Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Hal Murray]

tpie...@gmail.com said:
> Forget about accurate time from the RTC's registers.  The slowness and
> variably in I2C is quite poor and most of them don't even have sub-second
> resolution when read or written. 

If your RTC has a PPS output, you can read the registers to get the coarse 
time, then wait for the PPS to tick to get the low order bits.


-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Trent Piepho]

Some of the DS chips have a square wave input that can be fed from a GPS
PPS and the RTC will discipline itself.  It can also produce a PPS when the
GPS stops.

The question is, is NTP's holdover on the system clock better than NTP
disciplining the system clock with an RTC PPS.  I'd guess not.

Forget about accurate time from the RTC's registers.  The slowness and
variably in I2C is quite poor and most of them don't even have sub-second
resolution when read or written.


On Nov 1, 2017 4:22 PM, "MLewis"  wrote:

Is this a workable or worthwhile strategy?:
- RTC providing date & time to second to system on boot
- RTC frequency output driving a counter/divider to produce PPS
- GPS module providing UTC PPS
- GPS module's secondary PPS disciplining the RTC-counter-divider PPS by
resetting the RTC's counter/divider (I'm assuming there's one that will
rest fast enough to sync; I've never looked into these...)

If GPS PPS is lost, then the RTC counter/divider is producing a recently
disciplined PPS.

Valid or invalid?

And the DS3231 has:
- a 32K output, and
- an Active-Low-Interrupt/SQW output that can be set to PPS.
It's unclear to me how to sync the DS3231 PPS to the GPS PPS, or if that
can be done.

Thanks,

Michael


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Re: [time-nuts] Oscillators and Ovens

[jimlux]

On 11/1/17 1:38 PM, Hal Murray wrote:



In general, OCXOs have crystals with high Q -> low phase noise,  especially
compared to a TCXO, which *can't* have high Q, or the  temperature
compensation circuit can't do it's work.


I don't understand that.  Why can't I build a high Q TCXO?  I don't need to
change the compensation very fast.  Are good crystals high enough Q that it
would take too long?


Think of the oscillator as an amplifier and a high Q mechanical filter - 
it gets the Q from being mechanically stiff.


In order to move the frequency, you have to electrically push it, which 
is counter to the mechanical stiffness.  You just don't have the tuning 
range available.


We struggled on a project to build a high Q, but adjustable DRO, and 
that was the fundamental problem.





What's the time constant?  I'd guess it's Q/freq, maybe with factors of 2pi
or e or ???

That seems small relative to how fast temperature changes.  (but maybe fast
relative to FCC smearing or things like that)




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[time-nuts] Trimble UCCM weird behaviour

[Mark Sims]

That looks like the device is defective.   I'd see if the seller will replace 
it.
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Re: [time-nuts] Designing an embedded precision GPS time

[jimlux]

On 11/1/17 12:13 PM, Adrian Godwin wrote:

How do those compare with vectron's part : ?

https://www.vectron.com/products/ocxo/mx-503.htm



That part is interesting, but the phase noise (-124 dBc @ 100 Hz) isn't 
particularly impressive compared to a middle of the road OCXO (-155dBc 
at 100Hz)


The frequency stability is very nice, compared to a TCXO, and at a LOT 
less power (40mW) than the OCXO (2W after warmup).




There's also this patent.

http://www.google.sr/patents/US20020005765

I don't really know if that's valid - it seems to propose something similar
to the numerically-compensated oscillator in my rather old PM frequency
counter.



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Re: [time-nuts] Oscillators and Ovens

[Bob kb8tq]

Hi

As mentioned in Rick’s post, it’s not really Q, it’s the motional capacitance 
that is the issue. 
Even if you resonate out C0, you still have to deal with Cm. The only practical 
high Q designs
for crystals are very low Cm resonators. Yes, if you could do a design that had 
Rm of 0.1 ohms
it could be high Q without a high Cm. That’s not the way it works out. 

Since the electrical equivalent circuit is related to the physics of the 
resonator, simply changing 
this or that is not trivial. You might well have to change the basic material 
to impact some of this.
Simply as a side note, SC’s are “worse” (lower) for Cm than AT’s of similar 
frequency and overtone. 

> On Nov 1, 2017, at 6:44 PM, Dana Whitlow  wrote:
> 
> Bob,
> 
> This discussion is getting really interesting.  In thinking about the
> crystal Q versus
> tuning range conundrum, two (presumably-overlapping) concerns come to mind:
> 
> 1. The motional parameters of a high-Q crystal are such that the external
> network
>needed to pull it very far would be wholly impractical.

Depending on you definition of impractical … this is TimeNuts …. In the real 
world, yes indeed
impractical. 

> 
> 2. Varactors themselves probably have pretty limited Q over much of their
> range.

They have a very finite Q. That turns into loss in the oscillator circuit. Loss 
goes up as the
pull increases. The varactor is a bigger part of the circuit as the pull 
increases.  The Q of a real oscillator 
is only partly from the crystal. Tossing more loss into the oscillator will 
impact the overall Q. 
You might go from crystal / 2 to crystal / 5 (very arbitrary numbers and only 
for illustration … ) 

> 
> Is my thinking on the right track at all?

Pretty much.

Bob

> 
> Dana  K8YUM
> 
> On Wed, Nov 1, 2017 at 5:13 PM, Bob kb8tq  wrote:
> 
>> Hi
>> 
>> A high Q crystal by design is very difficult to tune. Putting it in a
>> circuit that will
>> swing it far enough to compensate it degrades the Q. In addition, thermal
>> noise
>> will come into the compensation circuit (even if it is noise free) and
>> degrade things.
>> 
>> Bob
>> 
>>> On Nov 1, 2017, at 4:38 PM, Hal Murray  wrote:
>>> 
>>> 
 In general, OCXOs have crystals with high Q -> low phase noise,
>> especially
 compared to a TCXO, which *can't* have high Q, or the  temperature
 compensation circuit can't do it's work.
>>> 
>>> I don't understand that.  Why can't I build a high Q TCXO?  I don't need
>> to
>>> change the compensation very fast.  Are good crystals high enough Q that
>> it
>>> would take too long?
>>> 
>>> What's the time constant?  I'd guess it's Q/freq, maybe with factors of
>> 2pi
>>> or e or ???
>>> 
>>> That seems small relative to how fast temperature changes.  (but maybe
>> fast
>>> relative to FCC smearing or things like that)
>>> 
>>> 
>>> --
>>> These are my opinions.  I hate spam.
>>> 
>>> 
>>> 
>>> ___
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>> mailman/listinfo/time-nuts
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>> 
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Re: [time-nuts] Oscillators and Ovens

[Richard (Rick) Karlquist]

On 11/1/2017 3:44 PM, Dana Whitlow wrote:

Bob,

This discussion is getting really interesting.  In thinking about the
crystal Q versus
tuning range conundrum, two (presumably-overlapping) concerns come to mind:

1. The motional parameters of a high-Q crystal are such that the external
network
 needed to pull it very far would be wholly impractical.

2. Varactors themselves probably have pretty limited Q over much of their
range.

Is my thinking on the right track at all?

Dana  K8YUM




Sorry your thinking is NOT on the right track.

What determines the pullability of a crystal is the ratio of the
motional capacitance to the static capacitance, commonly denoted
as C1/C0.  The Q of the crystal has nothing to do with it.  The
only thing significant about the Q is that it limits the
how QUICKLY you can change the crystal frequency.

What determines the noise of a crystal is the intrinsic
flicker of frequency noise.  The Q has nothing to do with it.
If the Q is degraded somewhat by adding varactors to pull the
frequency, it doesn't affect the noise.  It is true that if
varactors are used, it is possible that the noise will be
degraded if the tuning voltage is not clean enough.  The
HP smart clocks were always limited by this problem because
no realizable voltage source was good enough, at least 20
years ago.

In the 5071, I modified the 10811 to increase its tuning range
by an order of magnitude.  This did not affect its noise
at all, AKAIK.  The zener diode reference in the 10811 is
actually quite good.  This modification was done to eliminate
the need to tweak the coarse tuning of the 10811 as it aged.

Having said this, with currently available technology, I recommend
using frequency synthesizers to do a "virtual pull"
on crystal oscillators, rather than trying to pull them
with varactors.

Rick N6RK
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Re: [time-nuts] Oscillators and Ovens

[Dana Whitlow]

Bob,

This discussion is getting really interesting.  In thinking about the
crystal Q versus
tuning range conundrum, two (presumably-overlapping) concerns come to mind:

1. The motional parameters of a high-Q crystal are such that the external
network
needed to pull it very far would be wholly impractical.

2. Varactors themselves probably have pretty limited Q over much of their
range.

Is my thinking on the right track at all?

Dana  K8YUM

On Wed, Nov 1, 2017 at 5:13 PM, Bob kb8tq  wrote:

> Hi
>
> A high Q crystal by design is very difficult to tune. Putting it in a
> circuit that will
> swing it far enough to compensate it degrades the Q. In addition, thermal
> noise
> will come into the compensation circuit (even if it is noise free) and
> degrade things.
>
> Bob
>
> > On Nov 1, 2017, at 4:38 PM, Hal Murray  wrote:
> >
> >
> >> In general, OCXOs have crystals with high Q -> low phase noise,
> especially
> >> compared to a TCXO, which *can't* have high Q, or the  temperature
> >> compensation circuit can't do it's work.
> >
> > I don't understand that.  Why can't I build a high Q TCXO?  I don't need
> to
> > change the compensation very fast.  Are good crystals high enough Q that
> it
> > would take too long?
> >
> > What's the time constant?  I'd guess it's Q/freq, maybe with factors of
> 2pi
> > or e or ???
> >
> > That seems small relative to how fast temperature changes.  (but maybe
> fast
> > relative to FCC smearing or things like that)
> >
> >
> > --
> > These are my opinions.  I hate spam.
> >
> >
> >
> > ___
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Jim Harman]

On Wed, Nov 1, 2017 at 7:21 PM, MLewis  wrote:

>
> And the DS3231 has:
> - a 32K output, and
> - an Active-Low-Interrupt/SQW output that can be set to PPS.
> It's unclear to me how to sync the DS3231 PPS to the GPS PPS, or if that
> can be done.
>
>
>
The DS3231 has an 8 bit register that will change its frequency in
increments of about 0.1ppm. Thus you could discipline it to get its pps
aligned with your reference.

With an adjustment range of +/- 127, that's a maximum offset of 12.7 ppm.
In the worst case you would have to move its pps 0.5 sec, which by my
calculation would take about 41.667 seconds or about 11.5 hours.

-- 

--Jim Harman
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Bob kb8tq]

Hi

The normal NTP kernel does a pretty good job in “flywheel” mode. For modest 
outages, it’s quite adequate. 

Bob

> On Nov 1, 2017, at 7:21 PM, MLewis  wrote:
> 
> Is this a workable or worthwhile strategy?:
> - RTC providing date & time to second to system on boot
> - RTC frequency output driving a counter/divider to produce PPS
> - GPS module providing UTC PPS
> - GPS module's secondary PPS disciplining the RTC-counter-divider PPS by 
> resetting the RTC's counter/divider (I'm assuming there's one that will rest 
> fast enough to sync; I've never looked into these...)
> 
> If GPS PPS is lost, then the RTC counter/divider is producing a recently 
> disciplined PPS.
> 
> Valid or invalid?
> 
> And the DS3231 has:
> - a 32K output, and
> - an Active-Low-Interrupt/SQW output that can be set to PPS.
> It's unclear to me how to sync the DS3231 PPS to the GPS PPS, or if that can 
> be done.
> 
> Thanks,
> 
> Michael
> 
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Re: [time-nuts] HP 5335A Question

[Bob kb8tq]

Hi

The gate time on a 5335 can be set via GPIB. It can be set to some *very* long 
gates (well past 100 seconds).
When you do this, the data reported on GPIB does “stretch” to cover the added 
digits. The problem is that there
are internal register overflows. The designers did not anticipate needing quite 
so many bits. You loose the MSB’s
when you go to long gates. Compensating for this on a reasonable source is not 
a big deal.

Bob

> On Nov 1, 2017, at 6:14 PM, Dr. Frank  wrote:
> 
>> I have an HP 5335A and I am measuring the output of a Suzler 2.5.  I  >have 
>> a GPSDO 10 MHz input into the back of the 5335A. >There is 
> mention in the manual that the counter can display 11 or 12 >digits (in 
> addition to the two Exponent digits). I presently have it >displaying nine 
> digits. >Does anyone have one of these and could help me figure out how to 
> get 12 >digits displayed? The manual is not very helpful here. >Richard
> 
> Dear Richard,
> 
> the maximum resolution are 11 siginificant digits on the display. That means, 
> a leading '1' would then be the 12th digit. A leading 9 would give 11 digits. 
> Over GPIB, I think, it's always formatted to 9 significant digits.
> 
> The counter internally does not crop the result, but there's no way to output 
> this "infinite" resolution over the bus, nor to read the internal counting 
> and interpolation registers.
> 
> Anyhow, you'll get 10 digits with 10 sec Gate Time, and the maximum of 11 
> digits with > 100 sec gate time.
> 
> As the resolution is around 1ns / sec gate Time, maybe a bit lower than that, 
> it's useful to have slightly longer gate Times, like 12 sec or 150 sec, to 
> achieve this display resolution.
> 
> There are two ways.
> 
> At first, I replaced the 10µF tantalum capacitor C36 on A4 main board, which 
> controls the long gate time, with a higher one, I think a 100 or 150µF Elco. 
> That gave 12.5 sec maximum, and 10 digits.
> 
> Then you might also use the manual gate control and a stopwatch, to have > 
> 100sec gate time, and 11 digits.
> 
> That might also work under GPIB control, commands GO and GC, but you get no 
> GPIB readback, either.
> 
> If you want to characterize your Sulzer vs. GPSDO, I would instead recommend 
> Timelab (supports the 5335A) for the case of mid-to-longterm time constants, 
> but GPSDO is not able to catch the short term stability of the Sulzer.
> 
> Frank
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[MLewis]

Is this a workable or worthwhile strategy?:
- RTC providing date & time to second to system on boot
- RTC frequency output driving a counter/divider to produce PPS
- GPS module providing UTC PPS
- GPS module's secondary PPS disciplining the RTC-counter-divider PPS by 
resetting the RTC's counter/divider (I'm assuming there's one that will 
rest fast enough to sync; I've never looked into these...)


If GPS PPS is lost, then the RTC counter/divider is producing a recently 
disciplined PPS.


Valid or invalid?

And the DS3231 has:
- a 32K output, and
- an Active-Low-Interrupt/SQW output that can be set to PPS.
It's unclear to me how to sync the DS3231 PPS to the GPS PPS, or if that 
can be done.


Thanks,

Michael

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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[William H. Fite]

For this apparatus, a lepton is a lepton is a lepton. If the standard model
is complete/correct and neutrinos are massless, they can be ignored as a
potential threat to our project. They'll just zip through on their way to
wherever. The likelihood of a particle interaction is so minute that we can
disregard it. However, if the Super-Kamiokande group convincingly
demonstrates that neutrinos have mass >0. then they must also possess
magnetic moment. Therein lies the potential for variation in the error term
of our measurements.

Or so it seems to me.


On Wednesday, November 1, 2017, Magnus Danielson  wrote:

> Super-lumious or non-speeding neutrinos?
>
> Cheers,
> Magnus
>
> On 11/01/2017 04:30 PM, William H. Fite wrote:
>
>> Oh crap, I forgot about neutrinos.
>>
>> On Wednesday, November 1, 2017, William H. Fite > > wrote:
>>
>> Inside a three-meter thick sphere of lead and mu-metal, pressure
>> pulled down to .01 mTorr , temperature regulated to .001
>> degree C, operating in total darkness and absolute silence, aligned
>> with the galactic axis of rotation, and situated 20 light years from
>> the nearest star.
>>
>> That should satisfy even the most obsessive TN.
>>
>>
>>
>> On Wednesday, November 1, 2017, Magnus Danielson
>> > > wrote:
>>
>> Hi,
>>
>> On 11/01/2017 04:03 PM, jimlux wrote:
>>
>> On 11/1/17 7:37 AM, Magnus Danielson wrote:
>>
>> Hi,
>>
>>Silly people
>>
>> want a relative comfortable temperature and well,
>> building A/C is typically bang/bang regulated so you get
>> what you paid for.
>>
>>
>> wouldn't a true time-nut be in a basically isothermal cave
>> at 10C far underground, and just follow the classic mother's
>> advice "put on a sweater if you feel cold"
>>
>>
>> None of the NMI labs I've seen does this, but please, go ahead.
>>
>> Several of them was just underground, but not that cold.
>>
>> The two labs that where underground was relatively comfortable
>> temperatures. :)
>>
>> Cheers,
>> Magnus
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>> https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
>> 
>> and follow the instructions there.
>>
>>
>>
>> -- Patriotism is supporting your country all the time, and your
>> government when it deserves it.
>> --Mark Twain
>>
>> There is no one thing that is true. They're all true.
>> --Ernest Hemingway
>>
>> We may be surprised at the people we find in heaven. God has a soft
>> spot for sinners. His standards are quite low.
>> --Desmond Tutu
>>
>>
>>
>> --
>> Patriotism is supporting your country all the time, and your government
>> when it deserves it.
>> --Mark Twain
>>
>> There is no one thing that is true. They're all true.
>> --Ernest Hemingway
>>
>> We may be surprised at the people we find in heaven. God has a soft spot
>> for sinners. His standards are quite low.
>> --Desmond Tutu
>>
>>

-- 
Patriotism is supporting your country all the time, and your government
when it deserves it.
--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft spot
for sinners. His standards are quite low.
--Desmond Tutu
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Re: [time-nuts] Designing an embedded precision GPS time

[Leo Bodnar]

Regarding spread spectrum issues:

You might be lucky to find (or have) SSCG implementation that is reasonably 
stable.  I suspect most are - because it is easy to generate hershey kiss 
spectrum based on SM, LUT or some sort of multi-level LFSRs.  I don't know what 
this means - these are some random Internet acronyms so it must be true.

I have several Linux workstations with system board crystals replaced with GPS 
synchronised synthesiser.  Instead of trying to disable SSCG or spending time 
analysing it I have tweaked system clock frequency until local time drift of 
the freewheeling system (with NTP setup with disabled time adjustment and clock 
correction) was less than 1 microsecond per day.  This took several days and 
needed better than 10^-11 level frequency adjustments and stability.  With GPS 
synchronisation this is easily achievable. If I keep NTP local time corrections 
disabled (basically remove all the servers from ntpd configuration) the local 
time drift is better than 0.01ppb.  Interestingly, to achieve that what used to 
be 26.000M crystal has ended up being a system clock that looks more like 
26,000,001.7193Hz 

Leo
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[time-nuts] HP 5335A Question

[Dr. Frank]

I have an HP 5335A and I am measuring the output of a Suzler 2.5.  I  >have a GPSDO 10 MHz input into the back of the 5335A. >There is 
mention in the manual that the counter can display 11 or 12 >digits (in 
addition to the two Exponent digits). I presently have it >displaying 
nine digits. >Does anyone have one of these and could help me figure out 
how to get 12 >digits displayed? The manual is not very helpful here. 
>Richard


Dear Richard,

the maximum resolution are 11 siginificant digits on the display. That 
means, a leading '1' would then be the 12th digit. A leading 9 would 
give 11 digits. Over GPIB, I think, it's always formatted to 9 
significant digits.


The counter internally does not crop the result, but there's no way to 
output this "infinite" resolution over the bus, nor to read the internal 
counting and interpolation registers.


Anyhow, you'll get 10 digits with 10 sec Gate Time, and the maximum of 
11 digits with > 100 sec gate time.


As the resolution is around 1ns / sec gate Time, maybe a bit lower than 
that, it's useful to have slightly longer gate Times, like 12 sec or 150 
sec, to achieve this display resolution.


There are two ways.

At first, I replaced the 10µF tantalum capacitor C36 on A4 main board, 
which controls the long gate time, with a higher one, I think a 100 or 
150µF Elco. That gave 12.5 sec maximum, and 10 digits.


Then you might also use the manual gate control and a stopwatch, to have 
> 100sec gate time, and 11 digits.


That might also work under GPIB control, commands GO and GC, but you get 
no GPIB readback, either.


If you want to characterize your Sulzer vs. GPSDO, I would instead 
recommend Timelab (supports the 5335A) for the case of mid-to-longterm 
time constants, but GPSDO is not able to catch the short term stability 
of the Sulzer.


Frank
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Re: [time-nuts] Room temperature control (was: Holdover, RTC for Pi as NTP GPS source)

[Neville Michie]

You would be amazed by the effectiveness of installing a small fan, mounted 
parallel to the wall,
to create a slow whirlpool circulation in the room. Just a 10 w computer fan.
If the air velocity is below 0.3 m/s it is hardly perceptible. That takes about 
30 seconds to get 
around the room. The room becomes a well mixed volume of air. Heat sources 
distribute their burden 
of heat with only small local temperature rise. The room contains a 50 to 100kg 
mass of air and provides
an averaging effect on perturbations.
I have constructed 7 labs using this principle and have little difficulty 
keeping temperature swings below
1.0 degree C. The temperature control sensor must be very small (fast), exposed 
to the air flow, have zero 
hysteresis and be located on the wall that is opposite to the fan (and the AC 
unit).
The temperature sensor directly controls the AC unit, with the overriding logic 
that although a 0.1C deviation 
from the set point can switch the compressor on immediately (not even a second 
of delay), when switched off 
the compressor can not be switched on again for about 2 minutes, the time 
needed for the gas pressure in the AC 
to subside.
The natural cycle of about 2 - 3 minutes of on/off is attenuated by the 
integrated mass of the stirred air in 
the room. 1 Kw of heating or cooling is 1 kJ/s, air has an enthalpy of about 
1kJ/Kg per degree C, so 100Kg 
of air heats/cools at 1/100 degrees per second. The room temperature gently 
cycles by a fraction of a degree
as the AC cycles.
If anything, the service life (10 years or more) of the AC unit is longer than 
conventional hysteresis sensor
dominated chuggers.
All of this occurs because you stirred the air in the room!

cheers,
Neville Michie

> On 2 Nov 2017, at 6:32 AM, Poul-Henning Kamp  wrote:
> 
> 
> In message <20171101190841.366058d77e544d256b862...@kinali.ch>, Attila Kinali 
> w
> rites:
> 
>> The best we can do today is to have a well insulated room (no windows
>> with whith unknown power flows) and measure the temperature at a few
>> strategically choosen points. Then control the heat influx and
>> outflux using an approriate control loop. This will still result
>> in deviations of 1-2°C when somone walks in.
> 
> It is a matter of energy balance.
> 
> Humans emit heat on the order of a hundred watts and the only way
> to have that not affect the room temperture, is to "wash" it away
> in airflow with much higher energy content, as is typically done
> in clean-rooms.
> 
> That still leaves you with the thermal radiation imbalance from the
> higher temperature of the human skin, which is why "nano" laboratories
> sometimes are kept at an uncomfortably warm temperatures.
> 
> -- 
> Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
> p...@freebsd.org | TCP/IP since RFC 956
> FreeBSD committer   | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
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Re: [time-nuts] Oscillators and Ovens

[Bob kb8tq]

Hi

A high Q crystal by design is very difficult to tune. Putting it in a circuit 
that will 
swing it far enough to compensate it degrades the Q. In addition, thermal noise
will come into the compensation circuit (even if it is noise free) and degrade 
things. 

Bob

> On Nov 1, 2017, at 4:38 PM, Hal Murray  wrote:
> 
> 
>> In general, OCXOs have crystals with high Q -> low phase noise,  especially
>> compared to a TCXO, which *can't* have high Q, or the  temperature
>> compensation circuit can't do it's work. 
> 
> I don't understand that.  Why can't I build a high Q TCXO?  I don't need to 
> change the compensation very fast.  Are good crystals high enough Q that it 
> would take too long?
> 
> What's the time constant?  I'd guess it's Q/freq, maybe with factors of 2pi 
> or e or ???
> 
> That seems small relative to how fast temperature changes.  (but maybe fast 
> relative to FCC smearing or things like that)
> 
> 
> -- 
> These are my opinions.  I hate spam.
> 
> 
> 
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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

> On Nov 1, 2017, at 1:11 PM, tn...@joshreply.com wrote:
> 
>> While crystal curves are indeed cubic, there are higher order terms in 
>> the curve. The “why” is something people get to write papers on. If you 
>> are trying to compensate to tight specs, you will see all sorts of 
>> stuff. It is not at all uncommon to see >9th order curves residual curves. 
>> Indeed some of that is from residuals in the compensation circuit as well as 
>> from the crystal.
> 
> I’ve been trying to research this very topic!
> 
> Can you point to some of these papers?

The Frequency Control Symposium has papers going back > 50 years on crystals 
and how to 
cut them from raw quartz bars. Plan on spending a few million dollars to get 
set up to do this well. 
Something in the $5M is likely the “going rate” these days for a basic line, 
even with some of it
bought surplus. 

> 
> I am trying to build the most accurate fee running, low power time base I 
> can. I am using an MCU, 32768Khz watch crystals,


Which are very low precision crystals by their very nature…..

> 0.5C accuracy temp sensor, lots of thermal bringing between them, and mass 
> around them.

Which is pretty loose by todays standards. You can get parts that will do 
better than that. 

> The idea is to measure the frequency shift at all temps in the range, and 
> even in both directions (hopefully to capture some hysteresis)

Keep in mind that your hysteresis runs need to be at multiple speeds and over 
multiple temperature ranges. The results vary with 
both promoters.  

> for each unit and then use that database to compensate in software once the 
> system is free running. 

Assuming you have a “normal” watch crystal, you can get into slopes well over 
10 ppm / C without going to crazy extremes. That
and your 0.5 C resolution is going to have a pretty big impact. 

> 
> I am trying to beat existing products like the Dallas DS3231 and Micro 
> Crystal RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar 
> strategy. I’m hoping I can beat them by using more accurate temp tensing, 
> longer and more exhaustive calibration effort, and anything else possible! 

I’d say it’s unlikely. 

> 
> Can you give a quick explanation (or point to reference material) covering 
> the fundamental limits to XTAL compensation accuracy, and how to get there?

The FCS proceedings have a number of papers. The basics depend a lot on the 
type of crystal you have and the net result you are after. If you must 
have the output on frequency that leads in a different direction than if you 
just want an accurate one second tick. For example, with the one second tick, 
you can drop (or add) cycles in your divider. If the oscillator is at 10 MHz, 
the net result will always be within 100 ns. For something like NTP, there
is no real advantage going past that point. You can accumulate error over a 
long period. That allows very good stability over the long term. 

If you need low power and proper frequency, a thermistor resistor network 
driving a varicap diode is the classic answer. You generate a cubic voltage
function over temperature. It is the “required signal” to tune the oscillator 
on frequency. The components in the network are evaluated and adjusted with 
multiple 
temperature runs. The software to make it all work is generally the “top 
secret” IP of the people making the TCXO.

A brute force approach using a good temperature sensor (say an RTD) and a good 
ADC is possible. You can get into the low mili C range with this sort of
setup. Pairing that with a good low frequency overtone crystal can do pretty 
well. There are also multi mode ostillators that head in the same direction. 
There
are many twists and turns. A starting engineer generally takes something in the 
5 year range to come up to speed on most of them. 

> 
> That is, if I had an infinitely precise temp sensor and an infinite amount of 
> time to characterize an XTAL, what would be limits to how accurately I could 
> temp compensate it?

Well, aging is one obvious issue. ADEV is another. A reasonable goal for aging 
would be in the low parts in 10^-11 per day range. Good ADEV at 1,000 seconds 
would be at least that good. The electronics to make it all happen might use as 
much power as a low power OCXO …. Managing temperature in a TCXO like 
device below 1x10^-10 is unlikely.  Figure the crystal used would be > $100 in 
volume. Single piece (if you could buy one) might be over a thousand dollars. 

> 
> Also, what are the limits of characterizing and compensating for aging?

Low parts in 10^-11 per day on a “production” basis if cost is not a 
consideration. It also depends quite a bit on the constraints you put on the 
problem. (how many
days on power, what sort of environment, ….)

> 
> What other sources of inaccuracy would I need to consider?

There are *many*. How many years do you have to study the various topics? 
Thermal gradients are one that will bother you. 

Again, the FCS papers are a 

Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Magnus Danielson]

Super-lumious or non-speeding neutrinos?

Cheers,
Magnus

On 11/01/2017 04:30 PM, William H. Fite wrote:

Oh crap, I forgot about neutrinos.

On Wednesday, November 1, 2017, William H. Fite > wrote:


Inside a three-meter thick sphere of lead and mu-metal, pressure
pulled down to .01 mTorr , temperature regulated to .001
degree C, operating in total darkness and absolute silence, aligned
with the galactic axis of rotation, and situated 20 light years from
the nearest star.

That should satisfy even the most obsessive TN.



On Wednesday, November 1, 2017, Magnus Danielson
> wrote:

Hi,

On 11/01/2017 04:03 PM, jimlux wrote:

On 11/1/17 7:37 AM, Magnus Danielson wrote:

Hi,

   Silly people

want a relative comfortable temperature and well,
building A/C is typically bang/bang regulated so you get
what you paid for.


wouldn't a true time-nut be in a basically isothermal cave
at 10C far underground, and just follow the classic mother's
advice "put on a sweater if you feel cold"


None of the NMI labs I've seen does this, but please, go ahead.

Several of them was just underground, but not that cold.

The two labs that where underground was relatively comfortable
temperatures. :)

Cheers,
Magnus
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-- 
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government when it deserves it.
--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft
spot for sinners. His standards are quite low.
--Desmond Tutu



--
Patriotism is supporting your country all the time, and your government 
when it deserves it.

--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft spot 
for sinners. His standards are quite low.

--Desmond Tutu


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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Magnus Danielson]

That sloppy temperaturestabilization???

Also, we would need to care about the lead-poisoning of the mu-metal and 
the needed re-alignment... ehm... ah well.


Cheers,
Magnus

On 11/01/2017 04:29 PM, William H. Fite wrote:
Inside a three-meter thick sphere of lead and mu-metal, pressure pulled 
down to .01 mTorr , temperature regulated to .001 degree C, 
operating in total darkness and absolute silence, aligned with the 
galactic axis of rotation, and situated 20 light years from the nearest 
star.


That should satisfy even the most obsessive TN.



On Wednesday, November 1, 2017, Magnus Danielson 
> wrote:


Hi,

On 11/01/2017 04:03 PM, jimlux wrote:

On 11/1/17 7:37 AM, Magnus Danielson wrote:

Hi,

   Silly people

want a relative comfortable temperature and well, building
A/C is typically bang/bang regulated so you get what you
paid for.


wouldn't a true time-nut be in a basically isothermal cave at
10C far underground, and just follow the classic mother's advice
"put on a sweater if you feel cold"


None of the NMI labs I've seen does this, but please, go ahead.

Several of them was just underground, but not that cold.

The two labs that where underground was relatively comfortable
temperatures. :)

Cheers,
Magnus
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--
Patriotism is supporting your country all the time, and your government 
when it deserves it.

--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft spot 
for sinners. His standards are quite low.

--Desmond Tutu


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Re: [time-nuts] Designing an embedded precision GPS time

[Denny Page]

> On Nov 01, 2017, at 05:39, Attila Kinali  wrote:
> 
> 6-10µs is the interrupt latency of linux on ARM SoC. I guess, to get
> below that you'd have to tweak the kernel a bit. Which should not
> be that difficult. Definitly simpler than writing your own IP and NTP
> stack from scratch.

Just tweak the Linux kernel a bit? No. You would have to a rewrite substantial 
chunks of it. A tremendous effort. Low latency and accurate timing is not what 
Linux is designed for. This has been discussed extensively for years.

Writing your own IPv4 datagram stack for ICMP and NTP is rather trivial. It’s 
all static state, you don’t have to deal with fragments, you don’t have to deal 
with options, etc. There really is very little you actually have to do. IPv6 is 
more work because you have to maintain some dynamic state (routing), process 
options, etc., but it’s still nothing near like trying to turn Linux into a 
real-time system.


> Spread spectrum can usually be switched off, though requires at least a
> custom DTB or even patching of the kernel. There are a few boards, though
> that do not allow spread spectrum to be switched off.

Unfortunately is usually has to be done in the bios. The control space that 
would allow spread spectrum to be turned off is usually disabled prior to 
kernel load. For compliance, most bios implementations have removed spread 
spectrum as an option so you have to build a custom one. I can’t begin to tell 
you what kind of a chill comes over the conversation with a vendor (even a 
maker oriented one) when you tell them you want to create a custom bios to 
disable spread spectrum.

Denny

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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[MLewis]

This GPS/Pi server, with possibly a second copy, are intended to be the 
sole NTP sources for a local network. No going out to the internet.


My intent was to discipline the RTC, so it's current when needed. If I 
had to do a cold boot without GPS available, I'd want the RTC to be current.


I hadn't considered  'just riding through using the NTP estimate of the 
current system clock frequency', having assumed that a disciplined RTC 
would be a better source.


Hadn't heard about that issue with NTPsec.

I haven't looked at chrony. Something else to read tonight.

Thanks,

Michael

On 01/11/2017 4:57 PM, Chris Caudle wrote:

On Wed, November 1, 2017 3:17 pm, MLewis wrote:

hadn't got there yet

Your RTC is not likely to be tightly synchronized to NTP time, so there is
a high probability that trying to use RTC as a secondary time source will
actually make the system worse than just riding through using the NTP
estimate of the current system clock frequency.  If ntpd will even use it
as a secondary clock, and not reject it because it is too far off from the
preferred GPS source.

Is this system not connected to other NTP servers over the network?
NTPsec does not work very well in the case of only one GPS device
available to set time, that is documented as one of the use cases which
NTPsec does not currently handle well.  If there are no other servers
available for comparison you will probably want to use chrony.  If there
are other servers available then trying to switch over to the RTC will
definitely make your ntp server perform worse than just leaving well
enough alone.



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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Hal Murray]

>> How are you planning on making ntp use the RTC as a secondary
>> time source?  I don't see that as a supported refclk driver.

> hadn't got there yet likely using NTPsec, as the codebase is available if a
> driver or generic  driver won't work 

The PPS driver expects the pulse to be on the second.  You won't get that 
from a RTC.

If I was doing it, I use the SHM driver.  Then you can do all your work in a 
separate program and feed an offset to ntpd.

I haven't looked at RTC chips recently.  You want a pulse that you can feed 
to the kernel's PPS capture logic.  60Hz works on a PC.  32KHz is probably 
too fast.  (might be worth a try)  You might have to add a divider.

If you use the GPS HAT from Adafruit, it has a prototyping area where you 
could drop in a RTC and wire it up to various GPIO pins.  DIP package would 
be convenient.  The Ethernet on the PI is via USB, so it's crappy for making 
a nutty-good NTP server.  But the PI is convenient and good enough for all 
but nutty uses.


-- 
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Re: [time-nuts] Oscillators and Ovens

[Chris Caudle]

On Wed, November 1, 2017 3:38 pm, Hal Murray wrote:
>
>> In general, OCXOs have crystals with high Q -> low phase noise,
>> especially
>> compared to a TCXO, which *can't* have high Q, or the  temperature
>> compensation circuit can't do it's work.
>
> I don't understand that.  Why can't I build a high Q TCXO?

Compensation implies pulling the frequency away from the natural
resonance.  High Q implies that the frequency cannot be pulled very far
away from natural resonance.
The two items are directly contradictory.

-- 
Chris Caudle




 I don't need
> to
> change the compensation very fast.  Are good crystals high enough Q that
> it
> would take too long?
>
> What's the time constant?  I'd guess it's Q/freq, maybe with factors of
> 2pi
> or e or ???
>
> That seems small relative to how fast temperature changes.  (but maybe
> fast
> relative to FCC smearing or things like that)
>
>
> --
> These are my opinions.  I hate spam.
>
>
>
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Chris Caudle]

On Wed, November 1, 2017 3:17 pm, MLewis wrote:
> hadn't got there yet

Your RTC is not likely to be tightly synchronized to NTP time, so there is
a high probability that trying to use RTC as a secondary time source will
actually make the system worse than just riding through using the NTP
estimate of the current system clock frequency.  If ntpd will even use it
as a secondary clock, and not reject it because it is too far off from the
preferred GPS source.

Is this system not connected to other NTP servers over the network? 
NTPsec does not work very well in the case of only one GPS device
available to set time, that is documented as one of the use cases which
NTPsec does not currently handle well.  If there are no other servers
available for comparison you will probably want to use chrony.  If there
are other servers available then trying to switch over to the RTC will
definitely make your ntp server perform worse than just leaving well
enough alone.

-- 
Chris C


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Re: [time-nuts] Oscillators and Ovens

[Hal Murray]

> In general, OCXOs have crystals with high Q -> low phase noise,  especially
> compared to a TCXO, which *can't* have high Q, or the  temperature
> compensation circuit can't do it's work. 

I don't understand that.  Why can't I build a high Q TCXO?  I don't need to 
change the compensation very fast.  Are good crystals high enough Q that it 
would take too long?

What's the time constant?  I'd guess it's Q/freq, maybe with factors of 2pi 
or e or ???

That seems small relative to how fast temperature changes.  (but maybe fast 
relative to FCC smearing or things like that)


-- 
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[MLewis]

hadn't got there yet
likely using NTPsec, as the codebase is available if a driver or generic 
driver won't work

https://docs.ntpsec.org/latest/driver_howto.html

On 01/11/2017 12:38 PM, Chris Caudle wrote:

On Tue, October 31, 2017 9:27 pm, MLewis wrote:

I'm intending to add a "precision" (well, precision to the Pi world) RTC
to my Pi 3 to use for a holdover source when it hasn't got PPS from the
GPS module.

How are you planning on making ntp use the RTC as a secondary time source?
  I don't see that as a supported refclk driver.



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[time-nuts] Designing an embedded precision GPS time

[Mark Sims]

Trying to further compensate a TCXO can be a losing proposition.  Your control 
loop and the TCXO's control loop can easily get their panties all twisted in a 
wad.   

You are probably better off compensating a plain XO... which yields a TCXO that 
you can actually manipulate to your hearts content.   But beware,  thar be lots 
of dragons lurking in the swamp that is temperature compensation and oscillator 
disciplining.
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Re: [time-nuts] Designing an embedded precision GPS time

[Hal Murray]

tn...@joshreply.com said:
> I am trying to build the most accurate fee running, low power time base I can.

> I am trying to beat existing products like the Dallas DS3231 and Micro
> Crystal RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar
> strategy. I’m hoping I can beat them by using more accurate temp tensing,
> longer and more exhaustive calibration effort, and anything else possible!  

That should be easy.
  http://users.megapathdsl.net/~hmurray/time-nuts/Temp-tcxo32khz-a.png

Do you need an accurate output frequency, or just know what the actual 
frequency is so you can compensate when counting cycles to keep track of 
elapsed time?

I would start by bolting your setup to a large thermal mass and putting it 
inside an insulated box.  Not too well insulated or it will cook itself.  You 
want the thermal time constant to be long enough to give your control loop 
plenty of time to react.  You also want the thermal sensor at the same 
temperature as the crystal.



-- 
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Re: [time-nuts] Room temperature control (was: Holdover, RTC for Pi as NTP GPS source)

[Poul-Henning Kamp]

In message <20171101190841.366058d77e544d256b862...@kinali.ch>, Attila Kinali w
rites:

>The best we can do today is to have a well insulated room (no windows
>with whith unknown power flows) and measure the temperature at a few
>strategically choosen points. Then control the heat influx and
>outflux using an approriate control loop. This will still result
>in deviations of 1-2°C when somone walks in.

It is a matter of energy balance.

Humans emit heat on the order of a hundred watts and the only way
to have that not affect the room temperture, is to "wash" it away
in airflow with much higher energy content, as is typically done
in clean-rooms.

That still leaves you with the thermal radiation imbalance from the
higher temperature of the human skin, which is why "nano" laboratories
sometimes are kept at an uncomfortably warm temperatures.

-- 
Poul-Henning Kamp   | UNIX since Zilog Zeus 3.20
p...@freebsd.org | TCP/IP since RFC 956
FreeBSD committer   | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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Re: [time-nuts] Room temperature control (was: Holdover, RTC for Pi as NTP GPS source)

[Jeremy Nichols]

I remember visiting the metrology lab in HP Palo Alto (Glen Whatshisname)
when I worked for them in the early 70s. There was a set of toggle switches
on the wall; for every person inside, one switch was turned on. I don't
know how well it worked but apparently, well enough for 1970s-vintage
metrology.

Jeremy


On Wed, Nov 1, 2017 at 11:08 AM Attila Kinali  wrote:

> On Wed, 1 Nov 2017 15:37:38 +0100
> Magnus Danielson  wrote:
>
> >  Silly people
> > want a relative comfortable temperature and well, building A/C is
> > typically bang/bang regulated so you get what you paid for.
>
> My short stint in the HVAC business taught me, that it's surprisingly
> difficult to stabilize room temperature to better than 2-5°C.
> It starts at such simple things as measuring the temperature.
> The position of the sensor and its distance to the wall make a huge
> difference. Just 1cm further away from the wall, or 10cm up or down and
> you get 2-3°C difference. An A/C system usually controls the temperature
> of the air inlet, which is the simplest thing to do, but actually you want
> to control the (heat) power flow into the room. And this is something that
> has not been possible with standard equipment until recently.
>
> The best we can do today is to have a well insulated room (no windows
> with whith unknown power flows) and measure the temperature at a few
> strategically choosen points. Then control the heat influx and
> outflux using an approriate control loop. This will still result
> in deviations of 1-2°C when somone walks in.
>
>
> Attila Kinali
>
> --
> It is upon moral qualities that a society is ultimately founded. All
> the prosperity and technological sophistication in the world is of no
> use without that foundation.
>  -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] Designing an embedded precision GPS time

[Adrian Godwin]

How do those compare with vectron's part : ?

https://www.vectron.com/products/ocxo/mx-503.htm


There's also this patent.

http://www.google.sr/patents/US20020005765

I don't really know if that's valid - it seems to propose something similar
to the numerically-compensated oscillator in my rather old PM frequency
counter.



On Wed, Nov 1, 2017 at 5:11 PM,  wrote:

> >While crystal curves are indeed cubic, there are higher order terms in
> >the curve. The “why” is something people get to write papers on. If you
> >are trying to compensate to tight specs, you will see all sorts of
> >stuff. It is not at all uncommon to see >9th order curves residual
> curves. Indeed some of that is from residuals in the compensation circuit
> as well as from the crystal.
>
> I’ve been trying to research this very topic!
>
> Can you point to some of these papers?
>
> I am trying to build the most accurate fee running, low power time base I
> can. I am using an MCU, 32768Khz watch crystals, 0.5C accuracy temp sensor,
> lots of thermal bringing between them, and mass around them. The idea is to
> measure the frequency shift at all temps in the range, and even in both
> directions (hopefully to capture some hysteresis) for each unit and then
> use that database to compensate in software once the system is free running.
>
> I am trying to beat existing products like the Dallas DS3231 and Micro
> Crystal RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar
> strategy. I’m hoping I can beat them by using more accurate temp tensing,
> longer and more exhaustive calibration effort, and anything else possible!
>
> Can you give a quick explanation (or point to reference material) covering
> the fundamental limits to XTAL compensation accuracy, and how to get there?
>
> That is, if I had an infinitely precise temp sensor and an infinite amount
> of time to characterize an XTAL, what would be limits to how accurately I
> could temp compensate it?
>
> Also, what are the limits of characterizing and compensating for aging?
>
> What other sources of inaccuracy would I need to consider?
>
> Thanks!!!
>
> -josh
>
>
>
>
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[ewkehren via time-nuts]

Tbolt is a good oneBert Kehren


Sent from my Galaxy Tab® A
 Original message From: David J Taylor via time-nuts 
 Date: 11/1/17  12:07 PM  (GMT-05:00) To: 
time-nuts@febo.com Subject: Re: [time-nuts] Holdover, RTC for Pi as NTP GPS 
source 
From: Mark Sims

I have an analytical balance that reads down to micrograms.   The weigh 
chamber is surrounded by three layers of IR absorbing glass so that radiated 
body heat does not induce convection currents in the air.   I worked on a 
balance that had nanogram resolution (mostly wishful thinking in that spec). 
It operated in a vacuum.  30 bit mass-to-digital converters are rather 
finicky beasties.

It does not take all that good of a temperature sensor to detect changes in 
room temperature due to body heat (or fetching a beer from the fridge in the 
next room).  You are basically a 100 watt space heater... even larger for 
the more corpulent folks.
==

Temperature is indeed the killer.  My best Raspberry Pis for time-keeping 
are RasPi-1 and RasPi-4, both of which are in an unheated cupboard not 
exposed to sunlight, on the north side of the house with indoor patch GPS 
antennas.

  http://www.satsignal.eu/mrtg/performance_ntp.php

Cheers,
David
-- 
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 

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Re: [time-nuts] Oscillators and Ovens

[jimlux]

On 11/1/17 10:59 AM, Attila Kinali wrote:

Hi Jim,

On Wed, 1 Nov 2017 06:17:31 -0700
jimlux  wrote:


That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe
look for regular XO (no TC).  Those might have a more "pure" (read lower
order) freq vs temp characteristic.



It feels like I have asked this before, but I cannot remember and
cannot find the mailSo: What do you mean by "OCXO without oven?"




I often have requirements for "good phase noise" but no particular 
requirement for "good temperature stability" or Allan Deviation for tau 
> 200 seconds - we get frequency knowledge from other sources (e.g. 
just like a GPSDO, or it can be inferred from the measurement)
I often have a system where the environment is pretty benign - a typical 
on-orbit temperature variation might be a degree or two over 90 minutes 
(or longer), if that - we don't know what the temperature will be (in 
advance), but once it's up there, it's pretty stable.  Maybe the temp 
changes a bit due to relative orientation to the sun and similar 
effects, so there's a annual variation.  My current spacecraft is 
expected to change maybe 5-6 degrees over the year.


For instance, in a software defined radio, the input oscillator is often 
fed into some sort of NCO or DDS for tuning - knowledge of the frequency 
is what you really want, because the ultimate requirement is on the 
frequency accuracy at the input or output of the radio - the regulators 
care not a whit what kind of oscillator is inside (well, they DO ask on 
the license application, but it's really not relevant)


THere are also systems like Doppler radars - they need good pulse to 
pulse stability, and low phase noise to avoid reciprocal mixing 
degradation of the noise floor - but they care not what the exact 
frequency is.


In general, OCXOs have crystals with high Q -> low phase noise, 
especially compared to a TCXO, which *can't* have high Q, or the 
temperature compensation circuit can't do it's work.


So, it would be nice to have a *cheap* lowish power packaged part that 
has the Q of an OCXO, but without the power consumption of the oven 
(typically measured in watts).


yeah, I'd be operating it *way* far from the optimum turnover temp, so 
the tempco might be huge (in oscillator terms), but I don't really care 
- in fact, that might give me a way to measure the temperature of the 
system.


I have gotten quotes for OCXOs with the oven disabled-  but that's 
making it a custom part and as Bob has pointed out, the moment you 
deviate from the "catalog part", the cost (and often more importantly, 
the delivery time) goes up.


I think the ideal, of course, is to get an oscillator high performance 
crystal cut with the turnover temp around 20-30C - useless for a OCXO, 
but might be real useful for GPS/temp compensated oscillator where the 
oscillator drives a DDS (or or provides an output giving the estimated 
frequency).




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Re: [time-nuts] Designing an embedded precision GPS time

[Attila Kinali]

Hoi Leo,

On Wed, 1 Nov 2017 15:35:46 +
Leo Bodnar  wrote:

> > From: Attila Kinali 
> > What you should do is basically system identification and adaptive control. 
> 
> Thanks for your advice, Attila, I am going to google this tonight.  This is 
> exciting!

Before you dive into sys-id and adaptive control, make sure you
have at least basic knowledge of control theory. Otherwise nothing
will make sense. I recommend [1] usally, as I find it a nice
and easy to understand introduction. Phk recommended [2] some
time ago, which gives a more historical view (or "the motherlore" as
phk called it).


Attila Kinali

[1] "Fedback Control of Dynamic Systems",  by Gene F. Franklin,
J. Da Powell, Abbas Emami-Naeini

[2] "Radiation laboratory series" https://www.jlab.org/ir/MITSeries.html

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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[time-nuts] Room temperature control (was: Holdover, RTC for Pi as NTP GPS source)

[Attila Kinali]

On Wed, 1 Nov 2017 15:37:38 +0100
Magnus Danielson  wrote:

>  Silly people 
> want a relative comfortable temperature and well, building A/C is 
> typically bang/bang regulated so you get what you paid for.

My short stint in the HVAC business taught me, that it's surprisingly
difficult to stabilize room temperature to better than 2-5°C.
It starts at such simple things as measuring the temperature.
The position of the sensor and its distance to the wall make a huge
difference. Just 1cm further away from the wall, or 10cm up or down and
you get 2-3°C difference. An A/C system usually controls the temperature
of the air inlet, which is the simplest thing to do, but actually you want
to control the (heat) power flow into the room. And this is something that
has not been possible with standard equipment until recently.

The best we can do today is to have a well insulated room (no windows
with whith unknown power flows) and measure the temperature at a few
strategically choosen points. Then control the heat influx and
outflux using an approriate control loop. This will still result
in deviations of 1-2°C when somone walks in.


Attila Kinali

-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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[time-nuts] Oscillators and Ovens (was: Designing an embedded precision GPS time)

[Attila Kinali]

Hi Jim,

On Wed, 1 Nov 2017 06:17:31 -0700
jimlux  wrote:

> That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe 
> look for regular XO (no TC).  Those might have a more "pure" (read lower 
> order) freq vs temp characteristic.


It feels like I have asked this before, but I cannot remember and
cannot find the mailSo: What do you mean by "OCXO without oven?"

Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
 -- Miss Matheson, The Diamond Age, Neil Stephenson
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[David J Taylor via time-nuts]

From: David C. Partridge 


? RasPi-4? Not released until 2019 AFAIK ...  I have RasPi-3
===

My Raspberry Pi #1 and Raspberry Pi #4.  See:

 http://www.satsignal.eu/mrtg/performance_ntp.php

Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv
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Re: [time-nuts] Designing an embedded precision GPS time

[tnuts]

>While crystal curves are indeed cubic, there are higher order terms in 
>the curve. The “why” is something people get to write papers on. If you 
>are trying to compensate to tight specs, you will see all sorts of 
>stuff. It is not at all uncommon to see >9th order curves residual curves. 
>Indeed some of that is from residuals in the compensation circuit as well as 
>from the crystal.

I’ve been trying to research this very topic!

Can you point to some of these papers?

I am trying to build the most accurate fee running, low power time base I can. 
I am using an MCU, 32768Khz watch crystals, 0.5C accuracy temp sensor, lots of 
thermal bringing between them, and mass around them. The idea is to measure the 
frequency shift at all temps in the range, and even in both directions 
(hopefully to capture some hysteresis) for each unit and then use that database 
to compensate in software once the system is free running. 

I am trying to beat existing products like the Dallas DS3231 and Micro Crystal 
RV-8803-C7-32.768kHz-3PPM-TA-QC, which use (I think) a similar strategy. I’m 
hoping I can beat them by using more accurate temp tensing, longer and more 
exhaustive calibration effort, and anything else possible! 

Can you give a quick explanation (or point to reference material) covering the 
fundamental limits to XTAL compensation accuracy, and how to get there?

That is, if I had an infinitely precise temp sensor and an infinite amount of 
time to characterize an XTAL, what would be limits to how accurately I could 
temp compensate it?

Also, what are the limits of characterizing and compensating for aging?

What other sources of inaccuracy would I need to consider?

Thanks!!!

-josh




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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[David C. Partridge]

? RasPi-4? Not released until 2019 AFAIK ...  I have RasPi-3

-Original Message-
From: time-nuts [mailto:time-nuts-boun...@febo.com] On Behalf Of David J Taylor 
via time-nuts
Sent: 01 November 2017 16:08
To: time-nuts@febo.com
Subject: Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

From: Mark Sims

I have an analytical balance that reads down to micrograms.   The weigh 
chamber is surrounded by three layers of IR absorbing glass so that radiated 
body heat does not induce convection currents in the air.   I worked on a 
balance that had nanogram resolution (mostly wishful thinking in that spec). 
It operated in a vacuum.  30 bit mass-to-digital converters are rather finicky 
beasties.

It does not take all that good of a temperature sensor to detect changes in 
room temperature due to body heat (or fetching a beer from the fridge in the 
next room).  You are basically a 100 watt space heater... even larger for the 
more corpulent folks.
==

Temperature is indeed the killer.  My best Raspberry Pis for time-keeping are 
RasPi-1 and RasPi-4, both of which are in an unheated cupboard not exposed to 
sunlight, on the north side of the house with indoor patch GPS antennas.

  http://www.satsignal.eu/mrtg/performance_ntp.php

Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 

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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[David J Taylor via time-nuts]

From: Mark Sims

I have an analytical balance that reads down to micrograms.   The weigh 
chamber is surrounded by three layers of IR absorbing glass so that radiated 
body heat does not induce convection currents in the air.   I worked on a 
balance that had nanogram resolution (mostly wishful thinking in that spec). 
It operated in a vacuum.  30 bit mass-to-digital converters are rather 
finicky beasties.


It does not take all that good of a temperature sensor to detect changes in 
room temperature due to body heat (or fetching a beer from the fridge in the 
next room).  You are basically a 100 watt space heater... even larger for 
the more corpulent folks.

==

Temperature is indeed the killer.  My best Raspberry Pis for time-keeping 
are RasPi-1 and RasPi-4, both of which are in an unheated cupboard not 
exposed to sunlight, on the north side of the house with indoor patch GPS 
antennas.


 http://www.satsignal.eu/mrtg/performance_ntp.php

Cheers,
David
--
SatSignal Software - Quality software written to your requirements
Web: http://www.satsignal.eu
Email: david-tay...@blueyonder.co.uk
Twitter: @gm8arv 


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[time-nuts] Holdover, RTC for Pi as NTP GPS source

[Mark Sims]

I have an analytical balance that reads down to micrograms.   The weigh chamber 
is surrounded by three layers of IR absorbing glass so that radiated body heat 
does not induce convection currents in the air.   I worked on a balance that 
had nanogram resolution (mostly wishful thinking in that spec).  It operated in 
a vacuum.  30 bit mass-to-digital converters are rather finicky beasties.

It does not take all that good of a temperature sensor to detect changes in 
room temperature due to body heat (or fetching a beer from the fridge in the 
next room).  You are basically a 100 watt space heater... even larger for the 
more corpulent folks.   



> Surely he'd want to be in an isolating suit to avoid introducing a nasty
warm body into his nice stable cave ?
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Re: [time-nuts] Designing an embedded precision GPS time

[Leo Bodnar]

> From: Attila Kinali 
> What you should do is basically system identification and adaptive control. 

Thanks for your advice, Attila, I am going to google this tonight.  This is 
exciting!

Leo
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[William H. Fite]

Oh crap, I forgot about neutrinos.

On Wednesday, November 1, 2017, William H. Fite  wrote:

> Inside a three-meter thick sphere of lead and mu-metal, pressure pulled
> down to .01 mTorr , temperature regulated to .001 degree C,
> operating in total darkness and absolute silence, aligned with the galactic
> axis of rotation, and situated 20 light years from the nearest star.
>
> That should satisfy even the most obsessive TN.
>
>
>
> On Wednesday, November 1, 2017, Magnus Danielson <
> mag...@rubidium.dyndns.org
> > wrote:
>
>> Hi,
>>
>> On 11/01/2017 04:03 PM, jimlux wrote:
>>
>>> On 11/1/17 7:37 AM, Magnus Danielson wrote:
>>>
 Hi,

   Silly people
>>>
 want a relative comfortable temperature and well, building A/C is
 typically bang/bang regulated so you get what you paid for.


>>> wouldn't a true time-nut be in a basically isothermal cave at 10C far
>>> underground, and just follow the classic mother's advice "put on a sweater
>>> if you feel cold"
>>>
>>
>> None of the NMI labs I've seen does this, but please, go ahead.
>>
>> Several of them was just underground, but not that cold.
>>
>> The two labs that where underground was relatively comfortable
>> temperatures. :)
>>
>> Cheers,
>> Magnus
>> ___
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>> ailman/listinfo/time-nuts
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>>
>
>
> --
> Patriotism is supporting your country all the time, and your government
> when it deserves it.
> --Mark Twain
>
> There is no one thing that is true. They're all true.
> --Ernest Hemingway
>
> We may be surprised at the people we find in heaven. God has a soft spot
> for sinners. His standards are quite low.
> --Desmond Tutu
>
>

-- 
Patriotism is supporting your country all the time, and your government
when it deserves it.
--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft spot
for sinners. His standards are quite low.
--Desmond Tutu
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Re: [time-nuts] Designing an embedded precision GPS time

[MLewis]

Antenna is a Tallysman TW4722.

I'll try a different antenna later. If nothing else, right now I've got regular 
dropouts for failsafe testing...


But I feel a better solution is a reliable holdover capability, which I should 
have anyway for failsafe.


Thanks,

Michael


On 01/11/2017 10:22 AM, Bob kb8tq wrote:

Hi

Ok, local RF interference sounds like a significant part of the problem. I would
suggest that swapping antennas might make sense. Not all “super interference
rejecting” antennas are created equal.

Bob


On Nov 1, 2017, at 9:55 AM, MLewis  wrote:



I'm also too close to that tall building that is reflecting the sats over the Bering 
Strait at me. It's a military computer site, which I thought would be pretty tight on 
stray RF, but it has antennas. I asked a friend who works there about my GPS issues and 
if RF from the site may be influencing things. He hesitated, then said "'Yes'. 
That's all I can say."
For first power up I had obtained an active antenna for multi-constellation and a 
pre-filter that "provides protection from near  frequency or strong harmonic 
interfering signals."


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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[William H. Fite]

Inside a three-meter thick sphere of lead and mu-metal, pressure pulled
down to .01 mTorr , temperature regulated to .001 degree C,
operating in total darkness and absolute silence, aligned with the galactic
axis of rotation, and situated 20 light years from the nearest star.

That should satisfy even the most obsessive TN.



On Wednesday, November 1, 2017, Magnus Danielson 
wrote:

> Hi,
>
> On 11/01/2017 04:03 PM, jimlux wrote:
>
>> On 11/1/17 7:37 AM, Magnus Danielson wrote:
>>
>>> Hi,
>>>
>>>   Silly people
>>
>>> want a relative comfortable temperature and well, building A/C is
>>> typically bang/bang regulated so you get what you paid for.
>>>
>>>
>> wouldn't a true time-nut be in a basically isothermal cave at 10C far
>> underground, and just follow the classic mother's advice "put on a sweater
>> if you feel cold"
>>
>
> None of the NMI labs I've seen does this, but please, go ahead.
>
> Several of them was just underground, but not that cold.
>
> The two labs that where underground was relatively comfortable
> temperatures. :)
>
> Cheers,
> Magnus
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-- 
Patriotism is supporting your country all the time, and your government
when it deserves it.
--Mark Twain

There is no one thing that is true. They're all true.
--Ernest Hemingway

We may be surprised at the people we find in heaven. God has a soft spot
for sinners. His standards are quite low.
--Desmond Tutu
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Re: [time-nuts] Designing an embedded precision GPS time

[Bob Martin]

One can minimize temperature change which affects things like 
regulated voltages, CMOS transition points for those converting

sine to TTL with gates, etc. in addition to the oscillators.

The November issue of Nuts and Volts Magazine has an article on an
Arduino based PID controller which might interest someone who wants 
to experiment with reducing temperature effects by controlling 
temperature.


Bob M

On 10/31/2017 8:42 PM, jimlux wrote:

On 10/31/17 1:47 PM, Bob kb8tq wrote:

HI

TCXO is a very loosely defined term. A part that does +/- 5 ppm 
-40 to +85C
is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be 
a TCXO.


Dividing the total deviation of either one by the temperature 
range to come

up with a “delta frequency per degree” number would be a mistake. You
would get a number that is much better than the real part exhibits.

Working all this back into a holdover spec in an unknown temperature
environment is not at all easy.



Very much so - most of the TCXO curves I've seen tend to be "much" 
better than the spec over the central part of the frequency range 
(which makes sense, the underlying crystal is a cubic with temp, 
most likely)


Retrace and hysteresis might be your dominant uncertainty.
I've attached a typical TCXO data plot for your viewing pleasure..
(that's an expensive oscillator, because it's for space, but I don't 
think space or not changes the underlying performance)




Bob





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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Magnus Danielson]

Hi,

On 11/01/2017 04:03 PM, jimlux wrote:

On 11/1/17 7:37 AM, Magnus Danielson wrote:

Hi,


  Silly people
want a relative comfortable temperature and well, building A/C is 
typically bang/bang regulated so you get what you paid for.




wouldn't a true time-nut be in a basically isothermal cave at 10C far 
underground, and just follow the classic mother's advice "put on a 
sweater if you feel cold"


None of the NMI labs I've seen does this, but please, go ahead.

Several of them was just underground, but not that cold.

The two labs that where underground was relatively comfortable 
temperatures. :)


Cheers,
Magnus
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Adrian Godwin]

Surely he'd want to be in an isolating suit to avoid introducing a nasty
warm body into his nice stable cave ?


On Wed, Nov 1, 2017 at 3:03 PM, jimlux  wrote:

> On 11/1/17 7:37 AM, Magnus Danielson wrote:
>
>> Hi,
>>
>>  Silly people
>
>> want a relative comfortable temperature and well, building A/C is
>> typically bang/bang regulated so you get what you paid for.
>>
>>
>
>
> wouldn't a true time-nut be in a basically isothermal cave at 10C far
> underground, and just follow the classic mother's advice "put on a sweater
> if you feel cold"
>
>
>
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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[jimlux]

On 11/1/17 7:37 AM, Magnus Danielson wrote:

Hi,


 Silly people
want a relative comfortable temperature and well, building A/C is 
typically bang/bang regulated so you get what you paid for.






wouldn't a true time-nut be in a basically isothermal cave at 10C far 
underground, and just follow the classic mother's advice "put on a 
sweater if you feel cold"



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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Magnus Danielson]

Hi,

This is also true for labs. One NMI lab I visited had issues with their 
5071As and H-maser clocks. It took some time to correlate it, but it 
turned out to correlate with the florescent lamps in the lab and people 
walking in and out. Changing these lamps to milder labs removed that 
disturbance, so people needing things like light can be a problem even 
for higher end grade labs.


The draft has been a re-occurring problem that we have seen, so it was 
really nice to show that simple blockage of air helped to significantly 
reduce that impact. Again, people walking up to the lab-bench was an issue.


A third example was a couple of students doing stability measures. They 
had a long-term measurement that all of a sudden became much flatter, 
and they asked me what it could be. They had started the measurement at 
15:00 and three hours late the semi-cyclic variation in phase stopped 
and things became much more quiet. I could then quickly conclude that it 
was the A/C for the building turned off, not to heat it during night. 
They didn't believe me, so I popped out the board and showed how the 
air-stream passed directly over the crystal metal housing and hence had 
a good thermal connection to the surrounding air temperature. I later 
had them put a few strands of foam-tape over the crystal, and they 
measured the same stable behavior over the full work-day. Silly people 
want a relative comfortable temperature and well, building A/C is 
typically bang/bang regulated so you get what you paid for.


Cheers,
Magnus

On 11/01/2017 02:17 PM, Bob kb8tq wrote:

Hi



On Nov 1, 2017, at 12:14 AM, MLewis  wrote:

(I suspect this is drifting from the original thread too much, so new subject)

Temperature ranges from 65F to 78F, with the potential for drafts, but is more 
typically 76F.


The gotcha in a real environment often involves people. They walk by (creating 
a draft). They
turn on all the lights and equipment. They open or close the blinds to let in 
or block the sun.
They tend to do this in an unpredictable / chaotic fashion. All of this makes a 
correction
process based on “normal operation” a bit difficult. Something goes wrong, and 
the unit
goes into holdover. People suddenly start dashing around and the temperature is 
not
what it has been ….

Bob




I read about the NTPsec runs with insulating a Pi and running a load generating 
program to better maintain a stable core temperature.
Just today I've put my GPS module inside a case for an RF shield that is also 
semi insulated. It's feeding LH on a PC while I do the next step.
The Pi 3 is going inside a large enough tea tin and that will be lined with 
insulation.
I'm wondering about insulating the RTC...

The low cost for a 'precision' RTC means it is cheap to test.

I'd completely discounted coasting with the system clock, as I have fixed in my 
head the variable loads on my production machine mean that Window's time lags 
variable amounts, as the CPU load is variable with variable burst loads every 
1/8 of a second.

Michael

On 31/10/2017 11:45 PM, Hal Murray wrote:

I'm intending to add a "precision" (well, precision to the Pi world) RTC  to
my Pi 3 to use for a holdover source when it hasn't got PPS from the  GPS
module.
An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one  to
20 minutes.

Run some experiments to collect some data and play with the numbers.

How stable is the temperature in your environment?

The key to keeping sane time on a PC or Raspberry PI is to calibrate the
crystal.  Most CPUs have a register that counts at the CPU clock frequency -
or something in that range.  Most systems smear the clock to keep the FCC
happy...

Most OSes keep time by watching that register and dividing by the clock rate.
  The actual clock rate doesn't usually match the number printed on the
crystal.  It's close, but ntpd can easily measure the error and tell the
kernel so the kernel can use the right value.  If you turn on loopstats, ntpd
will log it and you can graph it.

If you are writing an embedded system, you will want that sort of logic too.

My guess is that in the under 30 minute range, you will get better results by
just coasting with the system clock rather that using a RTC.  It would be an
interesting experiment.  Implement both clocking schemes and compare them.







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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

Ok, local RF interference sounds like a significant part of the problem. I 
would 
suggest that swapping antennas might make sense. Not all “super interference
rejecting” antennas are created equal. 

Bob

> On Nov 1, 2017, at 9:55 AM, MLewis  wrote:
> 
> I wish.
> 
> It's using GLO and GPS now, yet gets reception dropouts.
> That's why I'm hoping to eventually get the firmware update that will add GAL 
> to the mix.
> 
> I had anticipated reception issues, which is why I went with the M8T for its 
> sensitivity, multi-constellation and it's a timing module so a good PPS on a 
> single sat - only to get surprised that my version didn't have GAL enabled. 
> But I didn't envision reception would be so bad that not having GAL would be 
> material.
> 
> I'm also too close to that tall building that is reflecting the sats over the 
> Bering Strait at me. It's a military computer site, which I thought would be 
> pretty tight on stray RF, but it has antennas. I asked a friend who works 
> there about my GPS issues and if RF from the site may be influencing things. 
> He hesitated, then said "'Yes'. That's all I can say."
> For first power up I had obtained an active antenna for multi-constellation 
> and a pre-filter that "provides protection from near  frequency or strong 
> harmonic interfering signals."
> 
> As it's just for NTP accuracy, I may be better off letting all the multipath 
> through than getting dropouts. I'm starting to think that part of my problem 
> is that I know the GPS is capable of getting better than I need, so instead 
> of working to get what I need, I want what it should be capable of.
> 
> But I feel a better solution is a reliable holdover capability, which I 
> should have anyway for failsafe. So perhaps the reception dropouts are a way 
> to make me address holdover properly rather than limp in and get surprised 
> later.
> 
> Hence adding a 'precision' RTC to the Pi.
> 
> Michael
> 
> On 01/11/2017 8:45 AM, Bob kb8tq wrote:
>> Hi
>> 
>> For NTP levels of accuracy Glonas is quite fine. Combining that with GPS 
>> should
>> get you a pretty good “time source” even under your extreme conditions.
>> 
>> Bob
>>> On Oct 31, 2017, at 11:14 PM, MLewis  wrote:
>>> 
>>> I'm stuck with a near ground level antenna site (~16" above grade?), with 
>>> half a sky view (thankfully to the SSE), less some low blocking buildings 
>>> with regular mutlipath, plus multipath bouncing off a taller building to 
>>> the SE that bounces sats from the NW at me from low over the Bering Strait. 
>>> The building I'm in is concrete with flat steel under each floor from the 
>>> construction method. As I write this I'm down to two green sats in LH.
>>> 
>>> A number of times a day, it will drop to one sat, and there's a few 
>>> dropouts a day where it goes to none of sufficient signal. How many times 
>>> and for how long varies by the day. It's worse when it's wet out, which it 
>>> is right now. If I lower the signal strength threshold, then I end up with 
>>> tons of multipath signals.
>>> 
>>> If I can ever get a bios update to my NEO-M8T, then I'll have GAL in the 
>>> mix and should experience fewer dropouts, potentially none.
>>> 
>>> An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one to 
>>> 20 minutes.
>>> 
>>> While I wrote this, LH was typically showing two or three green sats, once 
>>> up to five and once down to one. And I just hit a dropout... for a minute 
>>> and a half; the one remaining green sat went behind the corner of the 
>>> building's entrance canopy, then back out.
>>> 
>>> 
>>> On 31/10/2017 10:30 PM, Bob kb8tq wrote:
 Hi
 
 Under what conditions would you expect to loose GPS? I seem to be able to
 do just fine sitting in an armchair here in the family room. That’s hardly 
 a
 fancy setup.
 
 Bob
 
> 
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Re: [time-nuts] Designing an embedded precision GPS time

[MLewis]

I wish.

It's using GLO and GPS now, yet gets reception dropouts.
That's why I'm hoping to eventually get the firmware update that will 
add GAL to the mix.


I had anticipated reception issues, which is why I went with the M8T for 
its sensitivity, multi-constellation and it's a timing module so a good 
PPS on a single sat - only to get surprised that my version didn't have 
GAL enabled. But I didn't envision reception would be so bad that not 
having GAL would be material.


I'm also too close to that tall building that is reflecting the sats 
over the Bering Strait at me. It's a military computer site, which I 
thought would be pretty tight on stray RF, but it has antennas. I asked 
a friend who works there about my GPS issues and if RF from the site may 
be influencing things. He hesitated, then said "'Yes'. That's all I can 
say."
For first power up I had obtained an active antenna for 
multi-constellation and a pre-filter that "provides protection from 
near  frequency or strong harmonic interfering signals."


As it's just for NTP accuracy, I may be better off letting all the 
multipath through than getting dropouts. I'm starting to think that part 
of my problem is that I know the GPS is capable of getting better than I 
need, so instead of working to get what I need, I want what it should be 
capable of.


But I feel a better solution is a reliable holdover capability, which I 
should have anyway for failsafe. So perhaps the reception dropouts are a 
way to make me address holdover properly rather than limp in and get 
surprised later.


Hence adding a 'precision' RTC to the Pi.

Michael

On 01/11/2017 8:45 AM, Bob kb8tq wrote:

Hi

For NTP levels of accuracy Glonas is quite fine. Combining that with GPS should
get you a pretty good “time source” even under your extreme conditions.

Bob

On Oct 31, 2017, at 11:14 PM, MLewis  wrote:

I'm stuck with a near ground level antenna site (~16" above grade?), with half 
a sky view (thankfully to the SSE), less some low blocking buildings with regular 
mutlipath, plus multipath bouncing off a taller building to the SE that bounces sats 
from the NW at me from low over the Bering Strait. The building I'm in is concrete 
with flat steel under each floor from the construction method. As I write this I'm 
down to two green sats in LH.

A number of times a day, it will drop to one sat, and there's a few dropouts a 
day where it goes to none of sufficient signal. How many times and for how long 
varies by the day. It's worse when it's wet out, which it is right now. If I 
lower the signal strength threshold, then I end up with tons of multipath 
signals.

If I can ever get a bios update to my NEO-M8T, then I'll have GAL in the mix 
and should experience fewer dropouts, potentially none.

An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one to 20 
minutes.

While I wrote this, LH was typically showing two or three green sats, once up 
to five and once down to one. And I just hit a dropout... for a minute and a 
half; the one remaining green sat went behind the corner of the building's 
entrance canopy, then back out.


On 31/10/2017 10:30 PM, Bob kb8tq wrote:

Hi

Under what conditions would you expect to loose GPS? I seem to be able to
do just fine sitting in an armchair here in the family room. That’s hardly a
fancy setup.

Bob



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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

> On Nov 1, 2017, at 9:17 AM, jimlux  wrote:
> 
> On 11/1/17 6:01 AM, Bob kb8tq wrote:
>> Hi
>> Unfortunately not all TCXO’s are created equal. It depends a bit on the
>> original intended use. I’d bet it also depends a bit on the original target
>> price. Perturbations  (frequency jumps) over temperature are one “feature”
>> that might be present. Hysteresis at half the temperature spec is another
>> “feature”.
>> Even within the same batch or same test run, some will be much better
>> than others. You stop the compensation process when they get “good enough”.
>> That will mean that a few are right at whatever the production target is and
>> others exceed the target by quite a bit.
>> While crystal curves are indeed cubic, there are higher order terms in the
>> curve. The “why” is something people get to write papers on.If you are trying
>> to compensate to tight specs, you will see all sorts of stuff. It is not at 
>> all uncommon
>> to see >9th order curves residual curves. Indeed some of that is from 
>> residuals
>> in the compensation circuit as well as from the crystal.
>> Why all this yack? A lot of people come from a background using OCXO’s. An
>> OCXO generally has a low order temperature characteristic. It also is rare 
>> to see
>> things like frequency perturbations in an OCXO. Moving from one to the other
>> can be a bit interesting.
> 
> Indeed - I was looking at algorithmically compensating some cheap TCXOs and 
> there's an amazing spread in the "details" of the curves - sure, they all met 
> the spec (several ppm, as I recall), but it was clear after very little 
> testing that there was no "one algorithm to fit them all"
> 
> As you say, good grist for a paper or thesis project.
> 
> That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe look 
> for regular XO (no TC).  Those might have a more "pure" (read lower order) 
> freq vs temp characteristic.

The issue there is that the crystal is where the money is. The oven circuit is 
actually pretty far 
down the list cost wise. Poke at this spec, poke at that spec and you have a 
$50 crystal (in volume). 


> 
> The problem I see with regular XO is that they tend to be designed to a cost 
> point and there might be more of the hysteresis and mechanical effects - if 
> you're not claiming better than 100ppm, then 50 ppm of hysteresis isn't a 
> problem.
> 
> A 1ppb OCXO is going to have to be a better mechanical design - so that it 
> can hit that 1ppb every time when you turn the oven on and go from cold to 
> hot.

It very much has a crystal that spent more time on the production line 
(somewhere) being processed than 
it’s lower spec cousins. Time is money and that equipment isn’t cheap either. 
There then is the minor 
issue of yield. Toss in a more expensive package while you are at it ….

> 
> 
> Maybe this is just griping in general - why don't mass production 
> manufacturers make exactly the niche part I want to buy (that is of no use to 
> anyone else)for $3 each
> 
> I suppose if you were going to build little algorithmically compensated 
> modules, you'd bite the bullet and design a crystal oscillator and then YOU 
> get to choose what crystal in what mount etc.

If you are buying a few million of this or that a month, you most certainly can 
get people’s attention. Toss 
in a willingness to pay a few dozen bucks a piece on top of that and you will 
get a *lot* of people’s attention. 

Crystals are made and sold with characteristic data on them. The same is true 
of just about any type of
oscillator. Getting to the point that the data is *useful* takes a lot of 
engineering on both ends of the process. 
There are a number of companies that have set up to do the characterization 
once the device is in the
end end product. To a great extent that gets done to speed up the engineering 
process ….


> 
> When all is said and done, the production cost for a design that uses a 
> crystal in a can plus half a dozen discrete devices to make an oscillator is 
> probably not a lot different than the production cost for a design using an 
> oscillator in a can.  it's the "other stuff" in the design that will add up.

If the crystal is at some odd frequency or in an odd package … be careful. 
Experience counts in terms of making a good crystal. 
Experience at 5.00 MHZ in an HC-40 is unfortunately not the same as 
experience at 5.05 MHz in the same package. I have a lot
of data on this ….

Bob


> 
> 
> 
> 
> 
> 
> 
> 
> 
>> Bob
>>> On Oct 31, 2017, at 10:42 PM, jimlux  wrote:
>>> 
>>> On 10/31/17 1:47 PM, Bob kb8tq wrote:
 HI
 TCXO is a very loosely defined term. A part that does +/- 5 ppm -40 to +85C
 is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be a TCXO.
 Dividing the total deviation of either one by the temperature range to come
 up with a “delta frequency per degree” number would be a mistake. You
 would get a number that is 

Re: [time-nuts] Designing an embedded precision GPS time

[jimlux]

On 11/1/17 6:01 AM, Bob kb8tq wrote:

Hi

Unfortunately not all TCXO’s are created equal. It depends a bit on the
original intended use. I’d bet it also depends a bit on the original target
price. Perturbations  (frequency jumps) over temperature are one “feature”
that might be present. Hysteresis at half the temperature spec is another
“feature”.

Even within the same batch or same test run, some will be much better
than others. You stop the compensation process when they get “good enough”.
That will mean that a few are right at whatever the production target is and
others exceed the target by quite a bit.

While crystal curves are indeed cubic, there are higher order terms in the
curve. The “why” is something people get to write papers on.If you are trying
to compensate to tight specs, you will see all sorts of stuff. It is not at all 
uncommon
to see >9th order curves residual curves. Indeed some of that is from residuals
in the compensation circuit as well as from the crystal.

Why all this yack? A lot of people come from a background using OCXO’s. An
OCXO generally has a low order temperature characteristic. It also is rare to 
see
things like frequency perturbations in an OCXO. Moving from one to the other
can be a bit interesting.



Indeed - I was looking at algorithmically compensating some cheap TCXOs 
and there's an amazing spread in the "details" of the curves - sure, 
they all met the spec (several ppm, as I recall), but it was clear after 
very little testing that there was no "one algorithm to fit them all"


As you say, good grist for a paper or thesis project.

That's why I wish they'd sell OCXOs, cheap, without the oven. Or maybe 
look for regular XO (no TC).  Those might have a more "pure" (read lower 
order) freq vs temp characteristic.


The problem I see with regular XO is that they tend to be designed to a 
cost point and there might be more of the hysteresis and mechanical 
effects - if you're not claiming better than 100ppm, then 50 ppm of 
hysteresis isn't a problem.


A 1ppb OCXO is going to have to be a better mechanical design - so that 
it can hit that 1ppb every time when you turn the oven on and go from 
cold to hot.



Maybe this is just griping in general - why don't mass production 
manufacturers make exactly the niche part I want to buy (that is of no 
use to anyone else)for $3 each


I suppose if you were going to build little algorithmically compensated 
modules, you'd bite the bullet and design a crystal oscillator and then 
YOU get to choose what crystal in what mount etc.


When all is said and done, the production cost for a design that uses a 
crystal in a can plus half a dozen discrete devices to make an 
oscillator is probably not a lot different than the production cost for 
a design using an oscillator in a can.  it's the "other stuff" in the 
design that will add up.











Bob




On Oct 31, 2017, at 10:42 PM, jimlux  wrote:

On 10/31/17 1:47 PM, Bob kb8tq wrote:

HI
TCXO is a very loosely defined term. A part that does +/- 5 ppm -40 to +85C
is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be a TCXO.
Dividing the total deviation of either one by the temperature range to come
up with a “delta frequency per degree” number would be a mistake. You
would get a number that is much better than the real part exhibits.
Working all this back into a holdover spec in an unknown temperature
environment is not at all easy.


Very much so - most of the TCXO curves I've seen tend to be "much" better than 
the spec over the central part of the frequency range (which makes sense, the underlying 
crystal is a cubic with temp, most likely)

Retrace and hysteresis might be your dominant uncertainty.
I've attached a typical TCXO data plot for your viewing pleasure..
(that's an expensive oscillator, because it's for space, but I don't think 
space or not changes the underlying performance)



Bob

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Re: [time-nuts] Holdover, RTC for Pi as NTP GPS source

[Bob kb8tq]

Hi


> On Nov 1, 2017, at 12:14 AM, MLewis  wrote:
> 
> (I suspect this is drifting from the original thread too much, so new subject)
> 
> Temperature ranges from 65F to 78F, with the potential for drafts, but is 
> more typically 76F.

The gotcha in a real environment often involves people. They walk by (creating 
a draft). They 
turn on all the lights and equipment. They open or close the blinds to let in 
or block the sun. 
They tend to do this in an unpredictable / chaotic fashion. All of this makes a 
correction 
process based on “normal operation” a bit difficult. Something goes wrong, and 
the unit
goes into holdover. People suddenly start dashing around and the temperature is 
not 
what it has been ….

Bob


> 
> I read about the NTPsec runs with insulating a Pi and running a load 
> generating program to better maintain a stable core temperature.
> Just today I've put my GPS module inside a case for an RF shield that is also 
> semi insulated. It's feeding LH on a PC while I do the next step.
> The Pi 3 is going inside a large enough tea tin and that will be lined with 
> insulation.
> I'm wondering about insulating the RTC...
> 
> The low cost for a 'precision' RTC means it is cheap to test.
> 
> I'd completely discounted coasting with the system clock, as I have fixed in 
> my head the variable loads on my production machine mean that Window's time 
> lags variable amounts, as the CPU load is variable with variable burst loads 
> every 1/8 of a second.
> 
> Michael
> 
> On 31/10/2017 11:45 PM, Hal Murray wrote:
>>> I'm intending to add a "precision" (well, precision to the Pi world) RTC  to
>>> my Pi 3 to use for a holdover source when it hasn't got PPS from the  GPS
>>> module.
>>> An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one  to
>>> 20 minutes.
>> Run some experiments to collect some data and play with the numbers.
>> 
>> How stable is the temperature in your environment?
>> 
>> The key to keeping sane time on a PC or Raspberry PI is to calibrate the
>> crystal.  Most CPUs have a register that counts at the CPU clock frequency -
>> or something in that range.  Most systems smear the clock to keep the FCC
>> happy...
>> 
>> Most OSes keep time by watching that register and dividing by the clock rate.
>>  The actual clock rate doesn't usually match the number printed on the
>> crystal.  It's close, but ntpd can easily measure the error and tell the
>> kernel so the kernel can use the right value.  If you turn on loopstats, ntpd
>> will log it and you can graph it.
>> 
>> If you are writing an embedded system, you will want that sort of logic too.
>> 
>> My guess is that in the under 30 minute range, you will get better results by
>> just coasting with the system clock rather that using a RTC.  It would be an
>> interesting experiment.  Implement both clocking schemes and compare them.
>> 
>> 
>> 
>> 
>> 
> 
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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

> On Oct 31, 2017, at 11:33 PM, Leo Bodnar  wrote:
> 
>> From: Bob kb8tq 
>> Working all this back into a holdover spec in an unknown temperature 
>> environment is not at all easy.
>> Bob
> 
> 
> This is true, it is too easy to multiply figures from the datasheet and then 
> start believing in them.
> 
> We did extensive testing of real units in real life before committing to any 
> specification figures.  They are based on statistical measurements followed 
> by an expanded safety margin.
> Here is a typical holdover offset curve over 24 hours in non-DC environment 
> (i.e. 5-10 degrees ambient temperature change during day/night period.)
> 
> http://leobodnar.com/balloons/NTP/24hr-holdover.png 
> 

Looking at the data, the DUT did some sort of discrete frequency shift around 
4,000 seconds. The rest of the time it 
plodded along do nothing much ( = it was very stable). None of that is terribly 
unusual in terms of a holdover plot. 
The nasty question that always gets asked is “what if shift happened earlier?”. 
Depending on the test profile, that
may be unlikely or …. . Doing a lot of testing is about the only way to sort 
things out. 


> 
> Time drift over 24h on this particular unit was below 0.7ms. This is pretty 
> good for the device that consumes 1W of power (via PoE or USB) and fits in 
> the pocket.
> 
> I have used typical Raspberry Pi with a GPS add-on run-of-the-mill timeserver 
> as suggested by Attila to monitor relative offsets - this is why reported 
> timing is jittery and local (to RPi) 1PPS has an offset.
> 
> It is really puzzling why holdover has suddenly come into focus.

This is TimeNuts ….


>  Due to NTP redundancy feature it is trivial to put several inexpensive time 
> servers around the local or campus network and let clients do the standard 
> NTP sanity checking and server selection.  And those building an NTP system 
> able to cope with 24h+ global GPS outage know what they are doing anyway.

Based on some other posts, it appears that some of the applications are in 
*very* unusual environments. They are 
far more outage prone than one would normally expect. 

Bob

> 
> Leo
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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

Unfortunately not all TCXO’s are created equal. It depends a bit on the 
original intended use. I’d bet it also depends a bit on the original target
price. Perturbations  (frequency jumps) over temperature are one “feature”
that might be present. Hysteresis at half the temperature spec is another
“feature”. 

Even within the same batch or same test run, some will be much better 
than others. You stop the compensation process when they get “good enough”. 
That will mean that a few are right at whatever the production target is and
others exceed the target by quite a bit. 

While crystal curves are indeed cubic, there are higher order terms in the
curve. The “why” is something people get to write papers on.If you are trying
to compensate to tight specs, you will see all sorts of stuff. It is not at all 
uncommon
to see >9th order curves residual curves. Indeed some of that is from residuals
in the compensation circuit as well as from the crystal. 

Why all this yack? A lot of people come from a background using OCXO’s. An
OCXO generally has a low order temperature characteristic. It also is rare to 
see
things like frequency perturbations in an OCXO. Moving from one to the other
can be a bit interesting. 

Bob



> On Oct 31, 2017, at 10:42 PM, jimlux  wrote:
> 
> On 10/31/17 1:47 PM, Bob kb8tq wrote:
>> HI
>> TCXO is a very loosely defined term. A part that does +/- 5 ppm -40 to +85C
>> is a TCXO. A part that does +/- 5x10^-9 over 0 to 50C may also be a TCXO.
>> Dividing the total deviation of either one by the temperature range to come
>> up with a “delta frequency per degree” number would be a mistake. You
>> would get a number that is much better than the real part exhibits.
>> Working all this back into a holdover spec in an unknown temperature
>> environment is not at all easy.
> 
> Very much so - most of the TCXO curves I've seen tend to be "much" better 
> than the spec over the central part of the frequency range (which makes 
> sense, the underlying crystal is a cubic with temp, most likely)
> 
> Retrace and hysteresis might be your dominant uncertainty.
> I've attached a typical TCXO data plot for your viewing pleasure..
> (that's an expensive oscillator, because it's for space, but I don't think 
> space or not changes the underlying performance)
> 
> 
>> Bob
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Re: [time-nuts] Designing an embedded precision GPS time

[Bob kb8tq]

Hi

For NTP levels of accuracy Glonas is quite fine. Combining that with GPS should
get you a pretty good “time source” even under your extreme conditions.

Bob 
> On Oct 31, 2017, at 11:14 PM, MLewis  wrote:
> 
> I'm stuck with a near ground level antenna site (~16" above grade?), with 
> half a sky view (thankfully to the SSE), less some low blocking buildings 
> with regular mutlipath, plus multipath bouncing off a taller building to the 
> SE that bounces sats from the NW at me from low over the Bering Strait. The 
> building I'm in is concrete with flat steel under each floor from the 
> construction method. As I write this I'm down to two green sats in LH.
> 
> A number of times a day, it will drop to one sat, and there's a few dropouts 
> a day where it goes to none of sufficient signal. How many times and for how 
> long varies by the day. It's worse when it's wet out, which it is right now. 
> If I lower the signal strength threshold, then I end up with tons of 
> multipath signals.
> 
> If I can ever get a bios update to my NEO-M8T, then I'll have GAL in the mix 
> and should experience fewer dropouts, potentially none.
> 
> An RTC that +/- 3 PPM over 24 hours would be great for holdovers of one to 20 
> minutes.
> 
> While I wrote this, LH was typically showing two or three green sats, once up 
> to five and once down to one. And I just hit a dropout... for a minute and a 
> half; the one remaining green sat went behind the corner of the building's 
> entrance canopy, then back out.
> 
> 
> On 31/10/2017 10:30 PM, Bob kb8tq wrote:
>> Hi
>> 
>> Under what conditions would you expect to loose GPS? I seem to be able to
>> do just fine sitting in an armchair here in the family room. That’s hardly a
>> fancy setup.
>> 
>> Bob
>> 
>>> On Oct 31, 2017, at 10:27 PM, MLewis  wrote:
>>> 
>>> I'm intending to add a "precision" (well, precision to the Pi world) RTC to 
>>> my Pi 3 to use for a holdover source when it hasn't got PPS from the GPS 
>>> module.
>>> 
>>> On 31/10/2017 10:04 PM, Chris Caudle wrote:
 On Tue, October 31, 2017 7:19 pm, MLewis wrote:
> ...the "better" quality RTCs seem to be DS3231 based
> How does one translate that into an expected 24 hour holdover?
 For the RTC, or for an NTP server?  If the NTP server is running it will
 not make a difference, modern operating systems do not use the RTC for the
 system clock, only to get close to the correct time at startup.
 
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Re: [time-nuts] Designing an embedded precision GPS time

[Attila Kinali]

On Tue, 31 Oct 2017 22:13:01 -0700
Denny Page  wrote:

> Depends upon the results you are trying to achieve. Using Linux pretty
> much guarantees that your server clock will be off by 6-10us, with
> substantial variance. Even with a good nic that supports hardware
> timestamping, the variance will increase substantially as you go off box
> (spread spectrum is a big annoyance!).

6-10µs is the interrupt latency of linux on ARM SoC. I guess, to get
below that you'd have to tweak the kernel a bit. Which should not
be that difficult. Definitly simpler than writing your own IP and NTP
stack from scratch.

Spread spectrum can usually be switched off, though requires at least a
custom DTB or even patching of the kernel. There are a few boards, though
that do not allow spread spectrum to be switched off.

Attila Kinali


-- 
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the prosperity and technological sophistication in the world is of no 
use without that foundation.
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Re: [time-nuts] Designing an embedded precision GPS time

[Attila Kinali]

On Wed, 1 Nov 2017 04:06:06 +
Leo Bodnar  wrote:

> > From: Attila Kinali 
> > Basically, all you have to do is use an SBC that runs linux and has
> > a GPIO with an interrupt to act as a PPS input. Attach a GPS receiver
> > and you are almost done. The cheapest option are probably the i.MX233
> > based ones (go as low as €20). 
> 
> Thank you, Attila, this sounds like the way to go - perhaps I can
> repackage this solution in a smart attractive enclosure and market 
> it as a high performance product.  
> I was a bit behind the curve on recent developments - do you have a
> suggestion for the best linux running SBC and cheap GPS suitable for this?

Depending on your expertise and the volume you expect, I would probably
build my own board. Select a SoC that has fast 32bit timers so you
can accurately measure the PPS. The OSD3358 I mentioned is a good
compromise IMHO, as it allows you to build upon the Beaglebone community,
without having to deal with a complex board design. And the PRU allows
you to sample with 5ns precision. A simple interpolation like what
Nick Sayer did would also be a good idea, IMHO.


> > You should have a control loop somewhere, which explicitly or implicitly 
> > estimates the frequency of the TCXO. 
> > The time-nuts archives are full with discussions how to do such
> > control loops and improve hold over performance. Though there
> > weren't many in the last 2-3 years. John Vigs tutorial is also
> > a good start.
> 
> OK, so I need to introduce additional TCXO and a control loop to improve the 
> holdover performance?

Not an additional TCXO, but model its behaviour. What you should do
is basically system identification and adaptive control. The most
common way to do that is a Kalman filter. Though Marek Peka wrote 
a paper on the problems of Kalman filters for clock modeling
(mostly stability issues of the predicition) and presented it at
IFCS/EFTF in Prague in 2013.

> It is really puzzling why holdover has suddenly come into focus.  Due to
> NTP redundancy feature it is trivial to put several inexpensive time servers
> around the local or campus network and let clients do the standard NTP sanity
> checking and server selection.  And those building an NTP system able to cope
> with 24h+ global GPS outage know what they are doing anyway.

Well, that was me guessing what your goal was. Seems like I was off.


Attila Kinali


-- 
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the prosperity and technological sophistication in the world is of no 
use without that foundation.
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[time-nuts] Trimble UCCM weird behaviour

[Attila Kinali]

Hi,

I recently bought a Trimble UCCM from ebay and it shows a rather strange
behaviour. Beside the unit having a low sensitivity (the LEA-5 I have
considers satellites having a good C/N0 where the trimble cannot even
track them) it does not seem to properly lock. Periodically it steers
the DAC output to 0, then about half a minute later full down to the
negative maximum, another 15s later full up to the positive maximum.
It cycles this way for 2 to 6 times before settling again to "normal"
behaviour. As you can see from the first picture, the PPS offset
jumps up by multiple microseconds during those cycles. This behviour
seems to be related to how many satellites it sees, during times when
it has view of 5 satellites for long stretches, it seems to work fine,
but dropping down to 4 or less and it behaves crazy.

I first thought it was due to my GPS antenna (a cheap puck from ebay)
having too low signal levels, so I added an LNA4ALL[1] which uses
a MiniCircuits PSA4-5043+, to give it another 15dB of gain. It did
not really help.

What also puzzles me a bit is, that the UCCM seems to lose track of
satellites where it has clear view of them and thus strong signal.
Signal levels per satellite also vary quite a bit within a few minutes
it can go from ~35dB C/N0 to 45dB and back down... again, with clear
view of the satellite.


Does someone have an idea what could be the cause of this?
Or how I could debug this?


Attila Kinali


[1] http://lna4all.blogspot.de/

-- 
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the prosperity and technological sophistication in the world is of no 
use without that foundation.
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