[time-nuts] Re: +1/f of transistors

[Lux, Jim]

On 4/2/22 3:31 AM, Leon Pavlovic wrote:

Hello to all,

as Gerhard already mentioned, the 1/f data of transistors is really a
kind-of black magic stuff and almost never present in the datasheets.


Not only that, but it potentially changes from lot to lot.

Lately, I've been seeing papers using various microwave pHEMTs but, by 
the time you find and read the paper, the part is no longer available. 
And, of course, just like low noise MMICs (PGA-103, GALI-74) you have to 
measure them yourself to find out - because the mfr only measures from 
50 MHz and up.


A good example is the 2018 paper by Chen, et al. which references the 
ATF54143 - a 3 year old paper, and the part isn't available any more.  
The 2SK117 shows up a lot in some older articles and app notes (e.g. 
from Wenzel) - it's discontinued, but potentially available from some 
surplus/obsolete dealers.


Exp Astron (2018) 45:231–253 https://doi.org/10.1007/s10686-018-9576-3 
https://link.springer.com/article/10.1007/s10686-018-9576-3



There is a list in The Art of Electronics, but some of them won't be 
available.   Some datasheets do have the curve - the JFE150 from TI has 
its voltage noise curve right on the front page.

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[time-nuts] Re: Low Phase Noise 10 MHz bench signal source sought

[Lux, Jim]

On 4/1/22 2:12 PM, Joseph Gwinn wrote:

I'm looking for suggestions for AC-powered 10 MHz sinewave laboratory
signal sources with very low phase noise, having a noise floor below
-170 dBc/Hz.  Rubidium is desired, but not essential.  Reliability
and durability in lab use is essential.

Which makes and models should I consider purchasing?

I like the SRS model FS725, but its noise floor is too high at -150
dBm/Hz, 20 dB noisier than many things I may wish to measure.


Thanks,


Ultimately, what you're looking for is a "oscillator and distribution 
amp in a box with a power supply" - you can build it yourself, or you 
can contract it out to a variety of places, or you can go to an 
oscillator manufacturer.


Call Wenzel Associates (https://www.wenzel.com/) - they'll put some of 
their low noise sources/distribution amps into a box with a power 
supply. For a price. Their website is broken for these kinds of 
products, but it's something they do all the time.




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[time-nuts] disciplning natural phenomena

[Lux, Jim]

As I re-read Fleming's "You Only Live Twice" last night, which features 
a geyser that is "regulated" by a valve of some sort I was intrigued by 
this idea:


Can one discipline a geyser to an external source?

a) I assume there's some data somewhere on eruption timing - sure, Old 
Faithful is quite regular, sufficiently that they can say "the next 
eruption will occur at" and people will gather and watch it.  But what's 
the ADEV? As a kid in the early 70s, I didn't think to ask this 
question, and neither Allan nor Leeson seem to mention it in their 
papers from 1966.  There is a lot of variation in timing performance of 
various geysers, though.  Old Faithful *is* regular, and in a place 
where it's watchable.


b) Are periodic geysers actually regulatable ?- From the little I know 
about how they work, I would think the eruption frequency depends on 
things like the water temperature and flow. It's also possible that the 
valve in Fleming's novel is purely fictional, because it serves as part 
of a plot device at the end (much like closing off the safety valve on a 
boiler).


Or, are geysers an example of a chaotic system that is only seemingly 
regular in some sense? That is, it's not like a VCO, with a consistent 
and well defined relation between the control input and the period. 
Changing the control input may change the period in an unpredictable 
way. Some geysers stop working if the surrounding hydrology changes. Or 
is that simply that the Q of the oscillator is so high that it's easy to 
"get out of operating range".  That would be like trying to discipline a 
spurious parasitic oscillation in an amplifier.


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[time-nuts] Re: The STM32 GPSDO, a short presentation

[Lux, Jim]

On 3/31/22 2:40 PM, Bob kb8tq wrote:

Hi

The gotcha of looking at one GPSDO against another is that
the GPS side of things is “common mode” to all of the devices.

Bob



We all know that GPS is "truth" and has no errors, common or otherwise 
...




But seriously, three cornered hats work best if all the sources have 
similar statistics, but say you compare 2 GPSDOs against OCXO, even if 
the OCXO is an order of magnitude or two worse (at long tau), where does 
it fall in the "trying to discipline a wet noodle" territory.


One might also be able to do different tau ranges - GPS is not so hot in 
the short run (1000 seconds) but is quite good in the long run (days), 
while an OCXO is the reverse (which is why we make GPSDOs at all). So, 
if you're looking to measure the performance of a new GPSDO, can one do 
two sets of measurements - one against OCXO and one against GPS, and 
then see how you do against each of the component curves.  After all, 
the hope is that the GPSDO is better than either of the other two.

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[time-nuts] Re: Truetime DC-XL usable with Lady Heather?

[Lux, Jim]

On 3/25/22 12:55 PM, Greg Troxel wrote:

paul swed  writes:


I thought there was a thread on this a long time ago. Don't seem to find
it. But was Lady Heather usable with the truetime dcxl gps receiver. When I
probe the dc-xl port it doesn't seem like it has atypical nema stream that
would be useful.

Do you mean "XL-DC"?  I have one, and I used to get NMEA out of it for
ntpd.  ntpd decoded the NMEA just fine, starting in 1997 or so (on a
sparc IPX running NetBSD)

Are you getting NMEA but finding it deficient?   Or not getting NMEA?

It is possible NMEA was an option; Truetime and the XL series seems to
have lots of options.  (Mine has 1 PPS and a 10 MHz out.)



Lots of options is an understatement.  Those things had dozens and 
dozens of options, both in terms of the software and in terms of the 
various cards that can be plugged in. You could get low phase noise 10 
MHz, you could get Rb or a better OCXO, you could get a counter/timer, 
you could get a programmable trigger/rate output and on and on. And sort 
of mix and match among the 4(?) slots.  The one I used had the LN 10MHz 
with built in DA and the hot stuff OCXO and IRIG.  All of those were 
options.


It wouldn't surprise me if NMEA was an option either in software or a 
specific card.

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[time-nuts] Re: looking for MITREX modem manual/documentation

[Lux, Jim]

On 3/25/22 10:45 AM, jeanmichel.fri...@femto-st.fr wrote:

Thank you all for the very fruitful exchanges and providing the references
I was looking for (by private email).

FYI I indeed tried the brute force search as documented at
https://github.com/oscimp/gr-satre/tree/main/reverse_code
but wanted to try to find some formal documentation demonstrating the 
orthogonality
of the codes beyond "trust me, we selected the best codes for maximum 
orthogonality".

Also if anyone is interested, I am looking into receiving TWSTFT exchange using 
TV
parabola (of course receiving only !). That is for fun since, for work the 
modem is
being assembled indeed, but that is off topic :)

Thank you very much, Jean-Michel

--
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, 
France



There's a good chance that the codes are non maximal length. More likely 
they are composite, Gold or Kasami codes, for instance. They give good 
cross correlation properties (as well as good autocorrelation) - which 
is why GPS uses them.


One can prove the orthogonality in an analytical sense, but these days, 
for a short 10,000 bit sequence, brute forcing it is probably as good as 
anything. Back in the 80s, a mathematical proof was faster.


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[time-nuts] Re: looking for MITREX modem manual/documentation

[Lux, Jim]

On 3/25/22 5:55 AM, jeanmichel.fri...@femto-st.fr wrote:

Dear community,
I am trying to find historical documents describing the pseudo random sequences 
used
in the SATRE Two-Way Satellite Time and Frequency Transfer (TWSTFT) modem.
K. Imamura & F. Takahashi (Two Way Time Transfer Via a Geostationnary Satellite,
J. of the Comm. Research Lab., 39(1), March 1992) describe the code structure
(14-bit long pseudo random sequence truncated to 1 bit length) but the
generator polynomial coefficients are not given. I have not been able to find 
this
information in Timetech's SATRE manual nor in the publicly available literature.
This paper cites "P. Hartl, A modem for microwave time and ranging experiments
via telecommunication satellites, MITREX2500 Manual, Jan 1989" which I am unable
to locate.
Would anyone have such a document, or at least could tell me whether the 
generator
polynomials are described there? Alternatively, does anyone have a description 
of these
14-bit polynomial generators?

Thanks, Jean-Michel

--
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, 
France
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I ran across this:
https://ieeexplore.ieee.org/document/5168259


 SATER modem used in microwave time transfer

but it's only 2 pages and not very informative.


A paper by Hartl, et al.

https://doi.org/10.1016/0094-5765(85)90134-1 



Cites a paper for the MITREX modem, but also mentions it can do two PN 
sequences at the same time.

P. Hartl et al., High accuracy global time transfer via geo-
synchronous telecommunication satellites with MITREX " Z. FLugwiss., 
Weltraumforschung, v7, pp 334-342, 1983



https://ui.adsabs.harvard.edu/abs/1983ZFlWe...7..335H/abstract

1983ZFlWe...7..335H


Hartl has had a paper at PTTI in Dec 1983 "Spread spectrum time transfer 
experiment via INTELSAT"


There's also P. Hartl, et al., Mitrex 25000 documentation, Institute for 
Navigation, Univ. Stuttgart, Germany, Jan 1985



Looks like it was probably built at Univ Stuttgart:
https://www.ins.uni-stuttgart.de/institut/geschichte/

Im Jahr 1983 wurde Professor Dr.-Ing. Philipp Hartl auf die Professur 
berufen und übernahm die Leitung des Instituts. Während der Amtszeit von 
Prof. Hartl wurden insbesondere die Forschungsbereiche der 
Satellitenmesstechnik sowie der Satellitenfernerkundung aufgebaut. 
Hervorzuheben sind die Konzeption und Entwicklung des 
Satellitenmesssystems PRARE, des Zeitübertragungssystems MITREX sowie 
die grundlegenden Arbeiten zur Radar-Interferometrie.

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[time-nuts] Re: looking for MITREX modem manual/documentation

[Lux, Jim]

On 3/25/22 7:45 AM, Azelio Boriani wrote:

 From the paper "PRESENT STATE OF LONG DISTANCE TIME TRANSFER VIA
SATELLITES WITH APPLICATION OF THE MITREX - MODEM", Hartl specifies
that "The PN-code is a truncated maximum length sequence of period
10.000, instead of the 16.383 chips". In the Xilinx application note
XAPP052, we can find that the taps for an MLS of 14 bits should be
14,5,3,1. In the paper "GEOSTATIONARY SATELLITE POSITION DETERMINATION
FOR COMMON-VIEW TWO-WAY TIME TRANSFER MEASUREMENTS" there is an
introduction that suggests the MITREX 2500 modem has a variable
pseudorandom noise sequence. Maybe the modem was built in different
versions wrt the PN sequence generation.


There's a fair number of of maximal sequences of length 2^14-1 . And 
it's possible that they chose a non-maximal sequence that had "better" 
properties. Maximal sequences will have a run of N ones and (N-1) zeros, 
for instance, which might not have enough transitions per unit time to 
let the receiver get a good lock.  Or they picked a "good" 10kbit 
sequence in the middle of a maximal sequence.


It could also be a composite code (the XOR of multiple PN sequences) - 
none of the papers seems to say anything about tap configurations.


I'd suggest seeing if you could find an email address for Professor 
Hartl - He'd be 94 now.  I think he retired from Univ Stuttgart in 1990.


At 2 Mchip/second, you could pretty easily build a MITREX type modem today.




On Fri, Mar 25, 2022 at 2:37 PM  wrote:

Dear community,
I am trying to find historical documents describing the pseudo random sequences 
used
in the SATRE Two-Way Satellite Time and Frequency Transfer (TWSTFT) modem.
K. Imamura & F. Takahashi (Two Way Time Transfer Via a Geostationnary Satellite,
J. of the Comm. Research Lab., 39(1), March 1992) describe the code structure
(14-bit long pseudo random sequence truncated to 1 bit length) but the
generator polynomial coefficients are not given. I have not been able to find 
this
information in Timetech's SATRE manual nor in the publicly available literature.
This paper cites "P. Hartl, A modem for microwave time and ranging experiments
via telecommunication satellites, MITREX2500 Manual, Jan 1989" which I am unable
to locate.
Would anyone have such a document, or at least could tell me whether the 
generator
polynomials are described there? Alternatively, does anyone have a description 
of these
14-bit polynomial generators?

Thanks, Jean-Michel

--
JM Friedt, FEMTO-ST Time & Frequency, 26 rue de l'Epitaphe, 25000 Besancon, 
France
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[time-nuts] Re: Why Jan 6th?

[Lux, Jim]

On 3/22/22 5:43 PM, Bill Beam wrote:

You gotta start the clock some time.

0UTC Jan 6, 1980 is when the GPS clock was started.

Do a search on "0UTC Jan 6, 1980" and you will get lots of answers.

regards
Bill NL7F



Oh lots of "GPS zero = 6 Jan 1980 at 0 UTC" answers, but none that had 
Hal's explanation - it was the first Sunday in 1980.


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[time-nuts] Re: Why Jan 6th?

[Lux, Jim]

On 3/22/22 5:30 PM, Hal Murray wrote:

j...@luxfamily.com said:

I've been hunting around for the origin of GPS zero - Why is it 0UTC Jan  6,
1980?   Is it a subtle joke about "Twelfth Night"? Does it have some  useful
properties that "end of year" does not?

GPS weeks start on Sunday.  That was the first Sunday in 1980.

OK, that's too easy.. I was going for some connection to Twelfth Night. 
When trickery, deception, and all around tomfoolery occur. Or 
overconsumption of wassail.



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[time-nuts] Why Jan 6th?

[Lux, Jim]

I've been hunting around for the origin of GPS zero - Why is it 0UTC Jan 
6, 1980?   Is it a subtle joke about "Twelfth Night"? Does it have some 
useful properties that "end of year" does not?


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[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

On 3/6/22 1:36 PM, Krishna Makhija wrote:

Hello Tom,

Yes, the GPSDOs are working well. However, when I use each as a reference
to a separate radio, I find there is a slow phase change over time between
said radios. I imagine this is expected since there will always be some
error between two discrete oscillators. However, I am hoping to use the PPS
and FEE metadata to compute what the phase *should* have been in
post-processing. So far, it is not working out for me. I am wondering if
that is even possible or if my math is just wrong.

Bob,

The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +- 10
ns is >360 deg. With a common-mode reference I see a small phase change (+-
3-4 deg) but that is not an option for my application.

Where does the PPS offset come from? Isn't it from the positioning error?
Typical GPS receivers have 1-3 m of positioning error which should give
you +- 10 ns. Why is this a "dream" performance? It should be expected from
any modern GPS receiver.

Thanks for your inputs so far.

Krishna


The PPS offset can come from multiple sources - At some point, the GPS 
receiver has an oscillator and counts down to generate the PPS, so any 
given PPS transition is going to be synced to the edges of that internal 
oscillator.  That leads to things like hanging bridges and wander 
between the time quantization of the PPS and the actual GPS epoch.  A 
typical spec is that the 1pps is within 20-50 ns of "true" (e.g. 
corresponding to an internal clock of 50-20 MHz), but the statistics of 
that error are dependent on the receiver.


Very few GPS receivers these days actually lock an internal oscillator 
to the GPS signal - rather they compute observables like code phase and 
epoch phase in terms of the local reference, then compute what the 1pps 
should be (solving for position, etc.) and then set a register so that 
the receiver puts out a pulse at the right time.  On *some* receivers, 
they'll put out estimates of the difference between the 1pps time and 
their current estimate of code "true" 1pps.  After all, the actual 
signal from the satellites are all skewed from each other, so the "true 
epoch" is a mathematical construct.


You've got two 1pps sources - so as a first step, you might hook them up 
to a counter and look at their relative performance.


Indeed, if you are not moving, and you've got two receivers side by side 
(so ionospheric effects, and solid earth tides, and all the other "less 
than a meter" phenomena are common) you should be able to get a 
*calculated* uncertainty in the single digit nanoseconds between the 
receivers.


But you don't just have receivers - you've got GPSDOs, so what you're 
seeing is an oscillator, divided down to 1Hz, with the oscillator pushed 
around to match GPS.  The 1pps from this will have different statistics 
than a 1pps from a bare receiver.  Think of it as the 1pps from GPS 
(with its 20-50 ns somewhat uniformly distributed uncertainty) run 
through a not necessarily linear filter.


You sort of have two choices in front of you:

Understand the nature of the GPSDO filters and be able to de-embed their 
contribution algorithmically - this is not easy.


Measure the instantaneous output of the GPSDO against GPS observables, 
and use some post processing to figure out what it really is. Then you 
can remove the delta between "true" estimated time and "GPSDO estimated 
time"




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[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

On 3/6/22 10:48 AM, Bob kb8tq wrote:

Hi

How close are you trying to get?

How far apart are the GPSDO’s?

A “run of the mill” number would be out around 100 ns. A “pretty good”
number is in the 20 ns range. A “crazy good” number would be 2 ns. To
do better than this, you likely would need to go to a more exotic configuration
on the GPSDO.



it also depends on whether you can "post process"  as in a receiving 
array application.  For instance, you can let the GPSDO do what it 
wants, but you record the GPS observables against your oscillator for 
each one.  Then, you post process to determine what the GPSDO actually did.


We are doing that for the SunRISE interferometer mission - 6 independent 
SDRs digitizing the signals, with a GNSS receiver running off the same 
clock.  A sort of rudimentary disciplining ensures that all the 
receivers take samples at about the same time, but the fine adjustment 
is done later.  In our case, we are processing the GNSS and clock data 
through GIPSYx, which gives you a time offset and rate at periodic time 
ticks.


So, though we capture data and run everything with (six different) 50 
MHz clocks (20ns resolution), in post processing, we can get down below 
1 ns in ultimate uncertainty. And the data acquisition only needs to be 
within a microsecond (we capture 655.36 microseconds of data, and being 
"off" by a microsecond is only reducing the section with an overlap by 
1/655th)


The trick with SDRs, in general, is *really understanding* the clock 
distribution and processing.  If there's PLLs in the pathway, then you 
need to worry about whether there are uncertainties due to initial 
state, even if the frequency of the oscillator is known perfectly.  For 
example, say you had 2 receivers with a 10MHz oscillator each, and you 
capture data at 100 kHz,  There's 100 possible offsets between the 
samples, depending on the state of the 100:1 divider for the ADC clock.


When you have FPGAs with DPLLs in them to cross clock boundaries, you 
need to be really careful that you understand what's going on there.



In your case, you seem to have continuous recording, so maybe what 
you're seeing is the small scale independent frequency variations of the 
two oscillators.  A GPSDO affects the frequency at say, 10-1000 seconds 
and longer. There will be short term variations on time scales less than 
100 seconds that the GPSDO will likely do nothing about.


If it's possible, what helps is having a free running counter running 
off your oscillator, and you "snapshot" that on each GPS 1pps. That way, 
you are getting a more direct measurement of the oscillator frequency, 
which you can then model/smooth and back out.





Bob


On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:

Hello,

I am new to the whole precision time-keeping game (and to this mailing
list) so I apologize in advance if my question is too naive or has been
answered already in your mailing list.

Is it possible to have two separate GPSDOs, each with their own antennas,
be phase coherent to each other? I have a Jackson-Labs Fury
 and a Mini-JLT
. I am using each to
provide a 10 MHz reference to two separate software-defined radios (SDRs).
In my tests I find that the phase offset between said SDRs has a slow
time-varying behavior. I know the frequency errors of the GPSDOs are of the
order of parts per trillion which will show up as slow time-varying phase
offsets but I was hoping to use the PPS offsets and instantaneous frequency
errors that I get from these modules (using SCPI commands) to be able to
"back out" or predict what that time-varying phase offset would be. Is such
a thing possible? Currently, the time-varying phase change does not seem to
follow any discernible pattern and my attempts at backing out the phase
change do not match my measurements.

Here is the math I am using for calculating what I *think *the phase
*should* be:
[image: image.png]
[image: image.png]

[image: image.png]
Does any of this seem sensible? Any input is appreciated.

TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
possible but can you use PPS offsets and frequency errors metadata to
correct for it in post?

Regards,
Krishna
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[time-nuts] Re: HP z3816a DC power supply DIP switches

[Lux, Jim]

On 3/2/22 6:00 AM, paul swed wrote:

As Ed stated Switchers feeding switchers can be problematic. What I found I
had to do was put a relay in series with a slight time delay. This allows
the primary switcher to come up and then gets slammed into the secondary
SPS. Could be a fet/transistor switch. So linear is preferable.
Regards
Paul



Sequencing is important - You can also use the "shutdown" input on the 
cascaded switcher with an RC time delay (maybe), or an explicit timer 
(555 or counter or 8 pin FPGA programmed appropriately or...)


The challenge comes when your first stage switcher needs a minimum load 
to work properly - Shutting off the secondary switcher removes the load. 
A resistor might work, if you're not total power consumption constrained.


But yes, since switchers are "constant power" devices, they draw a lot 
of current when the input is low voltage, so you can get into a 
motorboating problem with the first one in the chain's overcurrent 
protection (or just regulation).


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[time-nuts] Re: GPSDO - GPS1300-10-1000 by RFX Ltd. UK

[Lux, Jim]

On 3/1/22 12:54 AM, John Moran, Scawby Design wrote:

David - thanks for the reply, but these seem designed for SDR and I wanted 
1pps. Nice and cheap though.

Paul - thanks too; it seems that you are saying that the performance of all 
GPSDOs are the same, but that wasn't the impression I had got from listening to 
discussion here. Fine when the thing is locked (except for sawtooth stuff and 
these hanging bridge things, and digital vs analogue (British) control loops, 
etc.) but what about hold-up performance when it's not.



Holdover performance is going to be determined by the oscillator's 
stability. The time error is essentially = tau * ADEV(tau)  - so if you 
have a ADEV(1000sec) of 1E-9, after 1000 seconds, the uncertainty of 
your 1pps would be 1E-6 seconds after 1000 seconds of holdover.


A more subtle effect is "what happens when the GNSS signal returns" - 
Does your 1 pps abrubptly snap back to GNSS epoch within a few ns? Or 
does it gradually drift back to sync. Downstream uses might care.


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[time-nuts] Re: HP Z3801A project update

[Lux, Jim]

On 2/24/22 11:31 PM, Larry McDavid wrote:

I give up! Where is this $5 USB-to-RS422 Adapter to be found?

Amazon has such a DTECH adapter for $22.


Indeed the price has gone up substantially - $15

https://www.amazon.com/DSD-TECH-SH-U10-Converter-Compatible/dp/B078X5H8H7




I am using a RS422-to-RS232 adapter and it has been totally reliable 
for many years. I am not trying to use the pps output via the serial 
link so any delay in the USB adapter would be irrelevant.


Is anyone successfully using a USB-to-RS422 adapter to communicate 
with a HP Z3801A? How does the Z3801A software need to be configured 
for USB or does the USB adapter simulate a RS232 port?


The one I have looks just like a serial port (i.e. it shows up as 
/dev/cu.SLAB_USBtoUART on my mac and is usable with pyserial ) . I 
changed the jumpers on my Z3801 to RS232 a long time ago, so I don't 
know if it would work, but I suspect it would - they're not doing any 
exotic protocols - it's just serial data.


And I've never depended on the timing of the messages - so the USB 
timing isn't important.





A lot was left unsaid in that simple statement below...

Larry

On 2/24/2022 8:53 PM, Lux, Jim wrote:

On 2/24/22 7:25 PM, ed breya wrote:
Paul and Hal, I think there may be some confusion here. I modified 
the Z3801A to RS-232 way back when I first got it. My recent post 
was just discussing some of what's in this demo/testing unit thing. 
It's just a translator for TTL/RS-232 for when the GPS RX board (TTL 
serial) is used by itself, hooked to a PC (RS-232), so the WinOncore 
SW can talk to it. When the RX board is inside the Z3801A, it's 
controlled in that environment.


These days, since PCs don't have serial ports, you can buy a RS422 
USB adapter for $5.  If I were doing it now, I wouldn't bother 
reconfiguring.



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[time-nuts] Re: HP Z3801A project update

[Lux, Jim]

On 2/24/22 7:25 PM, ed breya wrote:
Paul and Hal, I think there may be some confusion here. I modified the 
Z3801A to RS-232 way back when I first got it. My recent post was just 
discussing some of what's in this demo/testing unit thing. It's just a 
translator for TTL/RS-232 for when the GPS RX board (TTL serial) is 
used by itself, hooked to a PC (RS-232), so the WinOncore SW can talk 
to it. When the RX board is inside the Z3801A, it's controlled in that 
environment.


These days, since PCs don't have serial ports, you can buy a RS422 USB 
adapter for $5.  If I were doing it now, I wouldn't bother reconfiguring.




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[time-nuts] Re: PTTI archive - looking for volunteer to write html file

[Lux, Jim]

On 2/11/22 9:51 AM, Attila Kinali wrote:

Moin,

You might know that I am hosting a copy of the old PTTI papers from
1969 to 2012 on http://time.kinali.ch/ptti/
And thanks to Demetrios Matsakis the set is now complete with all papers from 
those years.
There is one catch, though, PTTI 2005 was held jointly with IFCS, which made
the old USNO server link to the, now broken, IEEE page. The pdf files are there
( http://time.kinali.ch/ptti/2005papers/ ) but there is no html page linking to 
them.

What needs to be done is to take the html file of one of the other years (e.g. 
2006)
and change it to contain the info and links for 2005. Unfortunately, as it is 
103 papers
it is quite a bit of work and I currently do not have the time for this. So, if 
someone
on time-nuts would have the time and willingness to do this, I would very much 
appreciate it.

Attila Kinali

Is there a database that can get the bibliographic references for 2005, 
to start with. Then it's a matter of linking the reference to the right pdf.


I can get a list from Xplore of all the papers, I think


https://ieeexplore.ieee.org/xpl/conhome/10510/proceeding

And the table of contents is public (although the blue hot links in the 
pdf aren't actually links)


https://ieeexplore.ieee.org/stamp/stamp.jsp?tp==1573893

so maybe we can reprocess that in some way?

I can export all the references as a text file, and maybe we can just 
fix up the references? (or RIS or bibtex formats..)


C. T. C. Nguyen, "MEMS technology for timing and frequency 
control,"/Proceedings of the 2005 IEEE International Frequency Control 
Symposium and Exposition, 2005./, Vancouver, BC, Canada, 2005, pp. 11 pp.-.

doi: 10.1109/FREQ.2005.1573895
keywords: {Micromechanical devices;Timing;Frequency control;Atomic 
measurements;Radio frequency;Resonator filters;Microelectromechanical 
systems;Oscillators;Cost function;Robust control},
URL: 
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp==1573895=33284 



J. Oaks, K. Senior, M. Largay, R. Beard and J. Buisson, "NRL analysis of 
GPS on-orbit clocks,"/Proceedings of the 2005 IEEE International 
Frequency Control Symposium and Exposition, 2005./, Vancouver, BC, 
Canada, 2005, pp. 12-18.

doi: 10.1109/FREQ.2005.1573896
keywords: {Global Positioning System;Space 
vehicles;Laboratories;Performance analysis;Satellite ground 
stations;Atomic clocks;Timing;Extraterrestrial measurements;Geophysical 
measurements;Data analysis},
URL: 
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp==1573896=33284 



J. Phelan, T. Dass, G. Freed, J. Rajan, J. D'Agostino and M. Epstein, 
"GPS block IIR clocks in space: current performance and plans for the 
future,"/Proceedings of the 2005 IEEE International Frequency Control 
Symposium and Exposition, 2005./, Vancouver, BC, Canada, 2005, pp. 19-.

doi: 10.1109/FREQ.2005.1573897
keywords: {Global Positioning System;Satellite 
broadcasting;Stability;Oscillators;Atomic clocks;Frequency;Satellite 
navigation systems;Aerospace industry;Payloads;Computer vision},
URL: 
http://ieeexplore.ieee.org/stamp/stamp.jsp?tp==1573897=33284 


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[time-nuts] Re: Tonga effect on GPS

[Lux, Jim]

On 2/7/22 4:57 PM, Erik E. Fair wrote:
I wonder if the Tonga volcano explosion had measurable effect on the 
Earth's rotation, and what that means for UTC leap seconds (or not) 
from the IERRS.


Erik 



The folks at JPL usually put out a press release if there is an effect, 
but it takes a month or so for them to do their work. They ingest a 
bunch of precision GNSS data from all over the world grind it through 
the models, and say "the Earth moved, under your feet, by 1 mm (or 
whatever)"


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[time-nuts] Re: Tonga effect on GPS

[Lux, Jim]

On 2/7/22 12:46 PM, Kevin Rowett wrote:

Interesting data…


Can someone put the disturbance in perspective?  How much would this affect 
navigation, or timing, and how large (or small) is this disturbance in 
magnitude as compared to other events (CMEs?).


looks like a small single digit variation in TECU.

Compare to CMEs which look more like ~10 of TECU variation


https://angeo.copernicus.org/preprints/angeo-2019-39/angeo-2019-39.pdf

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2008RS004029

It's not clear on a cursory reading of the above whether the change is 
due to the UV or the particles.


I'll ask the JPLer who did the analysis, if I can figure out who did it. 
They're not credited in the news release.




KR, K6TD



On Feb 7, 2022, at 11:43 AM, John Moran, Scawby Design 
 wrote:

Someone suggested this a while ago, this may be of interest -

https://phys.org/news/2022-02-tonga-eruption-ripples-earth-ionosphere.html

John
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[time-nuts] Re: PLL subharmonic spurs

[Lux, Jim]

On 2/7/22 6:05 AM, Attila Kinali wrote:

On Mon, 7 Feb 2022 09:51:27 +1100
glen english LIST  wrote:


If you are interested in harmonics of the reference frequency at the
input to the phase detector, this will be entirely defined by the
fourier series of the source. Get yourself a 2nd year EE student,
they'll have this info for you.

I do not think that this tone is warranted here.

First of all, nothing about PLL is trivial. Especially not their
non-linear and non-ideal behaviour. I know many EEs who do not fully
understand these, much less have mastered them. Heck, I barely understand
these and I am considered the local PLL-"expert" where I am.
And it gets only worse the more closely one models the parts of a PLL,
most of which are quite far from their usually modeled ideal behaviour.



This is very true. At some trivial level, the output of the phase 
detector is some sort of variable duty cycle square wave feeding into a 
low pass filter, so a combination of the theoretical spectrum of edges 
passed through the filter would get you started.  In any case, it would 
tell you where the spurs are likely to be.


However, real PLLs often have other (non-idealized) waveforms going into 
the loop filter. A lot have little pulses (charge pump designs), but the 
pulses change their size and shape depending on how the PLL is 
programmed.  There's also interesting effects if the inputs to the phase 
(or phase frequency) detector have harmonic content, or spurious from 
other places. It wasn't a PLL, but an ADC, where I had a problem with 
the CPU clock rate (at 66 MHz) leaking in to either the power or the 
clock a ADC running at a sampling rate of 50MHz. There's a nonlinear 
sampling process (just like in a PLL), and small changes in the sampling 
instant cause spurs.


Even harder is figuring out what the spur *output* looks like from the 
PLL.  You may have figured out how the spurs look coming out of the loop 
filter, but then, that goes into the VCO, and VCOs are hardly paragons 
of perfectly linear behavior - they have their own control voltage time 
and frequency domain behavior.


to summarize, I think you can get "close" to a decent prediction if 
you're using a well known PLL part in a well understood area of 
operation. Other than that, you'd best contemplate building a breadboard 
or prototype and changing it.  I've got some experience making 7 and 8 
GHz signals with the ADF4360, and there are dozens of parameters that 
can be adjusted, the results of which are not trivially predictable.  In 
our application, we needed to tune a 100 MHz range, and have manageable 
spur levels to meet the NTIA/SFCG masks (which are generally -60dBc far 
out).

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[time-nuts] Re: Dimensioned Drawing LUCENT-SYMMETRICOM Z3810AS KS24361

[Lux, Jim]

On 2/5/22 7:00 PM, Martin Flynn wrote:
Since my homemade version did not turn out as well as I had hoped, I 
was planning to have my local fab shop laser cut a rack panel for our 
"new" LUCENT-SYMMETRICOM Z3810AS (KS24361)
You might also check out frontpanelexpress.com - they do nice engraved 
and punched panels, remarkably inexpensively. (and basically flat plates 
of any kind - I've used them for breadboards and prototypes where you 
want to fasten down a bunch of minicircuits type parts).


Before break out the ruler, anyone have a dimensioned drawing  of the 
mounting and panel holes?



Martin A Flynn / W2RWJ
Computer Deconstruction Laboratory
2201 Marconi Road
Wall Township, NJ 07719
Tel: +01 732-456-5001
Email: martin.fl...@compdecon.org
Online: www.compdecon.org
Facebook: https://www.facebook.com/compdecon/
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[time-nuts] Re: Time Signal Transmitter (low power)

[Lux, Jim]

On 1/25/22 9:29 AM, Bob kb8tq wrote:

Hi

It’s not clear that the regs in Australia would care about
doing this in an area not covered by this or that time signal.
I don’t know *what* they do or don’t say. My only concern
is that we all stay out of jail :)

Bob



For test purposes, an improvised "screen box" of sheet steel might be a 
decent approach.


I like the "harmonics from the speaker" scheme.  I'm not sure that would 
work with the new PSK modulation, though.


And yes, checking the rules is nice.  A post with perp-walk footage of a 
time-nut would not be a "a good thing".



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[time-nuts] Re: Time Signal Transmitter (low power)

[Lux, Jim]

On 1/25/22 7:52 AM, Bob kb8tq wrote:

Hi

I would check the local rules and regs before you put money
into one of these gizmos. At least by the rules over here, this
sort of thing might get you in trouble.


Setting a watch 30 cm away would require *very* low power

At 20m, you'd probably be risking setting your neighbors' clocks (unless 
you live in the middle of a huge tract of deserted land, of which there 
are many in Australia)





Bob


On Jan 25, 2022, at 5:29 AM, Anthony Dunne  wrote:

G'day time nut friends

I wonder if one of you could point me in the right direction on how to purchase 
(if one exists) a Time Signal Transmitter for use at the local level (range 
20m) for synchronising watches and radio clocks using the various standard 
radio frequencies?

And for the transmitter itself to receive / synchronise it's time using NTP?

I am in Australia and we are out of range for the normal transmissions...


Kind regards to you all


Anthony Dunne

Sydney, Australia

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[time-nuts] Re: Time Signal Transmitter (low power)

[Lux, Jim]

On 1/25/22 2:29 AM, Anthony Dunne wrote:

G'day time nut friends

I wonder if one of you could point me in the right direction on how to 
purchase (if one exists) a Time Signal Transmitter for use at the 
local level (range 20m) for synchronising watches and radio clocks 
using the various standard radio frequencies?


And for the transmitter itself to receive / synchronise it's time 
using NTP?


I am in Australia and we are out of range for the normal transmissions...


Kind regards to you all


Anthony Dunne

Sydney, Australia 



This has been discussed on the list.  The trick is to use Google and 
"site:febo.com WWVB Transmitter" (or similar..)


I found this thread which you can take a look at..  There's probably 
others, and some googling will find it.


https://febo.com/pipermail/time-nuts_lists.febo.com/2018-August/093726.html

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[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

On 1/22/22 3:30 PM, ed breya wrote:
I'd vote for going with a transformer too, but not just any old 
transformer - I'll explain later.


You can indeed connect directly to line with an AC divider, and 
measure the signal. In fact, you can even build a very broad band 
probing system that can go all the way down to DC, and up to RF, by 
making something equivalent to a high (1 meg) impedance oscilloscope 
front end. There are complications though. Measurement-wise, what 
would be needed is actually a differential system, looking between the 
neutral and line, so two identical attenuators are required to get and 
preserve the signals for subtraction and processing. It is difficult 
to get the balance and symmetry needed for good CMRR at higher 
frequencies, so line frequency and plenty of its harmonics can be 
handled OK, for good waveform fidelity well into audio, but the higher 
frequency differential and common-mode junk will blow right through. 
You would need all sorts of filtering and clamping to control the 
signal quality and protect the instrumentation. The input resistance 
should be kept high, like 1 meg, to be safer against faults, and to 
prevent tripping GFCIs (RCDs). For instance, if you're looking at 120 
VAC on a 1 meg front, the hot side input current will be around 120 uA 
RMS, adding only a small amount to the earth ground loops, compared to 
a typical 5 mA GFCI trip point.


The big problem with this, as you can see from the comments, is doing 
it safely, with properly rated parts, fault protection, and circuit 
layout and construction (especially clearance and creepage distances). 
Anyway, it can be done, but tends to be a PITA to do it right.


An obvious question is how much fidelity is needed. No matter how good 
your measuring system is, the results are only valid at the point you 
sample, and only somewhat representative of what you'd see at another 
spot - even in your own home. The incoming mains at your load center 
may be pretty solid, but every branch circuit will look a little 
different, depending on the loads and distance and so on. When 
appliances turn on and off, things will change throughout the system, 
and there will always be transients, and ubiquitous HF and RF 
interference from all the electronic gear in the system. If you look 
at it with high bandwidth, it can appear pretty disgusting, but it 
works for the main purpose of distributing plenty of power.


So, in order to remain blissfully ignorant of how ugly it may be, and 
to rig up something simple, safe, and easy, a transformer is the way 
to go. Regardless of the chosen one, some basic protections are 
usually desired, like first a small fuse or PTC on the hot side,  to 
protect the transformer in case you accidentally short the secondary - 
a high probability event when designing and experimenting. If the 
setup is experimental, and gets connected or changed around a lot, or 
you're fooling around on the primary side, it's a good idea to fuse 
the neutral connection the same way, so getting a cord reversed or 
such, won't reduce the protection function. Next, transient protection 
like TVSSs or MOVs can help to protect the parts and measuring 
equipment. Transformers are built to handle all this, so don't really 
need it - it's mostly for the other stuff. BTW I noticed in your 
recent post that you were plugging into a power strip to get some 
protection. I'd recommend not bothering with this - it will tend to 
cause more distortion, and if it's shared with other loads, you'll be 
including their effects too, making it even more removed from the true 
mains signals. Build the protections into the unit itself, and you 
won't have to worry about placing it anywhere in the system.


Now for transformer selection. First, remember that power transformers 
are not built for signal integrity. They are optimized for maximum 
cheapness that adequately meets the specs required for power transfer 
and packaging. The biggest cost factor is the amount of core and 
winding material needed to get the job done, so there are all sorts of 
trade-offs involved. The main thing is to use the smallest core 
possible, running at the highest flux density possible, along with the 
least amount of copper in the windings, to provide the function with 
"acceptable" core and winding loss, which typically may be 5-10 
percent of VA rating. Often, a temperature rise spec is provided, 
indicating the total real power loss in operation.


The simplest, biggest improvement you can make in signal fidelity, is 
to get the flux level down. The easiest way is to use a transformer 
with much higher primary voltage rating versus the line signal size. 
IOW, for 120 VAC, use a transformer with 240 V primary. For a given VA 
size, this will give four times the magnetizing inductance, one fourth 
the magnetizing current, half the flux level, and one fourth the 
equivalent VA rating (if you were to use it as a power transformer), 
versus running at 

[time-nuts] Re: Looking at ultra precise 50 km-100 km Electronic Distance Measurement.

[Lux, Jim]

On 1/22/22 9:48 AM, Patrick Barthelow wrote:

Looking to increase accuracy of a microwave transceiver pair designed to
measure very long distances  (*up to even 100km, Line of sight)
They send a microwave carrier, modulated by HF radio modulation sine waves.
To an identical Transceiver at the far end. Which sends the modulated
carrier back. and phase comparisons of emitted and returned sigs are
compared and precisely quantified.   Most instrument specs from the old
days are still pretty good,  +-  a centimeter or so, and a ppm variation
with total distance. --In 40-60km  High accuracy stuff  needs good
meteorology data along the line measured, which affects c = speed of radio
waves at the time of measurement.
The Modulation frequencies need to be either monitored continuously and/or
need to be stable to assigned values.   Trying to modify internal frequency
reference of a pretty old system to  GPSDO accuracy.
Anyone here that has done  EDM work in surveying  or Geodesy, etc?



Isn't this basically the same as the ranging we do with deep space 
transponders and DSN?


For that, we send a signal to the other end, track it with a PLL and 
send a signal back with a precise ratio (880/749 for X band). Then, as 
you describe, phase comparison between outgoing and incoming signals. To 
disambiguate, we'll phase modulate either a PN or sine or square wave on 
the carrier (leaving a lot of carrier).  The ranging tone (or sequence) 
is either just reflected back around, or is decoded and regenerated 
(regenerative ranging).


Doing a coherent turnaround at the far end allows you to cancel out any 
frequency variations at the far end, so all the "work" is at the one 
station. The oscillator on board the spacecraft is a not particularly 
special TCXO.


1 cm in 100km is 1E-7, right?

Typically, with deep space, and long averaging, we can get down into the 
1E-14 or 1E-15 range (at 1000 second averaging time). - that's cm in 
range and mm/s in Doppler at ranges of 1E9 km.  Run of the mill 
performance is ~1 meter.


The challenge would be finding some way to measure the difference in 
speed of light. For ionized materials, there's a frequency dispersion, 
so using two frequencies allows you to correct for it. (I don't know how 
well you can do using the ranging sidebands.. is 10 MHz enough separation?)


But I don't know if air density and humidity have a strong enough 
frequency dispersion effect. For DSN, we measure humidity along the path 
with radiometry.


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[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

On 1/22/22 8:21 AM, Bob kb8tq wrote:

Hi

None of these transformers are designed for audio use. The same is true
of the various inductors and capacitors mounted up on power poles to try
to phase correct the distribution process. Pretty much all of them are “out
in the wild” and thus exposed to fairly large temperature swings.

All of that suggests that the typical power distribution line is pretty narrow
band / low pass. Indeed back in the day, that’s what we found when measuring
this and that.

I’d vote for sticking with the transformer.

Bob



As always, there's a standard for that..
ANSI/IEEE C62.41-2002, “Guide on the Surge Environment in Low-Voltage 
(1,000 V and less) AC Power Circuits.”
(which is inactivated as of 2021, but reserved, I think there's a 
replacement coming out)


This is probably a Category A or B location

https://www.eaton.com/content/dam/eaton/products/backup-power-ups-surge-it-power-distribution/complete-home-surge-protection/eaton-eatons-guide-surge-suppression-application-notes-sa01005003e-ca.pdf 


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[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

On 1/22/22 2:09 AM, Wilko Bulte wrote:

Given that it sounds that these capacitors  are mains connected I would recommend using Y 
rated capacitors for safety reasons. That should prevent them from going short circuit, Y 
caps being designed to "fail open".

Or did I misunderstand the proposed circuit?

Wilko


Yes, choosing the right components is important in any line connected 
device.


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[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

On 1/21/22 7:00 PM, Robert LaJeunesse wrote:

Stick with the transformer. The use of a capacitive divider is predicated on 
the line waveform always being a sine wave. Dream on! All it takes is one good 
spike down the line, maybe only 20-30V amplitude, and your capacitive divider 
passes it right on to that ADC that has a much lower (3.3V?) limit. Guess what 
goes poof?

Bob L.



diode clamps or "Tranzorb" (which is basically back to back zeners)

An awful lot of carrier current operated devices use capacitor 
isolation, so it's a "solved problem" (and, of course, carrier current 
means you need to pass higher frequencies)




Sent: Friday, January 21, 2022 at 8:48 PM
From: "Lux, Jim" 
To: time-nuts@lists.febo.com
Subject: [time-nuts] Re: Another reason to monitor line frequency :) - My AC 
measurement project & question

On 1/21/22 4:43 PM, willl will wrote:

Hi everyone,

I have an recently finished project that also measuring AC waveform, full
description here:
https://github.com/will127534/RaspberryPiAtomicNixieClock/wiki

Basically using an AC transformer and Ti's ADC8681 @ 50Khz sampling rate.

This year I'm working on a earthquake sensor + AC mains monitor system (In
an earthquake prone area, AC mains frequency will fluctuate by power
generator and machine emergency stop like this one:
https://twitter.com/kuriuzu/status/1360602496821911553).

I want to improve AC measurement. Apart from the ADC sampling speed upgrade
(previously bottlenecked by the SPI connection to FPGA). I'm currently
debating about whether or not to bypass the transformer. How does the
distortion of an AC transformer impact the accuracy of mains waveform and
frequency? I'm not sure if it is worth it to go through the mains voltage
safety requirements.

You can use a capacitorsget your galvanic isolation, and a CR voltage
divider with minimal waveform distortion. Pick a burden current (say, 1
mA) and for 120V line, you need 120k impedance at line frequency 60Hz

X = 1/(377*C) --> C = 1/(377 * 120E3)  = 22 nF

Say you want ~100:1 ratio? so 22 nF in series with 1.2k  (or 2.2uF)
would do nicely. Then feed your high Z ADC with a couple 0.1 uF
capacitors from the ends of the 1.2k.

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[time-nuts] Re: Another reason to monitor line frequency :) - My AC measurement project & question

[Lux, Jim]

On 1/21/22 4:43 PM, willl will wrote:

Hi everyone,

I have an recently finished project that also measuring AC waveform, full
description here:
https://github.com/will127534/RaspberryPiAtomicNixieClock/wiki

Basically using an AC transformer and Ti's ADC8681 @ 50Khz sampling rate.

This year I'm working on a earthquake sensor + AC mains monitor system (In
an earthquake prone area, AC mains frequency will fluctuate by power
generator and machine emergency stop like this one:
https://twitter.com/kuriuzu/status/1360602496821911553).

I want to improve AC measurement. Apart from the ADC sampling speed upgrade
(previously bottlenecked by the SPI connection to FPGA). I'm currently
debating about whether or not to bypass the transformer. How does the
distortion of an AC transformer impact the accuracy of mains waveform and
frequency? I'm not sure if it is worth it to go through the mains voltage
safety requirements.


You can use a capacitorsget your galvanic isolation, and a CR voltage 
divider with minimal waveform distortion. Pick a burden current (say, 1 
mA) and for 120V line, you need 120k impedance at line frequency 60Hz


X = 1/(377*C) --> C = 1/(377 * 120E3)  = 22 nF

Say you want ~100:1 ratio? so 22 nF in series with 1.2k  (or 2.2uF) 
would do nicely. Then feed your high Z ADC with a couple 0.1 uF 
capacitors from the ends of the 1.2k.


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[time-nuts] Re: Dense pressure data of the last 24h and the next 48h

[Lux, Jim]

On 1/19/22 3:50 PM, Hal Murray wrote:

anders.e.e.wal...@gmail.com said:

AFAIK we only have 10 minute data...

What is the wavelength (in seconds) of an eruption?  What's the Nyquist
sampling rate?


From the shock wave?  I'm not sure wavelength is the appropriate 
measure - a shock wave starts as a step function, and then eventually 
relaxes to some sort of slope.





Is there dispersion of audio signals in the atmosphere?  I don't remember
hearing about it.  Maybe signals going around-the-world are different due to
interactions with the ground or open top,


Yes, there is dispersion - not in the variation of speed with frequency, 
but the atmosphere has different temperatures at different heights, so 
that causes dispersion, as do surface winds.





--

What sort of sampling rates do metrologists want or get?

--

What sort of gear do people use if they want to be able to see this sort of
thing?

What is the noise like?  Is it mostly from wind?  How many bits of signal are
left on a windy day?  Or what should I be asking?



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[time-nuts] Re: Vectron 380 teardown

[Lux, Jim]

On 1/15/22 8:30 AM, Bob kb8tq wrote:

Hi

Ummm ….. e ….. somebody … errr … got you a deal on
those parts … errr ….

Bob


Indeed.  But they were also, I think, some of the first 100MHz 
versions.  $400 each, in 2015.  They worked just fine.


Same mission we had the CSAC on (one goal was to compare CSAC, GPS, and 
EX-421.. as it happens, our temperatures were pretty stable, so it's not 
exactly a challenging test - as I recall, over the 6 months of data, 
there was no real change in frequency of either CSAC or OCXO compared 
against GPS, and the GPS was a simple Novatel OEM widget, so no ball of 
fire time-nuts wise)





On Jan 15, 2022, at 10:01 AM, Lux, Jim  wrote:

On 1/14/22 3:40 PM, Bob kb8tq wrote:

Hi

The design traces back to the Efratom “EMXO” from the 1980’s.
Vectron bought the rights to that design and produced examples
of it for about a decade or so.

Bob


I flew a 100MHz EX-421 in space on a cubesat - same series of tiny OCXOs - 
except the EX421 is a bit smaller foot print and taller. it's about the size of 
a US sugar cube (13x13 mm and 10mm tall). Low power for an OCXO, and small, 
both of which were important. And the phase noise was decent. They weren't too 
expensive (a few hundred $ each, as I recall - non space grade).
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[time-nuts] Re: High precision OCXO supplier for end costomers

[Lux, Jim]

On 1/10/22 9:56 AM, Bob kb8tq wrote:

Hi

An equally important part of this:

What are you driving with this OCXO and what is it’s measured noise floor
at 1 Hz (or 10 Hz or what ever ….). In some cases a “crazy” OCXO is actually
quieter than the device it is driving. That means that the last 5 or 10 db in 
phase
noise improvement really has zero impact on the system performance. I’ve run
into this a *lot* of times over the years.

Bob


This comes up a lot with ADCs.. Wideband driver amplifiers on the clock 
inputs may put more noise on the digitized signal.



An-756 from Analog Devices

Sampled Systems and the Effects of Clock Phase Noise and Jitter


https://www.analog.com/media/en/technical-documentation/application-notes/AN-756.pdf

and
Clocking the RF ADC: Should you worry  about jitter or phase noise?

https://www.ti.com/lit/an/slyt705/slyt705.pdf

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[time-nuts] Re: Clock specs for audio (was: High precision OCXO supplier for end costomers)

[Lux, Jim]

On 1/10/22 7:12 AM, Bernd Neubig wrote:

We are receiving such inquiries from "Audio nuts" rather frequently, but
also from professional high-end audios-studio equipment makers. There
argument is often, that the spatial transparency of the sound, i.e. how
exactly you can locate the sound source (instrument in an orchestra) would
be noticeably improved by such ultra-low noise OCXO sources. So it should be
more about time or phase (jitter?) than about frequency

As the customer and his belief is "king" at AXTAL - as long as doable and
payable - we have developed our AXIOM45ULN series, where the best phase
noise option guarantees a PN level of -115 dBc/Hz @ 1 Hz. But this kind of
performance can only be achieved by a crystal selection with rather low
yield. Therefore, as a manufacturer you need enough customers who accept
that "less is sufficient" and will buy the OCXO made from the other
crystals. We also are getting a few parts with -120 dBc/Hz @ 1 Hz out of a
larger lot, but we rather keep them than selling them to everybody.

Best regards
Bernd



And I suppose this is why it's worth talking to the mfr than looking 
through the catalogs. There might well be some key requirement that if 
relaxed slightly would work out quite well in terms of availability.


We run into this all the time in the space business - someone does a 
structured requirements flowdown, allocating design margin to subunits, 
and winds up with a performance spec that is difficult to meet, and 
nobody wants to go back up the flowdown chain and ask if the requirement 
can be changed.  Indeed, the cost of doing the waiver might be more than 
just buying the unnecessarily expensive part.


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[time-nuts] Re: High precision OCXO supplier for end costomers

[Lux, Jim]

On 1/10/22 12:26 AM, Norman Reitz via time-nuts wrote:

Hi, everyone,
I am looking for suppliers of high-quality OCXO in 10 Mhz (sine / square) and 
25 Mhz sine wave output. I can only find changes below my quality requirements 
for phase noise at Mouser or Digikey. E.g. a 10Mhz should be better than 
-120dbc @ 1Hz (-140dbc @ 10Hz) - a 25mHz better than -115 dbc@10Hz.I want to 
use them in high quality audio application. Unfortunately, the minimum order 
quantities of the providers that I have found are not available for private 
customers - or you only sell with proof of use or a trade license. Since i dont 
want to start a business in space or defence-business, this is a problem for 
me. Do you have a tip or contact person who also does business with 
non-lucrative end customers? I am already aware that the quality of OCXO cannot 
be obtained for 100 bucks.
best regards
Norman




I assume you've tried the usual suspects like Vectron (part of 
Microchip, now), Bliley, Wenzel, Q-Tech, Abracon, MTI-Milliren.


You might check to make sure you're not edging close to the limits in 
the US Munitions List (ITAR) - that will tend to make things harder - 
even if it's not space qualified or for radar: (15) Space-qualified 
oscillator for radar in paragraph (a) of this category with phase noise 
less than −120 dBc/Hz + (20 log10(RF) (in GHz)) measured at 2 KHz* RF 
(in GHz) from carrier;


Most of these places only sell through distribution, not through 
Mouser/Digikey/Farnell kinds of paths. But small quantities should be no 
problem (I've bought single or 3-5 oscillators at one time from most of 
these mfrs, and gotten quotes from all of them). Delivery time will be 
long (months, if not a year).

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[time-nuts] Re: Where do people get the time?

[Lux, Jim]

On 1/2/22 12:39 PM, Peter Vince wrote:


I used to work in broadcast television, and find that most people would
notice a couple of "frames" (80ms here in the UK) - we, trained and looking
for it, would notice one frame (40ms).  But it is more disturbing if the
sound is early, as that is so unnatural.  At that sort of level though, it
is quite hard to tell if it is early or late, especially as the actors
won't just stand there saying the same thing time and again :-)

Because the sound and video don't use the same frame rates, MPEG codecs
have a hard time getting it right.  We developed a test signal which
measured the time between a blip on the sound and a flash on the vision,
and used it regularly when lining up links.



The curse of the USB 8 kHz frame rate (derived from telephony, of course).

Neither 24 nor 60 divides into 8000 evenly. 50 does, but that's cold 
comfort for the US centric industry.  And let's not even talk about 
59.94 or 29.97 for NTSC video.


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[time-nuts] Re: Where do people get the time?

[Lux, Jim]

On 1/2/22 9:57 AM, Hal Murray wrote:

j...@luxfamily.com said:

So the  *sound* of the clap has to propagate to the sound recording equipment
(some ten miliseconds away if the mic is on a fishpole or boom).

How far off does the audio have to be before it doesn't look/sound right?  How
accurately does the typical movie process get things aligned?

We are used to a delay of several to low 10s of ms.  Somewhere in the 20-100
ms range (depending on how good your eyes are) you can't see the speakers face
well enough to help decode what they are saying.

Are people sensitive to the sound being early?


I don't know today, but back in the day, probably 1/24th of a second, 
even though they're projected at 48 fps (each frame is projected twice, 
so the flicker frequency is higher).


A lot depends on the image size - you're used to a shorter delay when 
you're closer to the person (i.e. their image is larger), but when 
they're across the room you expect a longer delay.  This is one of the 
things that makes the audio sound different when watching a movie on a 
small screen up close rather than in a theater - the psycho acoustic 
cues are different.  As to how they do it - skilled editors use their 
judgement.


I'd say what people are sensitive to is lip movements not synced with 
sounds, whether early or late.  I suppose obvious simultaneous events 
(gunshot sound + flash) would be weird if the sound occurred before the 
image. But a bit late would just be like real life.  In fact, the 
simultaneous explosion and sound in movies is something that bugs me, 
because if you were actually there, the delay of, say, 100 ms, is very 
noticeable - and for big things like rocket launches (and, I suppose 
nuclear explosions) which you're watching from miles away it's very 
noticeable.  I watched a Titan IV launch from about 10km away. You saw 
the ignition, a 5-10 seconds later you felt the ground shaking, and the 
rocket was maybe 300-400 meters up and hitting the scattered clouds 
before the sound got to you 30 seconds later, and then you have the 
weird phenomenon of the rocket getting smaller and farther away, while 
the sound gets louder.

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[time-nuts] Re: Where do people get the time?

[Lux, Jim]

On 1/1/22 8:48 PM, Tom Van Baak wrote:

Hal Murray wrote:
> where do you get the time?

The new years celebrations last night remind me of another source of 
time: time balls.


Although a bit of nostalgia these days or even a joke, time balls were 
clever, precise, and a critical part of naval infrastructure in the 
19th century, lasting well into the 20th century, and still 
operational daily in Greenwich (not right now due to refurbishment). 
It falls into the category of "time dissemination"; you know, WWV, 
GPS, UTC, NTP, and all.


The sound of church bells gave approximate time, partly because their 
use case didn't require high precision and partly because sound 
travels only one foot per millisecond. Related: starter pistols at 
track events; clapperboards on movie sets; timing thunderstorms with 
lightning. 



Clapper  boards even automatically compensate for the acoustic time 
delay, especially in a multi channel recording scenario. The board is 
placed near the people being recorded, the camera and sound gear is 
started (Rolling, Speed,...)


Then the clapper board is actuated (electronically today, with timecode 
displayed on an electronic board) (a process called "slating"). So the 
*sound* of the clap has to propagate to the sound recording equipment 
(some ten miliseconds away if the mic is on a fishpole or boom).  In 
editing, the sound and picture are lined up, (i.e. the sound is started 
later) using the sync from the clapper.


Obscure note - if they forgot to slate at the beginning, or they 
couldn't do it for some reason (focus, zoom, blocking), they do it at 
the end of the take, and hold it upside down.


Slating is done even if there's no sound, so you can identify the piece 
of film. And, if there's multiple cameras, there will be multiple slating.


I sort of left the business before modern digital cameras were 
universal, I assume they still do it, even if there's no need to 
physically identify a piece of film.

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[time-nuts] Re: Derivation of time from celestial sight

[Lux, Jim]

On 12/27/21 12:18 PM, Brent wrote:

My understanding (and I could be wrong) is that one could derive 'stellar'
time from a start sight/fix on polaris or another well tracked celestial
object.  I was once told that early editions of Bowditch provided the
process (for the moon I was told) although one of the relatively old
edition's that I have doesn't provide it.


Occultation of stars by the Moon provides a "universal" time source 
(assuming you can see the Moon and stars). It was the competitor to 
Harrison's clock based approach to timekeeping for the measurement of 
longitude. For stars sufficiently far away, the Moon passes in front of 
it at the same time everywhere on Earth. There are some problems - the 
Moon has a rough edge, etc.  And, of course, you need accurate 
ephemerides for the Moon and accurate celestial position of the stars.


Observations of a star can only give you local time; you need two things 
moving at different speeds to get the time at a specific longitude.





Some theodolite manufacturers provided attachments to aid the process (for
the high zenith where a theodolite experiences reduced accuracy), and those
attachments were dated and calibrated for their year of manufacture and
came with tables for use in future years.

That's about all I know or can find on the subject.  Can anyone here point
me to any published literature?  Anyone have experience trying?  Any idea
what type of accuracy can be expected?


There is a CD-ROM published by ION (Institute of Navigation) with 
hundreds of papers on such things.


https://www.ion.org/publications/upload/CelestialNavTOC.pdf

https://www.ion.org/publications/order-publications.cfm

$50 worth of reading on all manner of topics navigation and time related 
- Papers on "how Vikings navigated" "how Columbus navigated" (and papers 
refuting the theories in the previous papers, etc.)






Got some new toys coming and need something to do with them



And, of course, you can get yourself a copy of the Nautical Almanac and 
some sight reduction tables (get the airplane ones, not the ship ones) 
and do some celestial nav. You can make an artificial horizon with a pan 
of water at some distance, so you can sight your object of interest and 
the reflection at the same time.




Brent
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[time-nuts] Re: NIST NTP servers way off for anyone else?

[Lux, Jim]

On 12/15/21 7:53 AM, Magnus Danielson via time-nuts wrote:

Hi,

Expect network routes to be more dispersed these days, as it is needed.

While the wedge plot is a classic for NTP, it may be interesting to 
plot forward and backward path histograms independently.


Cheers,
Magnus 



I assume someone, somewhere has run some recent tests and maybe 
published them. All those plots and behaviors from the early days of NTP 
might have significantly changed, due to the plethora of new kinds of 
network routes.  Two things strike me as being "very different" from, 
say, 10-20 years ago - 20 years ago, most routers were "store and 
forward" - the entire packet would be received, and then decoded, and 
sent onward.  These days, many routers start sending the packet to the 
destination before the entire packet has been received.  To do S would 
take too much memory with multi Gbps speeds and long packets.  I recall 
being at a conference at least 10 years ago where they were talking 
about the sophistication required in 10G routers - cut through routing, 
adaptive equalization, etc.


The other thing that has changed is a modern diversity of kinds of 
networks. 20 years ago, it was basically wired connections of some kind 
with concentrators/deconcentrators/switches/routers - all of which have 
moderately well defined latency and statistics.


Now, though, there's a lot of over the air (cell phones, WISP, 5,6,7G 
nanocells injected surreptitiously - at least my neighbor claims that's 
what they're doing).  The latency on a WiFi connection, in a busy 
environment - It's 8PM, and all the neighbors are streaming "The Wheel 
of Time" (appropriately, for time-nuts) - varies wildly over a short 
time. (I will say that WiFi latency improves dramatically during a power 
failure in a residential neighborhood when you have backup power, and 
your neighbors do not)


Imagine NTP running over Starlink, especially when there are multi hop 
crosslinks between satellites.  At 7 km/s orbital velocity, the range is 
changing as much as 21 microseconds/second to a "stationary" observer.  
Now consider two satellites in different orbital planes. The dynamics of 
the latency get quite complex.


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[time-nuts] Re: NIST NTP servers way off for anyone else?

[Lux, Jim]

On 12/15/21 7:53 AM, Magnus Danielson via time-nuts wrote:

Hi,

Expect network routes to be more dispersed these days, as it is needed.

While the wedge plot is a classic for NTP, it may be interesting to 
plot forward and backward path histograms independently.


Cheers,
Magnus 



I assume someone, somewhere has run some recent tests and maybe 
published them. All those plots and behaviors from the early days of NTP 
might have significantly changed, due to the plethora of new kinds of 
network routes.  Two things strike me as being "very different" from, 
say, 10-20 years ago - 20 years ago, most routers were "store and 
forward" - the entire packet would be received, and then decoded, and 
sent onward.  These days, many routers start sending the packet to the 
destination before the entire packet has been received.  To do S would 
take too much memory with multi Gbps speeds and long packets.  I recall 
being at a conference at least 10 years ago where they were talking 
about the sophistication required in 10G routers - cut through routing, 
adaptive equalization, etc.


The other thing that has changed is a modern diversity of kinds of 
networks. 20 years ago, it was basically wired connections of some kind 
with concentrators/deconcentrators/switches/routers - all of which have 
moderately well defined latency and statistics.


Now, though, there's a lot of over the air (cell phones, WISP, 5,6,7G 
nanocells injected surreptitiously - at least my neighbor claims that's 
what they're doing).  The latency on a WiFi connection, in a busy 
environment - It's 8PM, and all the neighbors are streaming "The Wheel 
of Time" (appropriately, for time-nuts) - varies wildly over a short 
time. (I will say that WiFi latency improves dramatically during a power 
failure in a residential neighborhood when you have backup power, and 
your neighbors do not)


Imagine NTP running over Starlink, especially when there are multi hop 
crosslinks between satellites.  At 7 km/s orbital velocity, the range is 
changing as much as 21 microseconds/second to a "stationary" observer.  
Now consider two satellites in different orbital planes. The dynamics of 
the latency get quite complex.


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[time-nuts] Re: Clock displays -- eye response

[Lux, Jim]

On 12/10/21 12:31 PM, Brooke Clarke via time-nuts wrote:

Hi Hal:

There has been some recent research into illusions related to sight 
and sound.

https://en.wikipedia.org/wiki/McGurk_effect - related to speech
and search "audio optical illusion"

I like a crisp "tick" for clock human synchronization.

I wonder why there has not been more done with military "Have Quick" 
for time synchronization?

https://en.wikipedia.org/wiki/HAVE_QUICK


HAVE QUICK was a very early hopping spread spectrum system and was long 
since superseded by SINCGARS and other systems in the 1980s.  I don't 
know that it is a "source" of time.


There was a whole set of systems for loading "time" into these frequency 
hopping systems, since all manner of their synchronization behavior 
depended on time.  And in usual DoD/NATO fashion, forward and backward 
compatibility was often required, so you'd have a way to sync any two 
random radios in a battlefield situation.  Cryptographic keying is also 
often time based, as are frequency nets (local interference, keeping the 
other guy guessing, or propagation changes)


Synchronization is the "hard part" of most spread spectrum systems both 
Direct Sequence (PN codes) and Frequency Hopping, and that's where most 
of the classified stuff is - how do you synchronize reliably, how do you 
prevent the synchronization from being spoofed or jammed. A naive FH 
approach is to have a "hailing channel" and the first person transmits 
there, and the other person hears it, listens to a sync pattern, and 
then commence hopping together. This works for point to point between 
two stations, but doesn't work very well when you have multiple 
stations, not all of which can hear each other.


GPS would have been a godsend back then (although it's not very good 
from an Anti Jam standpoint).



Dixon, in the seminal tome "Spread Spectrum Systems" kind of makes an 
offhand comment that synchronization is the challenging part, and then 
moves on "assuming we have synchronized".



It's been a part of the PLGR and DAGR GPS receivers and I expect also 
for the military embedded versions for a long time.

https://prc68.com/I/PLGR.shtml#Time
https://prc68.com/I/DAGR.shtml#HQ1PPS
Also things like the O-1814/GRC-206 Reference Frequency Rb Oscillator 
make us of it.

https://prc68.com/I/O1814.shtml

I don't know that they make use of it, rather, they can provide sync TO 
a HAVE QUICK radio.


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[time-nuts] Re: Clock displays -- eye response

[Lux, Jim]

On 12/10/21 12:09 PM, Hal Murray wrote:

Does anybody have numbers for how long it takes for a visual signal to get
into your brain?

I think it's around 250 ms for a human to push a button when a light goes on.
Less if the penalty for false pushes is low.  I don't have a handy URL to back
that up.

But that's in and back out.  I assume the "in" step is only part of that.

Are flashes out of the corner of your eye that might indicate danger faster?

If 2 lights go on at close to the same time, how far apart do they have to be
before you can notice that one goes on first?


Oddly, something I have some practical experience with, see [1]. Using 
electrodes on your scalp, and flashing a light (or pattern), you can 
measure how long it takes for a response to show up.  The latency, 
particularly, comparing the two hemispheres, has diagnostic value. As 
children get older, the response time gets faster (perhaps some 
fundamental maturational thing?)


It takes about 80-120 milliseconds for your visual cortex to respond to 
a sudden change. We flashed checkerboard patterns, because your visual 
system tends to filter out uniform stimuli - blurry images have less 
response amplitude than sharp, but the time is the same. To actually 
interpret the stimulus takes about another 50-100 milliseconds (you get 
what's sometimes called a "recognition peak" at around 300 milliseconds).


Button pushing can actually be faster, because you are "primed" for the 
stimulus - think drag racing christmas tree, or Jeopardy, where you can 
only buzz in after the cuing lights have come on. I'm not sure if it's 
actually a faster response, or whether you're really responding to the 
priming stimulus. (I'm sure someone has studied it, I did my thing >40 
years ago).  Also, if the light is repetitively flashing, your brain 
will sync to it, sometimes quite impressively - It's some form of 
oscillator locking, and I'm sure you could analyze it as such. (for my 
research, we did random intervals, so that the flash response is random 
against the larger background activity)


Flicker fusion/distinction is a whole other set of things. That is 
substantially shorter time (i.e. you can tell the difference between two 
flashing lights with much shorter delays).


The physiological response time is independent of where it occurs in the 
visual field, at least for the 100 millisecond P1 response.


All of these "decision making" kinds of things (as opposed to the raw 
cortical processing, which looks for edges) are VERY dependent on things 
like brightness, attention, fatigue. There was a wealth of research into 
people looking at things like radar displays. One thing that humans are 
*really* good at is seeing patterns in noise (even if they aren't real - 
your brain wants to see structure), so blinking lights with slight 
irregularities in the pattern are easily detected.


https://colorusage.arc.nasa.gov/flashing.php


[1]


 Detection of Learning Disabilities Using the Visually Evoked Cortical
 Potential

Lux, James P.*Journal of Pediatric Ophthalmology and Strabismus**; 
Thorofare* Vol. 14, Iss. 4, (Jul/Aug 1977): 248-253.

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[time-nuts] Re: Clock display on Linux systems?

[Lux, Jim]

On 12/8/21 2:15 PM, Bill Dailey wrote:

You can also set them up so they don’t write to the SD once everything is set.  
SD’s will last forever like this.  Basically read only and RAM disk.



yes indeed - these days, with lots o'RAM on a rPi, you should boot off 
the SD (or eMMC) and run out of RAM.  For a "clock" application, you 
could probably structure your writes to SD (for nonvolatile storage of 
logs, etc.) so that you limit the number of writes. If you log once an 
hour that's just under 9000 writes/year.


Typical MLC flash is good for at least 10,000 erase cycles on a page. 
Writing data to an erased page (or the part that's not already written) 
doesn't wear it out, but changing data in the middle of a file does, 
because you have to erase it (consuming life), and then rewrite.


There are Journaling File Systems that deal with this, but I doubt 
they're compatible with the wear leveling systems in commercial SD 
cards. Basically, the SD card has a controller that exposes a 
generalized interface, with the wear leveling hidden from you, and if 
it's hidden, then the JFS doesn't really know how to manage the device.


I don't know, though, it's a fertile ground - and someone may have a 
nice JFS for a common distro for RPi and SD card.



If you want to get real down and dirty, there are also clever schemes 
that write all ones or zeros (depending on the device), instead of 
erasing, and then the reader of the file knows that this means "not 
used" - Much like the RUBOUT character on paper tape, or a similar 
scheme used with PROMS where you don't want to erase it.


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[time-nuts] Re: Clock display on Linux systems?

[Lux, Jim]

On 12/6/21 11:34 AM, John Miller via time-nuts wrote:

This is something I have explored from time to time in order to find
a good local display option for Pis running as GIS-disciplined NTP
servers. I've done a lot of exploring and fiddling around with different
options but have yet to find a solution that I am fond of, for the
reasons that you describe. 



When last I was looking at this, it turns out there's a bunch of 
non-traditional display shapes available for things like "Shelf edge 
displays" and such.  That is the google-able phrase to turn up stuff.


https://www.sunul.com/pdf/lcdr/sr21.5-200.pdf is an example - 2" high 
1920x200 pixels with HDMI (and other) interfaces.



They aren't super cheap, in general. (expect >$100)

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[time-nuts] Re: Preprogrammed fixed-frequency relatively lower noise oscillator chip

[Lux, Jim]

On 12/4/21 10:31 AM, Jeff Blaine wrote:
I'm wondering what the modern version of a fixed-frequency oscillator 
module is.


I had used the Silicon Labs (now Skyworks) Si570 in the past which has 
a great close-in noise footprint - but that requires I2C and some 
glue.  The Seiko distributor-programmable type of generic parts are 
functionally ideal - comes preprogrammed and requires minimal 
connections to function - but they have miserable close in noise 
characteristics.  I realize that "miserable" here is subjective of 
course.


The project at hand is a single frequency oscillator to serve as a LO 
in a receiver.  The noise profile I'm looking for would be in the 
range of "better than the generic PLL type $5 oscillators" and "not as 
good as the modern equivalent to the Si570 & similar."  Absolutely 
non-critical but the Seiko part in there now looks terrible starting 
to flatten out at -40 dBc a Khz or so spaced.


This answer seemed (from my recollection) easy to find back in the 
early 2010s when I had messed with it last.  Then Abercon had some 
decent offerings - but that is a long time ago in Electronics Progress 
Time scale.



Probably you're thinking Abracon -they're still in business - and all 
sorts of oscillators, including MEMS.


I don't know if they make a programmable one (even "program at 
factory/distributor")


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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/29/21 1:57 AM, Hal Murray wrote:

j...@luxfamily.com said:

And a lot ofsources may have a low flat spot in the curve, but it
eventually trends up. Except for primary standards like Cs beam.

What's magic about "primary standard" or "Cs beam" that keeps the ADEV from
trending up?

Their ultimate accuracy is dependent only on a invariant physical 
property that is independent of time.


A quartz crystal ages. No matter how good your oven is, the frequency 
will change over a long time.


Mercury ions, Cs ions (perhaps all ions?) have frequencies that are a 
fundamental property of the ion.  There might be practical 
implementation limits or side effects that limit the lowest achievable 
uncertainty - hot atoms will have higher variance than cold atoms, for 
instance. So not all Cs primary standards have the same ultimate 
performance.



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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/28/21 8:03 PM, Tom Van Baak wrote:

Jim,

For state of the art numbers and plots, here's a recent (2020) paper:

"A Review of Contemporary Atomic Frequency Standards",
by Bonnie Schmittberger, David Scherer
https://arxiv.org/abs/2004.09987
https://arxiv.org/pdf/2004.09987.pdf

It also has plots comparing clock stability vs. size and vs. power. 
Highly recommended reading.


There is also a power point version with similar content as the paper:

https://www.gps.gov/cgsic/meetings/2019/scherer.pdf

/tvb


On 11/28/2021 2:05 PM, Lux, Jim wrote:
Speaking of which, does anyone have a link to a "current state of the 
art" graph. 

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Awesome, these are a good start.

It's nice to have a figure to put in to reports and presentations.  
(Larry Young, recently retired from JPL who is a GNSS guru, had a hand 
drawn graph that he'd update over the years pasted into his notebook)


DSAC is shown as 10 liters (which is about how big it actually is) - I 
remember when John Prestage was talking about the 1 liter atomic clock 
more than 10 years ago, but that's the size of the physics package on 
the bench, and comparing to USOs.  That was before it was "flight ready".

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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/28/21 9:37 AM, Bernd Neubig wrote:

-Jim wrote-

I wouldn't actually think there's a Cs on ISS.  What purpose would it serve?  We as 
time-nuts think "of course you'd have a precise source of time", but really, 
there's not much need for timing on ISS on a scale smaller than seconds, if that.  NTP to 
timestamp files, for instance.

You are probaly right about the actual situation. However there is the ESA ACES 
project idling since years without being launched yet:
"ACES is an ESA ultra-stable clock experiment, a time and frequency mission to 
be flown on the Columbus module of the ISS (International Space Station), in support 
of fundamental physics tests. The mission objectives are both scientific and 
technological and is of great interest to two main scientific communities:
• The Time and Frequency (T) community; which aims to use ACES as a tool for 
high precision Time and Frequency metrology
• The Fundamental Physics community; which will benefit from the use of ACES 
data for accurate tests of general relativity.

See https://earth.esa.int/web/eoportal/satellite-missions/i/iss-aces

The ACES development was initiated in the 1990s. However, the decision to 
complete the development of the project has been achieved only at the ESA 
council at Ministerial Level of November 2008.
The launch was planned in 2018, but,  as said, the clock ensemble (which BTW 
includes two AXTAL OCXO 100 MHz) is still sitting on the test bench and waiting 
and waiting.

B



Right now, for high performance in space, trapped mercury ion clocks 
seem to be the ticket.  Deep Space Atomic Clock is working well, and 
there's a DSAC2 in the works.


A fundamental problem for this kind of thing is that "infrastructure" 
(faster communications, better time) doesn't get a lot of support unless 
it enables answering a science question that the community as a whole 
deems important. In the case of NASA, it's the decadal studies that 
drive a lot (Astrophysics 2020 just came out)- The decadal study says 
"it is of great interest to answer question X" and if your technology 
helps with that, great, it might get flown.


Good independent time keeping in deep space is an enabling technology 
for autonomous navigation and rendezvous, for which there hasn't yet 
been a really compelling science need.  Perhaps when we need to do 
auto-nav around moons of planets or something like that.


For all that NASA does human exploration, it all is in service of 
answering some science question. So NASA doesn't really spend a big 
amount on problems like "how do you allow a dozen astronauts on the Moon 
to know where they are" - sure, they do studies (I've participated in 
some), they keep up on current technology, but they're not going to 
invest $100-500M in building a Position, Nav, Timing infrastructure - 
that's viewed more as an "operational thing" to "be done by others".  
Everyone sort of assumes that something with GPS-like performance will 
be available if needed, one just needs to write the check.


I'm basically a radio and computer guy at heart, so to me, one of the 
things which good timekeeping (and PNT in general) enables is large 
distributed RF sensors - radio telescopes/interferometers in space. 
Large physical extent (with precise knowledge of time and position) 
gives you good angular resolution   Precision metrology also lets you do 
things like GRACE and GRAIL - measuring the gravitational field of a 
body by measuring the distance between paired orbiters - that distance 
is measured by, you guessed it, RF and optical links, based on 
ultrastable oscillators.  Things like mercury ion clocks have the 
potential to replace USOs - and just like in terrestrial timekeeping, 
standards that rely on the fundamental physics are desirable over "fine 
artisanal craftsmanship" which is what quartz clocks are - you start 
with 1000 blanks, pick the best, mount them, pick the best, age them, 
pick the best.  And the whole time you pray that you didn't "lose the 
recipe".  USOs (and atomic clocks) are invaluable for radio science and 
gravity experiments - precisely measuring the orbit of something a long 
ways away, or sending phase coherent signals at different frequencies to 
a receiver and measuring the relative amplitude and phase for 
occultations, or just the interplanetary medium - You want something 
that has really good ADEV at tau>1000 seconds, because integration time 
is important, for both the radio science and the ranging/gravity science.


This is part of why I got selected to be project manager for SunRISE - a 
10km scale interferometer in space.  I know how these kinds of things 
work, or, even better, I know when and where to go ask questions, so I 
know what I don't know.  What I don't get to do as PM (and is somewhat 
frustrating) is design the system that does it. (The phrase from the JPL 
Chief Engineer, Rob Manning, is "When you become a manager you give up 
your SME card")

[time-nuts] Re: Project Great

[Lux, Jim]

On 11/28/21 2:11 AM, Andy Talbot wrote:

I would imagine there are already several caesium clocks on board the ISS,
anyway.
Don't forget there is a velocity component in relativistic time shift, as
well as gravitational, so using a moving platform like an aircraft or the
ISS complicated things a lot

Andy
www.g4jnt.com

I wouldn't actually think there's a Cs on ISS.  What purpose would it 
serve?  We as time-nuts think "of course you'd have a precise source of 
time", but really, there's not much need for timing on ISS on a scale 
smaller than seconds, if that.  NTP to timestamp files, for instance.


As a practical matter, there's not a lot of "infrastructure" on ISS, 
i.e. there's no "house 10 MHz" - the experiments tend to be self 
contained.  When I was working on SCaNTestbed, which launched to ISS in 
2012, there wasn't even an onboard real-time GNSS time/position feed. We 
had a software GPS receiver as part of the testbed.  What you would get 
is a "playback" of ground predicts done by GSFC Flight Dynamics for 
position and time over MIL-STD-1553 as part of Broadcast Ancillary 
Data.  And knowing precisely where you are on ISS is tricky anyway - 
it's the size of a football stadium and flexes and moves on the scale of 
meters. The BAD data was for some presumed "center of mass", as I recall.

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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/27/21 11:08 PM, Tom Van Baak wrote:

Hi Thomas,

Good to hear the experiment was contagious for you. If you have 
additional questions let me know.


Your suggestion about Mount Evans and Pikes Peak are excellent. You 
will enjoy this 2017 paper:


"An Undergraduate Test of Gravitational Time Dilation"
https://arxiv.org/abs/1710.07381
https://arxiv.org/pdf/1710.07381.pdf

---

As for CSAC, the news is not so good. I've worked with several groups 
to explore CSAC for gravitational time dilation experiments. Those 
clocks are so cute and small, it's irresistible; but the numbers just 
don't add up. Over a day their stability is in the low e-12's vs. a 
"real" cesium clock like a 5071A in the low e-14's. So when you are 
doing a relativity experiment trying to detect a frequency shift 
that's on the order of e-13's you reach for a 5071A instead of a CSAC. 
The performance is nearly 100 to 1.


One solution is a taller mountain. The best on the planet is Mauna Kea 
(Big Island, Hawaii) where you can literally drive from sea level to 
the summit (13,800 ft, 4200 m) in a few hours. The frequency shift up 
there is 4.5e-13, which is 40 ns per day. But still, to have even the 
slightest chance of success you'd want your clocks to be good to 1e-13 
or better. CSAC aren't even close, and probably neither are telecom Rb.


I'm currently involved with another solution -- a HAB (High Altitude 
Balloon) CSAC flight. Getting to 100,000 ft altitude is quite common. 
Up there, clocks run a whopping 3.3e-12 faster, which is 280 ns/day, 
or 12 ns/hour. This is a clear case where the amazing low mass and low 
power of a CSAC is a  critical advantage. However, the numbers still 
aren't working out and the logistic and environmental conditions are 
brutal. I won't say it's impossible, but it may take years and a huge 
bag of tricks before it works or it's proved too impractical.


---

Jim, I'd be interested in any Cubesat / CSAC results. They don't 
exactly land in one piece so the typical round-trip clock comparison 
method wouldn't work. A direct frequency comparison might. In that 
case the drift and re-trace specs of a CSAC are probably more 
important than the stability.


/tvb



The CHOMPTT folks were trying to do time transfer using optical, but 
they also flew a CSAC (maybe even two).   One problem is that "what do 
you compare to", as you noted.  One could compare to on board GPS 1pps 
or to an onboard OCXO.  Both the CSAC and the OCXO would speed up 
relative to surface. But you also have the velocity problem (7 km/s) so 
they "apparently" run slow.  I don't know that CSAC vs GPS would 
actually be able to do the measurement - the uncertainty in the GPS is 
perhaps too high.  Maybe with post processed GPS - GIPSYX/RTGx should 
give you position and time to <1ns.


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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/27/21 3:20 PM, Magnus Danielson via time-nuts wrote:

Hi,

There is an overemphasis on the atom being used, and especially on 
cesium as that is what is used for SI definition. However, actual 
implementation means actual physical devices, and the physical devices 
have a physics package, for which details will be important to the 
actual performance. Various atoms have been more or less well adapted 
to different types of physical package types. The beam type of device 
can be made to have very little perturbation, and cesium was well 
suited for that, while rubidium ended up being very well suited for 
the gas cell type. The CSAC is really a cesium based gas cell, but the 
original benefit of rubidium filtered optical pumping has been 
replaced with semiconductor lasers for pumping. Today both cesium and 
rubidium gas cells with the same mechanism exists. With gas cell you 
get wall shift from atoms banging around the wall, but also gas shift 
as buffer gas makes the atoms hit the buffer gas most of the times. 
With a bit of selection of gas mixture, these can be made to balance 
each other.


So, what is the claim to fame for CSAC? It's actually not being 
cesium, but for the stability it provides for the small amount of 
power it consumes. That's also where it finds actual applications. If 
you can afford more power, there is cheaper alternatives available. 



Not only is the CSAC low power (~ 100 mW) it's physically small, which 
is attractive for some applications (inside a cubesat, for instance).  
It used to be price competitive with a Rb, too ($1000-1500, as I recall) 
but now they're about $5k.  Microsemi also has the NAC (which is more a 
conventional Rb, but small)



Parameter    NAC1        CSAC
Aging        3E-10/mo    9E-10
    1E-9/yr        10E-9

ADEV        2E-11 @ 100sec    2.5E-11
    8E-11 @ 10    8E-11
    2E-10 @ 1 sec    2.5E-10

phase noise
    -86 @ 10    -70
    -120 @ 100    -113
    -138 @ 1000    -128
    -143 @ 10k    -135
    -148 @ 100k    -140
    -150    floor

max chg        1E-9 (-20 to 65) 5E-10  (-10 to 35)

pwr        1.2W op        0.12
    1.8W warm    0.14
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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/27/21 12:37 PM, Thomas Valerio wrote:

I think that Tom's GREAT adventure is kind of what sealed the deal making
me a time-nut or at least a time-nuts lurker, a lot of this stuff is still
little over my head, but I keep reading.

If anyone is inclined and has the clocks and the kids ( I don't have
either ), there is always Mount Evans and Pikes Peak, although you may
have to leave the clocks behind overnight.  Mount Evans is still on my
bucket list but without clocks and two or three days of time to monitor
them, I don't think I will be doing the Mount Evans edition of GREAT.  For
anyone that is flush enough to afford or can beg, borrow or steal access
to a Microsemi chip scale atomic clock, I think a Mount Evans edition
would be an awesome addition to Tom's original work.

Thomas Valerio


I don't think a CSAC would be good enough.

Tom's experiment was 22 ns out of 42 hours or about 1.45E-13. That's 
quite a bit smaller than a CSAC adev over that period.


There's a variety of roads that go to ~12,000 ft in Colorado, about 
~10,000 in CA (Tioga Pass isn't closed yet), so you can get about 3x 
change, but still you're talking <1E-12.


Mammoth Mtn has a gondola to the top, but it's only 11,000. There may be 
a ski resort in CO that's higher.




For newcomers to time-nuts, Andy is asking about my DIY gravitational
time dilation experiment(s).

  > What am I missing?

It looks like you used the wrong value (or wrong units) for "h".

The summit of Mt Rainier is 14411 ft (4400 m), but the highest point on
Mt Rainier that is accessible by road is the Paradise visitors center at
5400 ft. Our house is at 1000 ft elevation so the net difference in
elevation of the clocks was 4400 ft (1340 m).

The clock(s) on the mountain ran fast by gh/c² = 9.8 × 1340 / (3e8)² =
1.5e-13. Fast clocks gain time. We stayed for about 42 hours so the net
time dilation was 42×3600 × gh/c² = 22 ns.



For more information see the Project G.R.E.A.T. 2005 page:

http://leapsecond.com/great2005/

Better yet, these two recent talks from 2018 and 2020 cover all 3 GREAT
experiments:





Lots of time nutty photos in both of those!

/tvb


On 11/27/2021 7:33 AM, Andy Talbot wrote:

Just been reading your adventures with 3 Cs clocks, a mountain and 3
kids,
but I can't make the estimate of time dilation work out.
You measured ~ 23ns and say it agrees with calculation

The equation quoted in a related reference, for "low elevations" is
g.h/c²
which if you plug in g = 9.81 m/s²  and h = 4300m for Mt Rainer gives
an
expected value of 4.7 * 10^-16.
Over 2 days, 2 * 86400s, that would be 81 ns in total, four times your
value

What am I missing?

Was just speculating what Ben Nevis at a mere 1340m height might offer

Andy
www.g4jnt.com
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[time-nuts] Re: Project Great

[Lux, Jim]

On 11/27/21 7:33 AM, Andy Talbot wrote:

Just been reading your adventures with 3 Cs clocks, a mountain and 3 kids,
but I can't make the estimate of time dilation work out.
You measured ~ 23ns and say it agrees with calculation

The equation quoted in a related reference, for "low elevations" is  g.h/c²
which if you plug in g = 9.81 m/s²  and h = 4300m for Mt Rainer gives an
expected value of 4.7 * 10^-16.
Over 2 days, 2 * 86400s, that would be 81 ns in total, four times your value

What am I missing?

Was just speculating what Ben Nevis at a mere 1340m height might offer



Considering the parking lot is around 200m, carrying the clocks and 
power to the top might be moving from a moderate walk to the strenuous 
category. One might also want to do this in summertime, so you've got a 
few months to plan the expedition.


I wonder what the highest road in the UK is?

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[time-nuts] Re: Frequency Standard - Where Can I Get One.

[Lux, Jim]

On 11/22/21 7:51 PM, Bill Notfaded wrote:

There's no substitute for a few good rubidiums.  OCXO and Rb are
different.  It's really hard to beat a really good GNSS diciplined Rb!
Extremely good holdover.  We're timenuts after all right?  Where's the fun
if you don't try them all?  There isn't any silver bullet or perfect
solution but I've found with some good measurement and comparisons you too
can be in the search of better and better stability.  Half the fun is
finding a better OCXO or finding a really stable Rb.  Testing them all
against each other is part of the journey.  Letting timelab run all night
every night.  Flipping back and forth between graphs.  For me it's a hobby
but I've gotten many many hours of great happiness from it.  The first time
I got into 10 ^ -13 how can you explain what that's like?  The huge digits
on an SR620 you can read from across the room.  I guess it's different for
each of us but it's something we all share.

Bill



When you get to where you can see the air conditioning or heat in the 
room cycling on and off, with TCXO and OXCOs.. "What's that bump at 1000 
seconds?"


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[time-nuts] Re: function generator

[Lux, Jim]

On 11/22/21 10:25 AM, Andy Talbot wrote:

But you don't need a DDS route to get 1 PPS from any frequency that is an
exact multiple of 1Hz.
I clock a PIC with 10MHz from a master reference.  An interrupt is
generated at an exact submultiple of this, and additional code outputs a
pulse every 25 clocks (clock freq = Fosc / 4)

Or from 10MHz, four 74AC390 CMOS counters will do it

Or am I missing some fundamental issue here?


The challenge is doing it with an off the shelf piece of test equipment, 
a Rigol DG1022Z (about $350) that may or may not be able to do it.


https://www.rigolna.com/products/waveform-generators/dg1000z/





Andy
www.g4jnt.com



On Mon, 22 Nov 2021 at 18:20, Gerhard Hoffmann 
wrote:



Am 22.11.21 um 18:31 schrieb Erik Kaashoek:

Some time ago I needed a output at 10,00.001Hz so I tried to do that
with a SI5351.
Using pure integer math (as the PLL and divider register are integers) i
search for a combination of 3 divider/multipliers that gave the least
error.
If the reference frequency is not integer related to the internal PLL
frequency and  multiplier/divider registers you always will have limited
accuracy as there is a fractional error.
The amount of error will depend on the number of digits in the
multiplier/divider register lengths and the care you take to search for
the best solution.
If the DDS has a PLL driven clock this could be the cause.

There once was a BCD DDS chip made by Standard Telecom.
Doing that in an FPGA would be an easy exercise.

Cheers, Gerhard


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[time-nuts] Re: function generator

[Lux, Jim]

On 11/22/21 10:19 AM, Gerhard Hoffmann wrote:



Am 22.11.21 um 18:31 schrieb Erik Kaashoek:
Some time ago I needed a output at 10,00.001Hz so I tried to do 
that with a SI5351.
Using pure integer math (as the PLL and divider register are 
integers) i search for a combination of 3 divider/multipliers that 
gave the least error.
If the reference frequency is not integer related to the internal PLL 
frequency and  multiplier/divider registers you always will have 
limited accuracy as there is a fractional error.
The amount of error will depend on the number of digits in the 
multiplier/divider register lengths and the care you take to search 
for the best solution.

If the DDS has a PLL driven clock this could be the cause.


There once was a BCD DDS chip made by Standard Telecom.
Doing that in an FPGA would be an easy exercise.

Cheers, Gerhard



All of this stuff is straightforward, but the challenge is that your 
stuck with the implementation of a particular manufacturer. Which is 
often not particularly well documented or understood. (when I talked to 
Agilent about the 33622 it was pretty obvious that the external 
reference was sort of an afterthought)

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[time-nuts] Re: function generator

[Lux, Jim]

On 11/22/21 8:52 AM, Jeremy Elson wrote:

I did not see any such setting in the Rigol, but I'll check again. in April
I did write to Rigol to report the problem and had the following (abridged)
conversation with support:

Me:

"I recently tried to use your DG1022Z signal generator to generate one
pulse-per-second (pulse mode, frequency 1.00hz, width 10 microseconds).
However, it appears there is a small frequency error of one part in 1e11,
i.e. the pulse per second gets later by about 6 nanoseconds every 1,000
seconds." [More technical description abridged, including a link to a
graph.]

Rigol:

"Please find the following datasheet for DG1000Z, the accuracy is +/-1ppm.
If your pulse is 1second with 10us width, 6ns per 1000s is in the accuracy
range."  [They attached an image of a page from the DG1000Z datasheet,
showing a line that said "Accuracy: +/- 1ppm of the setting value"]

Me:

"Is this the specification even when the unit is provided with an accurate
external clock?"

Rigol:

"I would say Yes. The internal processing circuit will effect the clock
signal,  harmonics and phase noise will result the frequency variance."

-Jeremy

On Sun, Nov 21, 2021 at 11:54 PM Poul-Henning Kamp 
wrote:



I've noticed that a lot of modern test equipment (Keysight 33622 
function generator as a concrete example) handles an external reference 
differently than one might expect.   Historically, you'd think a signal 
generator or counter or TBD would have a 10 MHz oscillator as a time 
base, and feeding in an external reference replaces that.



Today, though, their internal time base could be almost anything at any 
frequency. What they do is compare that against the external reference, 
and implement either some sort of frequency locked loop. This might be 
analog pushing of their internal reference smoothly, or it might be a 
period stepwise correction (e.g. reprogramming a DDS), or something else.


In any case, I would NOT expect the test equipment to improve its phase 
noise or ADEV to that of the external reference (unless the manual 
explicitly says that).  I would expect that the dial reading would 
follow the external source.  If the internal source had drifted 10ppm, 
then 10.00 MHz on the display could be 10.000,100 MHz at the output 
jack, but with an external source, it would be 10.00 (or maybe +/- 
10 Hz, if 1ppm is the spec).


In addition, one cannot assume that there is any consistent phase 
relationship between the external source and the output of the test 
equipment.  Connecting a pair of 33622 generators to the same source, 
and expecting the outputs to be synchronized will be unsuccessful. They 
will be syntonized to within the tolerance of the device, but 
synchronization depends on using the sync inputs, with the precision of 
synchronization determined by the internal oscillators which are fairly 
high frequency, but not phase locked to the external source. So if the 
internal clocks were, say, 200 MHz, then the sync between two generators 
is no better than the period of 5 ns.

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[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/11/21 4:00 PM, djl wrote:
I've used, wait for it, beeswax as a potting compound. Gouda cheese 
comes coated with it (some has paraffin, get the best,) in a lovely 
red. I also found out some years ago that Catholic churches use pure 
beeswax for large (not votive) candles and may give you the stubs. 
Nice, clean white.  Or, dear ol' Amazon has a huge assortment for 
around $1.00 / oz, in various stages of "purification". For expensive 
beeswax with some unknown sticky additives, use toilet mounting 
rings... (good also for preserving dry milsurp gunstocks, according to 
Anvil.)
73, Don 



Beeswax, if perfectly dry, and no carbon residue, is pretty good RF wise 
- at 1 MHz, epsilon is around 2.5, tan d is around 0.01 which is ok, but 
not great. You could mix it with microballoons to lower epsilon and 
dissipation.


It does shrink and, of course, it's pretty soft.


Pointing back to a previous suggestion 3M DP270 - that's 3.5 epsilon and 
0.018 tan d, but at 1kHz.  The graph in the datasheet does show pretty 
constant 0.020 up to 1 MHz.


http://www.emesystems.com/pdfs/parts/DP270.pdf

At least it's available in less than a gallon quantities.

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[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/10/21 2:40 PM, Richard (Rick) Karlquist wrote:

I am looking for help choosing a potting compound that
has the following properties:

1.  Good for 5,000VAC @ 1 MHz
2.  Low RF losses.
3.  Low permittivity is preferred
4.  Low tempco of permittivity is a want.
5.  Something I can implement in my home shop
without access to a vacuum pump etc. is a want.

Thanks in advance

Rick Karlquist N6RK 



After consulting the experts at work, what they use for this kind of 
thing (HVPS, high power RF, etc.) is:



https://www.elantas.com/pdg/products/tooling-composites-materials/gel-encapsulants-sealants.html

Elantas EN-11 (Conap) Casting, Potting and Molding Compound.

epsilon of 2.9 @ 1 MHz

610 V/mil breakdown

tan d of 0.009 at 1 MHz

8 hour cure at 80C, more than a week at 25C. 50 min pot life


your challenge may be finding it in small quantities - a casual search 
shows gallon cans at just under $200, and I'm not sure that you don't 
need to buy a part a and a part b.

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[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/10/21 5:31 PM, Gerhard Hoffmann wrote:

A customer of mine uses Solitane, another one Mupsil.
I just wrote down the names in case I might need it.
Probably more for coating boards in space apps, no idea
if it fits.


Am 10.11.21 um 23:40 schrieb Richard (Rick) Karlquist:

I am looking for help choosing a potting compound that
has the following properties:

_



Yeah, the solithane (that's the name we use) is more used to repair 
conformal coatings, stake fasteners, stick wires down to the board, glue 
components to the board so it will survive vibe (think tall skinny 
things, with the vibe in the plane of the board).  Fairly fluid, cures 
fairly quickly, low outgassing, and most important for space - someone 
else used it and it worked without causing a disaster.   There probably 
is a potting version of it, and I'll ask one of the M folks at work 
tomorrow what they think about Rick's need.


I've not heard of Mupsil, but we use a lot of Nusil - silicone 
elastomers, often with alumina particles in it, as a thermal bonding 
material. Say you've got a box with a fairly flat surface that you want 
to clamp to another fairly flat surface. The problem is that tightening 
the fasteners deforms both surfaces (unless you've got a zillion of 
them) so the thermal contact area is just around the fastener, and there 
is a perhaps a gap everywhere else. Spaceflight people hate "perhaps" so 
they say, ok, put a thermal gasket in there (hey, many of us have used a 
mica washer and silicone grease between part and heat sink, right?).  
You can get elastomeric thermal gaskets from Chomerics and similar 
companies, but they actually have the same problem with clamping force. 
You tighten the fasteners, but to get the required clamping force over 
the WHOLE gasket, you need a lot of fasteners, or a lot of force, and 
you're back to the deformation problem.


So the answer is "thermally conductive glue" - you slather a thin layer 
on, tighten the fasteners, which then causes the alumina particles to 
poke into the surfaces on both sides, and hey - good thermal 
conductivity.  Of course, if you need to take it off, you need to get in 
there with a wire saw and that's "not fun".


I will say the nifty-est thermal connection was a sort of velvet made of 
carbon fibers. Carbon fibers have very high thermal conductivity. You 
bond that furry velvet to both surfaces, and when you put it together, 
the fibers slide along each other and make good contact along their 
length, and there's millions of them. You aren't depending on clamping 
force - it's the springyness of the very stiff fibers that provides the 
contact force, and as you can imagine, it can tolerate a lot of 
misalignment and gaps.


The actual stuff was developed originally to make a very optically 
absorbing black coating over wide bandwidths - all those fibers bounce 
the light around. And as a laser load (instead of the proverbial stack 
of razor blades.  It was then was used to coat mannequin forms, for 
displaying lingerie for Victoria's Secret, of all places, because it was 
very rugged and didn't shed lint.  There's a whole exotic trade secret 
about how they make the velvet - there's some sort of electrostatic 
technique to making the fibers stand on end while they're bonded, and 
some other exotic trick to getting them all the same length, and so 
forth. I kept trying to use it in space (it is *so* much easier than 
glue, gaskets, or zillions of fasteners), but it never took -> 1) nobody 
else had used it before and 2) everyone was worried about little 
conductive fibers shedding and floating around into places they 
shouldn't be.  Again, in the space world, no matter how tedious and 
painful, if it worked before, we can do it again. thermally conductive 
glue may be a pain, but it's "known to work".



For those of you doing bolted joints..  thermal conductances are around 
0.1 to 1 W/K -


You want to google a chapter called "Mountings and Interfaces" by Gluck 
and Baturkin - It's in Spacecraft Thermal Control Handbook Volume 1. but 
there's tons of copies floating around the web, and it's a great 
handbook reference for "just what is the thermal resistance with a 4-40 
screw through that TO-220 tab onto an aluminum chassis"


It's one of those references which everyone cites.
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[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/10/21 4:37 PM, Brooke Clarke via time-nuts wrote:

Hi Jim:

Be careful with RTVs.  Some out gas acid that attacks metal, even gold 
plated metal.  Guess how I know that.


Oh yes.. one definitely needs to read the data sheets.. RTV12 is 2 
part.  Most 2 part RTVs don't use acid.  And the one part that cure in 
an oven at 70C, likewise.

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[time-nuts] Re: Potting compound advice needed

[Lux, Jim]

On 11/10/21 2:40 PM, Richard (Rick) Karlquist wrote:

I am looking for help choosing a potting compound that
has the following properties:

1.  Good for 5,000VAC @ 1 MHz
2.  Low RF losses.
3.  Low permittivity is preferred
4.  Low tempco of permittivity is a want.
5.  Something I can implement in my home shop
without access to a vacuum pump etc. is a want. 


What about curing? Is temperature cure (put it in an oven) ok? or do you 
need room temp cure?



Silicones are usually pretty good, RF wise. But you need to check the 
filler and exact composition.


I found a two component silicone that has epsilon 2.5 used for RF 
potting, 15kV/mm breakdown.


https://vitrochem.com/wordpress/wp-content/uploads/2020/10/Two-Component-Condensation-Silicone.pdf

they say nothing about the dissipation.


Aha.  RTV12 from Momentive - clear - epsilon 3.0, tan d (at 1kHz) is 
0.001, 400 V/mil - This stuff is pretty common, but I can't find any 
higher frequency permittivity info, which is odd. Someone somewhere 
probably built something and measured it.



Diallyl Pthalate is what they use in connectors - it's a thermosetting 
resin with good electrical properties.


https://www.cosmicplastics.com/products/dap/

Picking the first one in the list 224 DAP - 360 V/mil, so for your 5kV, 
you'd need ~14 mils. (most plastics are in this range)


Epsilon is kind of high 3.5, tan D is 0.01?  Is that good enough for you 
dissipation wise?  There's lots of kinds with various fillers.


A common way to reduce epsilon and tan d is to mix in microspheres.


Some epoxies are also good.  Rogers not only makes laminates for 
circuitboards they also produce the epoxy from which they are made



We use tons of arathane and solithane at JPL (both are urethanes), but I 
don't know if we pot RF circuits in araldite. Huntsman makes the 
"ara???" materials


https://huntsman-pimcore.equisolve-dev.com/Documents/US_2019_High_Performance_Components_Selector_Guide.pdf

one thing is that we store this stuff at -80C, but I don't know if 
that's after mixing or if it's shipped that way (in dry ice).


masterbond.com  -> give them a call or email

EP110F80-1 is a 2 part epoxy with e=2.69@1MHz, so it's probably 
reasonably low loss.

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[time-nuts] Re: GPS Elevation Mask Values.

[Lux, Jim]

On 11/10/21 7:29 AM, Bob kb8tq wrote:

Hi

Your GPS antenna does not have a clue where signals are coming from. The only
way things get plotted is based on the almanac data. If it sees sat Id 21, it 
looks in
the almanac for id 21. If the almanac says it’s over India, that’s where it 
goes on the
resulting plot.

Best guess: you are seeing intermod of some sort that your receiver interprets 
as a
signal from a sat that can’t actually be in view.



I don't know that an intermod would do that (PN sequences are different) 
- multipath, for sure.

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[time-nuts] Re: Ryzen mobos with serial port for Garmin GPS?

[Lux, Jim]

On 11/5/21 11:32 AM, Andy Talbot wrote:

Use FTDI USB serial ports - you can't go wrong with them.
https://ftdichip.com/

I have, at the last count, used something like 200 of their FT232 device in
one form or another on the shack PC.   I know that, because device manager
has registered up to COM200.  Every time a new one is plugged in, a new COM
port is set up.

Andy
www.g4jnt.com



On Fri, 5 Nov 2021 at 18:26, Alec Teal  wrote:


A friend of mine who lives and breaths this stuff (I wont tell you what
he does - but suffice to say he's authoritative) basically said to me on
something about serial ports that you can't go wrong with USB stuff,
even on Linux.

Would that work?

Serial ports certainly are getting scarce! You'd get 2 to a board an
embarrassingly long time ago!


A caution - FTDI has several series of chips (FT23x, FT245) that wind up 
in commodity USB to Serial/RS-422/etc products. Some of them do not have 
driver support for MacOS post Mojave (MacOS pulled the FTDI driver into 
the kernel, but it only accepts some PID/VID values, etc).


I can't speak to Windows 10 etc since I've not tried it.

I've got a dozen or so RS232 dongles that are now useless in a general 
sense - sure, they work with the old PC, but they lurk waiting to 
frustrate you when you plug them into another computer that doesn't 
support them.



And then, of course, there are the famous FTDI clones which "sometimes" 
work.


For Linux, you may or may not care - depending on libusb, etc. But I've 
had version compatibility issues there, too (Ubuntu, various LTS 
versions we use at work).


Just budget for potential replacements.

And, because this is timenuts - any modem control signal (e.g. RTS) 
going across USB is going to have a 125 microsecond uncertainty due to 
the 8kHz frame rate of USB.


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