Re: [time-nuts] GPS discipline oscillator vs phase lock

[Brent]

Jim -

Your first thread in this post was fascinating to me - stuff I'd never been
exposed to.  It seems like the 'tricks of the trade' for so much of how
things actually get done are so often only accessible to those who work
closely with them.  I was about to shoot you an email to ask if there was
any reference (other than piles of journal articles) that cover some of
these topics, when I scrolled down and found this post with the Descanso
links and references.  What a trove!  There's more info in the links on
that page than I could ever hope to comprehend.

Many thanks for this post.  It will remain marked and in the back of my
brain as "stuff that you should learn".  If we only ever had enough time.
Why didn't anyone ever expose me to this stuff when I was young and just
starting in RF?

Brent



On Sun, Jun 18, 2017 at 1:36 PM, jimlux  wrote:

> On 6/18/17 7:10 AM, Attila Kinali wrote:
>
>> On Sat, 17 Jun 2017 06:29:02 -0700
>> jimlux  wrote:
>>
>> Well, at JPL we regularly lock two crystal oscillators together that are
>>> over a billion km apart with added Allan deviation of less than 1E-15 at
>>> 1000 seconds with a radio link at 7.15 GHz.  It's how we measure the
>>> distance and velocity to spacecraft (a few cm in range and mm/s in
>>> velocity) and from that figure out the gravitational fields (among other
>>> things)
>>>
>>
>> This sounds interesing. What do I have to google for to learn more?
>>
>>
> It's just how we do radio science/ranging - you transmit a spectrally pure
> signal from earth (typically oscillator locked to a maser), at the
> spacecraft you have a very narrow band PLL (traditionally a VCXO) that
> locks to the received signal, and you generate the downlink signal from
> that same oscillator, transmit it back to earth, and compare.
>
> The transmitted signal is precisely in a specified ratio with the received
> signal (880/749 for X-band 7.15 GHz from earth, 8.4 GHz coming back). For
> Ka-band, the earth signal goes up at 34 GHz, and comes back at 32 GHz
>
> A typical spec is that the transponder introduce no more than 4E-16 ADEV
> at 1000 sec.
>
>
> https://descanso.jpl.nasa.gov/ has links to a whole bunch of useful
> references
>
> https://descanso.jpl.nasa.gov/monograph/mono.html
> specifically volume 1 by Thornton and Border talks all about radiometric
> ranging.
>
> The various design and performance series describe the specific
> implementations.
>
> Joe Yuen's "Deep Space Telecommunications Engineering"
> https://descanso.jpl.nasa.gov/dstse/DSTSE.pdf
> Chapter 3 covers receiver design
> Chapter 4 covers radio tracking
>
> --
>
> Then you can look for papers on "deep space transponder"  The classic
> design papers are in the 90s.   IEEE MTT, and the JPL IPN progress reports.
>
> The Cassini Deep Space Transponder is sort of a progenitor of them - then
> there's the Small Deep Space Transponder (SDST) designed in the 90s, flying
> 2000 through now.
>
> Somewhere around 2000, the design started moving away from trying to lock
> the oscillator to doing the phase lock and phase/frequency turnaround in a
> digital loop, with a fixed oscillator driving DDS or NCO.  At JPL, this
> would be the "Advanced Deep Space Transponder", but Thales Alenia Space
> Italia (TASI) uses a similar approach for their deep space transponders
> (look for Juno and BepiColombo)
>
>
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/20/17 4:22 PM, Hal Murray wrote:


jim...@earthlink.net said:

sequential tone ranging: by putting a "ranging tone" at, say, 1 MHz,  on the
carrier


Thanks.  The part that attracted my attention was your "spectrally pure
signal" for the VCO.



Typically a maser at the ground station - that probably gets you fairly 
good close in phase noise characteristics.



Deep space uses very simple modulations (none of this fancy 64QAM)  - 
BPSK direct modulation or BPSK on a 25 or 32 kHz subcarrier.  They 
adjust the mod index of the data and ranging signals to leave 
significant power in the carrier (unless they're in a very good SNR 
situation).


But you can see why people would like PN ranging - it takes NO power 
from the carrier, so you can get a really good doppler estimate.


In reality, of course, you can post process to remove the BPSK data 
(just like removing the PN ranging) and essentially get all the power 
back in the carrier.  But you can really only do that on the downlink. 
On the other hand, the uplink usually has much better SNR than the 
downlink (80kW transmitter into a 34 or 70 m dish gives you a *healthy* 
EIRP), and the data rate on uplink is usually fairly low (nobody uplinks 
at megabits yet)




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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

jim...@earthlink.net said:
> sequential tone ranging: by putting a "ranging tone" at, say, 1 MHz,  on the
> carrier

Thanks.  The part that attracted my attention was your "spectrally pure 
signal" for the VCO.

I think the answer I was fishing for is that the modulation has to be easy to 
filter out.


Many years ago, I did some work on radar.  The only part I remember was the 
range-velocity ambiguity.  It's the radar version of Heisenberg for signal 
processing.  You can't measure both frequency and time of a signal with high 
accuracy.  Radar uses distance and velocity rather than time and frequency.

The plot I remember was 3D, range and velocity error in X-Y and probability 
or signal-power or something like that in Z.  For a simple radar pulse of a 
given duration, the plot is a bell curve.  Make the pulse longer and the 
curve gets narrower in velocity but wider in distance.  Make the pulse 
shorter and you get better distance but poorer velocity.  The volume was 
constant.

In radar, you can do things like chirp and pulse trains, but they just push 
the ambiguity over to someplace else.

> Tone ranging also requires that you have a  good a-priori estimate to pick a
> suitable set of tones.

That lets you pick a signal that puts some of the ambiguity someplace where 
you can ignore it.

-

I still have the little red book from those days.  I wonder how long it would 
take me to get back to where I could understand most of it.

P M Woodward: Probability and Information Theory with Applications to Radar.  
1953

Small world.  There is a more direct time-nuts connection.
  https://en.wikipedia.org/wiki/Philip_Woodward#W5_clock
  https://en.wikipedia.org/wiki/Philip_Woodward#Achievements_in_horology





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Re: [time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/18/17 9:29 PM, Hal Murray wrote:


jim...@earthlink.net said:

Well, at JPL we regularly lock two crystal oscillators together that are
over a billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the
distance and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)



It's just how we do radio science/ranging - you transmit a spectrally  pure
signal from earth (typically oscillator locked to a maser), at the
spacecraft you have a very narrow band PLL (traditionally a VCXO) that
locks to the received signal, and you generate the downlink signal from
that same oscillator, transmit it back to earth, and compare.


A "spectrally pure signal" gets you a frequency offset for velocity, but it
doesn't get any timing info.  How do you get range?


Well, you start by just measuring the round trip light time - we know 
(having measured it on the ground) the time delay through the 
spacecraft, and we know, very precisely, the time delay from the 
transmitter on the ground to the radiating aperture, and, equally 
precisely, where that aperture is.  (the latter is, in its own right, an 
impressive feat - if you want to measure the distance to something at 
Jupiter or Saturn (1E9 km) to the nearest mm, which is what 1E-15 ADEV 
implies)



Once you know that (I'm not sure what kind of uncertainty that is, maybe 
a microsecond?) you do the precise measurement in two different ways:
1) sequential tone ranging: by putting a "ranging tone" at, say, 1 MHz, 
on the carrier, which gets turned around in the transponder.  Remember 
that there is typically an "approximate" estimate of the range, so if 
you look at the phase difference of the ranging tone, you can figure out 
where you are within the "cycle".  Use a sequence of tones to resolve 
ambiguities.
2) PN ranging - put a PN code on the transmitted signal which is turned 
around by the transponder and measure the difference in code phase - 
some do regeneration, some don't


And finally, you measure carrier phase.



In general, PN ranging is considered better, but we still use tone 
ranging for most JPL missions - heritage  and availability of spacecraft 
radios that support it is a powerful force.


https://cwe.ccsds.org/css/docs/CSS-SM/Meeting%20Materials/2008/March-2008/DS%20Ranging%20type%20comparison%202001-11-03.pdf

Tone ranging takes power away from the telemetry (limiting the data 
return), while PN ranging doesn't (after despreading, you've got your 
whole signal power there). Tone ranging also requires that you have a 
good a-priori estimate to pick a suitable set of tones.


A historical note.. before Apollo, tone ranging what what people did 
(back in the 900 MHz Microlock days).  Apollo introduced PN ranging as 
part of the "unified s-band", but the acquisition time for the PN codes 
was apparently very, very long, even with the composite codes in 
use(called, oddly, JPL Ranging codes), and apparently everyone hated it 
(so I am told, I was a little kid and didn't know PN from tones) - so 
they went back to tone ranging for the most part.  ESA makes a lot of 
use of regenerative PN ranging, NASA does not - the SDST (Small Deep 
Space Transponder) doesn't do regen PN ranging, the BepiColombo 
transponder does.



https://deepspace.jpl.nasa.gov/dsndocs/810-005/203/203C.pdf - tone ranging
https://deepspace.jpl.nasa.gov/dsndocs/810-005/214/214A.pdf  - PN ranging
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

jim...@earthlink.net said:
>> Well, at JPL we regularly lock two crystal oscillators together that are
>> over a billion km apart with added Allan deviation of less than 1E-15 at
>> 1000 seconds with a radio link at 7.15 GHz.  It's how we measure the
>> distance and velocity to spacecraft (a few cm in range and mm/s in
>> velocity) and from that figure out the gravitational fields (among other
>> things)

> It's just how we do radio science/ranging - you transmit a spectrally  pure
> signal from earth (typically oscillator locked to a maser), at the
> spacecraft you have a very narrow band PLL (traditionally a VCXO) that
> locks to the received signal, and you generate the downlink signal from
> that same oscillator, transmit it back to earth, and compare. 

A "spectrally pure signal" gets you a frequency offset for velocity, but it 
doesn't get any timing info.  How do you get range?


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Re: [time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/18/17 7:10 AM, Attila Kinali wrote:

On Sat, 17 Jun 2017 06:29:02 -0700
jimlux  wrote:


Well, at JPL we regularly lock two crystal oscillators together that are
over a billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the
distance and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)


This sounds interesing. What do I have to google for to learn more?



It's just how we do radio science/ranging - you transmit a spectrally 
pure signal from earth (typically oscillator locked to a maser), at the 
spacecraft you have a very narrow band PLL (traditionally a VCXO) that 
locks to the received signal, and you generate the downlink signal from 
that same oscillator, transmit it back to earth, and compare.


The transmitted signal is precisely in a specified ratio with the 
received signal (880/749 for X-band 7.15 GHz from earth, 8.4 GHz coming 
back). For Ka-band, the earth signal goes up at 34 GHz, and comes back 
at 32 GHz


A typical spec is that the transponder introduce no more than 4E-16 ADEV 
at 1000 sec.



https://descanso.jpl.nasa.gov/ has links to a whole bunch of useful 
references


https://descanso.jpl.nasa.gov/monograph/mono.html
specifically volume 1 by Thornton and Border talks all about radiometric 
ranging.


The various design and performance series describe the specific 
implementations.


Joe Yuen's "Deep Space Telecommunications Engineering"
https://descanso.jpl.nasa.gov/dstse/DSTSE.pdf
Chapter 3 covers receiver design
Chapter 4 covers radio tracking

--

Then you can look for papers on "deep space transponder"  The classic 
design papers are in the 90s.   IEEE MTT, and the JPL IPN progress 
reports.


The Cassini Deep Space Transponder is sort of a progenitor of them - 
then there's the Small Deep Space Transponder (SDST) designed in the 
90s, flying 2000 through now.


Somewhere around 2000, the design started moving away from trying to 
lock the oscillator to doing the phase lock and phase/frequency 
turnaround in a digital loop, with a fixed oscillator driving DDS or 
NCO.  At JPL, this would be the "Advanced Deep Space Transponder", but 
Thales Alenia Space Italia (TASI) uses a similar approach for their deep 
space transponders (look for Juno and BepiColombo)


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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Attila Kinali]

On Sat, 17 Jun 2017 06:29:02 -0700
jimlux  wrote:

> Well, at JPL we regularly lock two crystal oscillators together that are 
> over a billion km apart with added Allan deviation of less than 1E-15 at 
> 1000 seconds with a radio link at 7.15 GHz.  It's how we measure the 
> distance and velocity to spacecraft (a few cm in range and mm/s in 
> velocity) and from that figure out the gravitational fields (among other 
> things)

This sounds interesing. What do I have to google for to learn more?

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] GPS discipline oscillator vs phase lock

[Arnold Tibus]

sorry about, but

who is 'lifespeed', a robot or a real person with a natural name?

many thanks,

73, Arnold, DK2WT


Am 17.06.2017 um 23:07 schrieb Lifespeed via time-nuts:

Yes, one has to lock them at a high reference frequency so as to avoid 
multiplied-up phase noise.  I can manage the tracking loop design.  Some 
applications aren't line-of-sight, so the radio link doesn't solve every 
situation.  Fiber optic backup plan, but everybody hates cords.

This is my application as well, phase measurement of the signals separated by 
some distance.  Not a billion km, but even a few km requires similar 
considerations.

Lifespeed

-Original Message-
From: jimlux [mailto:jim...@earthlink.net]

Well, at JPL we regularly lock two crystal oscillators together that are over a 
billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the distance 
and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)

So it is *doable*

The performance depends ultimately on the noise within your tracking loop 
bandwidth.



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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Lifespeed via time-nuts]

Yes, one has to lock them at a high reference frequency so as to avoid 
multiplied-up phase noise.  I can manage the tracking loop design.  Some 
applications aren't line-of-sight, so the radio link doesn't solve every 
situation.  Fiber optic backup plan, but everybody hates cords.

This is my application as well, phase measurement of the signals separated by 
some distance.  Not a billion km, but even a few km requires similar 
considerations.

Lifespeed

-Original Message-
From: jimlux [mailto:jim...@earthlink.net] 

Well, at JPL we regularly lock two crystal oscillators together that are over a 
billion km apart with added Allan deviation of less than 1E-15 at
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the distance 
and velocity to spacecraft (a few cm in range and mm/s in
velocity) and from that figure out the gravitational fields (among other
things)

So it is *doable*

The performance depends ultimately on the noise within your tracking loop 
bandwidth.



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Re: [time-nuts] GPS discipline oscillator vs phase lock

[jimlux]

On 6/16/17 10:55 PM, Lifespeed via time-nuts wrote:

Not too surprising to read locking two crystal oscillators together
without using a physical cable is difficult to impossible.
Essentially what I am looking for is the phase alignment accuracy
(and phase noise) one would get PLL’ing one oscillator to the other
using a cable, but over a longer distance.  Some modest phase noise
degradation might be acceptable, but not an order of magnitude.
Clearly not a trivial problem. Yes, the jitter (phase noise)
typically accomplished from a PLL phase comparing at 100MHz is better
than what one could get “locking” to GPS.  It was just a thought,
apparently not a realistic one.  Thanks for disabusing me of that
notion.



Well, at JPL we regularly lock two crystal oscillators together that are 
over a billion km apart with added Allan deviation of less than 1E-15 at 
1000 seconds with a radio link at 7.15 GHz.  It's how we measure the 
distance and velocity to spacecraft (a few cm in range and mm/s in 
velocity) and from that figure out the gravitational fields (among other 
things)


So it is *doable*

The performance depends ultimately on the noise within your tracking 
loop bandwidth.


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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi

If you also need the phase noise of the OCXO’s to be quite good when operating, 
the PLL approach has some issues. If you 
are after -100 doc / Hz sort of numbers at 1 Hz offset at 100 MHz, then a PLL 
to GPS is not your friend. At GPS will degrade
that by many 10’s of db. If the phase similarity requirement of the two OCXO’s 
extends out into the 100’s of Hz (you need them
phase coherent to 10 MHz maybe..), you will not be able to do that with any PLL 
approach. Again, the “record a set of modulated
signals” approach is more likely to do what you need to get done. In some ways 
the modulation *helps* in this case. It lets you better
estimate cycle slips and other odd occurrences. 

Bob


> On Jun 17, 2017, at 1:55 AM, Lifespeed  wrote:
> 
> Not too surprising to read locking two crystal oscillators together without 
> using a physical cable is difficult to impossible.  Essentially what I am 
> looking for is the phase alignment accuracy (and phase noise) one would get 
> PLL’ing one oscillator to the other using a cable, but over a longer 
> distance.  Some modest phase noise degradation might be acceptable, but not 
> an order of magnitude.  Clearly not a trivial problem. Yes, the jitter (phase 
> noise) typically accomplished from a PLL phase comparing at 100MHz is better 
> than what one could get “locking” to GPS.  It was just a thought, apparently 
> not a realistic one.  Thanks for disabusing me of that notion.
>  
> Sorry I can’t go into a lot of detail about the overall system block diagram, 
> but this one aspect of the design does just reduce to phase-locking two 
> oscillators over a distance.
>  
> Bob, I think I understand your post processing method refers to the reality 
> that all broadcast signals from which phase information could be extracted 
> are modulated, introducing complications that would not be present with a 
> simple carrier.
>  
> Lifespeed
>  
> Hi
>  
>  
>> On Jun 16, 2017, at 7:24 PM, life speed > > wrote:
>>  
>> That sounds like phase-locking the oscillators to a local radio transmitter. 
>>  Not sure there is any difference post-processing vs. real time.
>  
> The advantage is that you capture a much wider bandwidth signal than you can 
> lock to. That lets you extract better “instantaneous phase” information. With 
> the narrow band loop normally used for locking, loop dynamics get into the 
> picture. That on top of the RF propagation issues is a bit of a mess. It also 
> is quite possible to capture multiple radio (or TV or …) transmissions and 
> post process against all of them.
>  
> The bottom line is still that “many degrees” at 100 MHz is far more practical 
> than “tenth of a degree”. There are very few options if your application 
> really does need roughly a tenth of a degree. 
>  
> Bob
>  
> 
> 
>  
> - Lifespeed
>  
> 
> Hi
>  
> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> radio link at the level you are looking for.
>  
> Bob
>  

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

time-nuts@febo.com said:
> Not too surprising to read locking two crystal oscillators together without
> using a physical cable is difficult to impossible.  Essentially what I am
> looking for is the phase alignment accuracy (and phase noise) one would get
> PLL’ing one oscillator to the other using a cable, but over a longer
> distance.

Have you investigated getting a fiber connection?

I think you said the target distance was a mile.  That's easy for a fiber 
without any repeaters.  If you (try to) get the fiber from the local giant 
telephone company, they will probably run to the CO and back to the other 
site because that's the way they do things.  You might get through to 
somebody who will understand and cooperate.

Maybe you have a cooperative municipality or small telco or a site out in the 
woods where you can run your own fiber.  Or...

I didn't say it would be cheap.

You can prototype it in the lab.


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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Lifespeed via time-nuts]

I guess that is the obvious answer, and sort of how the problem is currently 
addressed up to the limits of realistic cable length, which actually cannot be 
very long for my application.  I am trying to come up with a better way.  What 
you describe quickly becomes impractical.  Again, apologies for not spelling 
out the entire system.

Lifespeed

-Original Message-
From: Mike Cook [mailto:michael.c...@sfr.fr] 

Why not just have ONE frequency generator locked to GPS if you want, and just 
distribute the output with equal length cables. It would mean a cable roll to 
store at one end, but you would be assured of phase coherency at both. No? 



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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Charles Steinmetz]

life-sp...@yahoo.com wrote:


Perhaps I could implement an ISM band radio link for the purpose of locking the 
two oscillators.  Of course that wouldn't reach a couple miles either.


There appears to be some amount of talking past each other going on here.

First, I think you may have a fundamental misconception of phase locking 
as it applies in your proposed case.  If there are two GPSDOs, the 
oscillators are *already* phase locked -- each one to the GPS network as 
received at its actual location.  If you were to try to do some other 
phase-locking, at least one of them wouldn't be a GPSDO any more.  (That 
may not be a bad thing, if common-view GPSDOs can't achieve the required 
accuracy.)


The two GPSDOs would, ideally, produce clock "ticks" identical to each 
other within picoseconds, which would be plenty sufficient for the vast 
majority of applications.   Of course, there are inevitably various 
errors, so in reality we do not achieve the full theoretical precision 
of the system.


The largest contributors to the differential errors (i.e., the phase 
difference between the two oscillators) are (or should be)  (i) mismatch 
in the cable delays due to differences between the lengths of the coax 
connecting each receiver to its antenna and/or the propagation 
velocities of the antenna cables (including the temperature coefficients 
of the cables), and (ii) differences in the GPS "solutions" in use at 
the two locations, which includes differences between the satellite 
constellations being used moment-to-moment by the two GPS receivers and 
the local reception conditions (quality of sky view, multipath, etc.).


Then there is (iii) the jitter of each GPSDO, which is not synchronous 
one to the other.  This includes the ionospheric path distortion [maybe 
this should be its own item], the GPS receiver electronics, and the 
locked oscillators themselves (including noise on the EFC line and the 
different instabilities of the two OCXOs).


Item (i) cable delay differences due to the cable lengths and/or 
isothermal propagation velocities result in a static offset.  Most 
timing-grade GPS receivers have a "cable length" setting that allows one 
to compensate for the cable delay (although that will not correct for 
the cable temperature coefficients).  This is all avoided if you use 
integrated GPSDOs (GPSDO built into an antenna housing).


It may be possible to reduce item (iI) by operating the GPS receivers in 
"single satellite" mode, both looking at the same satellite.  If long 
observations are required (such that the satellite in use must be 
changed during the measurements), this would become messy.  The tradeoff 
in "single satellite" mode is that while you eliminate errors due to the 
different satellite constellations being used moment-to-moment by the 
two receivers, but the the RF path errors and noise may increase, giving 
back at least some of the gain.


To reduce item (iii) errors, use identical GPSDOs with the very best 
OCXOs available.  You may need to select samples of the GPSDOs to 
minimize these errors.


All that will hopefully get you down to a differential phase of a few 
nS, at least for substantial stretches of time (using single-frequency 
receivers).  Times of day with low ionospheric distortion will produce 
lower differential phase than times with higher ionospheric distortion.


If you need better differential phase than this, you may want to 
consider solutions like White Rabbit (note that there may be issues with 
portable-type [movable] applications).


Best regards,

Charles


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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Lifespeed via time-nuts]

Not too surprising to read locking two crystal oscillators together without 
using a physical cable is difficult to impossible.  Essentially what I am 
looking for is the phase alignment accuracy (and phase noise) one would get 
PLL’ing one oscillator to the other using a cable, but over a longer distance.  
Some modest phase noise degradation might be acceptable, but not an order of 
magnitude.  Clearly not a trivial problem. Yes, the jitter (phase noise) 
typically accomplished from a PLL phase comparing at 100MHz is better than what 
one could get “locking” to GPS.  It was just a thought, apparently not a 
realistic one.  Thanks for disabusing me of that notion.

 

Sorry I can’t go into a lot of detail about the overall system block diagram, 
but this one aspect of the design does just reduce to phase-locking two 
oscillators over a distance.

 

Bob, I think I understand your post processing method refers to the reality 
that all broadcast signals from which phase information could be extracted are 
modulated, introducing complications that would not be present with a simple 
carrier.

 

Lifespeed

 

Hi

 

 

On Jun 16, 2017, at 7:24 PM, life speed  wrote:

 

That sounds like phase-locking the oscillators to a local radio transmitter.  
Not sure there is any difference post-processing vs. real time.

 

The advantage is that you capture a much wider bandwidth signal than you can 
lock to. That lets you extract better “instantaneous phase” information. With 
the narrow band loop normally used for locking, loop dynamics get into the 
picture. That on top of the RF propagation issues is a bit of a mess. It also 
is quite possible to capture multiple radio (or TV or …) transmissions and post 
process against all of them.

 

The bottom line is still that “many degrees” at 100 MHz is far more practical 
than “tenth of a degree”. There are very few options if your application really 
does need roughly a tenth of a degree. 

 

Bob

 





 

- Lifespeed

 

  _  

Hi

 

A far more common approach is to let the two oscillators free run and to record 
something like a local broadcast station. 

You then post process all of the data to give you the phase accuracy. One of 
several gotcha’s is the stability of any 

radio link at the level you are looking for.

 

Bob

 

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi


> On Jun 16, 2017, at 7:24 PM, life speed  wrote:
> 
> That sounds like phase-locking the oscillators to a local radio transmitter.  
> Not sure there is any difference post-processing vs. real time.

The advantage is that you capture a much wider bandwidth signal than you can 
lock to. That lets you extract better “instantaneous phase” information. With 
the narrow band loop normally used for locking, loop dynamics get into the 
picture. That on top of the RF propagation issues is a bit of a mess. It also 
is quite possible to capture multiple radio (or TV or …) transmissions and post 
process against all of them.

The bottom line is still that “many degrees” at 100 MHz is far more practical 
than “tenth of a degree”. There are very few options if your application really 
does need roughly a tenth of a degree. 

Bob


>  
> - Lifespeed
> 
> 
> Hi
> 
> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> radio link at the level you are looking for.
> 
> Bob

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[life speed via time-nuts]

That sounds like phase-locking the oscillators to a local radio transmitter.  
Not sure there is any difference post-processing vs. real time. - Lifespeed

 Hi
A far more common approach is to let the two oscillators free run and to record 
something like a local broadcast station. You then post process all of the data 
to give you the phase accuracy. One of several gotcha’s is the stability of any 
radio link at the level you are looking for.
Bob   
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Attila Kinali]

On Fri, 16 Jun 2017 18:29:31 -0400
Bob kb8tq  wrote:

> > Well, we still don't know what the requirements are.
> 
> 
> …. well, we do. A requirement of 0.1 degree at 100 MHz was stated earlier on.
> That’s where the ps stuff comes in.

Yes, that's _a_ requirement. Not _the_ requirement.

It's like marketing saying we need a 32bit ADC resolution.
Yes, we can do that. No, the ADC will not deliver 32bit
and it doesn't need to either.

Without knowing what the application is, it is very hard
to say whether the 0.1 degree is just a number out of thin
air or really a requirement of the system.

Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Attila Kinali]

On Sat, 17 Jun 2017 00:21:54 +0200
Attila Kinali  wrote:

> Another reference value: Time transfer using calibrated GPS receivers
> achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
> several 10km with single frequency receivers. Dual frequency receivers
> have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).

Yet another reference value:

Using LEA6-T it is possible to achieve sub-cm relative positioning precision
when recording phase data and doing postprocessing over several hour
blocks and with distances of a few km (<5km IIRC). Stability is good
to several months in high-alpine environments.

Attila Kinali

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi

> On Jun 16, 2017, at 6:21 PM, Attila Kinali  wrote:
> 
> On Fri, 16 Jun 2017 17:46:45 -0400
> Bob kb8tq  wrote:
> 
>> A far more common approach is to let the two oscillators free run and to 
>> record something like a local broadcast station. 
>> You then post process all of the data to give you the phase accuracy. One of 
>> several gotcha’s is the stability of any 
>> radio link at the level you are looking for.
> 
> Well, we still don't know what the requirements are.


…. well, we do. A requirement of 0.1 degree at 100 MHz was stated earlier on.
That’s where the ps stuff comes in.

Bob

> Though I doubt that a GPSDO cannot deliver the needed stability.
> Unless the goal is to build a multi-static radar that can locate
> a plane down to sub-cm range, ps level stabilty is not required.
> Even something as demanding as VLBI is in the couple ns range.
> (But they measure over long intervals (hours) which puts higher
> demands on the local oscillator)
> 
> Another reference value: Time transfer using calibrated GPS receivers
> achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
> several 10km with single frequency receivers. Dual frequency receivers
> have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).
> 
> 
>   Attila Kinali
> 
> [1] "Comparing a GPS time link calibration with an optical fibre
> self-calibration with 200ps accuracy", by Jlang et al 2015
> http://dx.doi.org/10.1088/0026-1394/52/2/384
> 
> -- 
> You know, the very powerful and the very stupid have one thing in common.
> They don't alters their views to fit the facts, they alter the facts to
> fit the views, which can be uncomfortable if you happen to be one of the
> facts that needs altering.  -- The Doctor
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Attila Kinali]

On Fri, 16 Jun 2017 17:46:45 -0400
Bob kb8tq  wrote:

> A far more common approach is to let the two oscillators free run and to 
> record something like a local broadcast station. 
> You then post process all of the data to give you the phase accuracy. One of 
> several gotcha’s is the stability of any 
> radio link at the level you are looking for.

Well, we still don't know what the requirements are.
Though I doubt that a GPSDO cannot deliver the needed stability.
Unless the goal is to build a multi-static radar that can locate
a plane down to sub-cm range, ps level stabilty is not required.
Even something as demanding as VLBI is in the couple ns range.
(But they measure over long intervals (hours) which puts higher
demands on the local oscillator)

Another reference value: Time transfer using calibrated GPS receivers
achieves an absolute accuracy of about 1-2ns(RMS) over base lines of
several 10km with single frequency receivers. Dual frequency receivers
have been reported to do <200ps[1] over 400km (TDEV <60ps up to 5days).


Attila Kinali

[1] "Comparing a GPS time link calibration with an optical fibre
self-calibration with 200ps accuracy", by Jlang et al 2015
http://dx.doi.org/10.1088/0026-1394/52/2/384

-- 
You know, the very powerful and the very stupid have one thing in common.
They don't alters their views to fit the facts, they alter the facts to
fit the views, which can be uncomfortable if you happen to be one of the
facts that needs altering.  -- The Doctor
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[life speed via time-nuts]

I was afraid of that, I guess it doesn't hurt to ask.  Perhaps I could 
implement an ISM band radio link for the purpose of locking the two 
oscillators.  Of course that wouldn't reach a couple miles either. - Lifespeed

 
Hi

A *lot* depends on your definition of “phase locked”.  If indeed you are after 
0.1 degree at 100 MHz, that gets into the “no can do” range. To put some 
numbers on it, 0.1 degree at 100 MHz is 2.7 ps. GPS time as received simply is 
not stable to that level … If you drop back to about 20 degrees, you start to 
get into the “maybe can do” range. 

Bob


   
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Hal Murray]

> I suspect the result of a GPSDO is not the same as phase-locking two
> oscillators together.

Each GPSDO is phase locked to GPS time.

In case you didn't catch it in Attila Kinali's reply...

> The analysis above is under the assumption that you have good sky view, with
> little multi-path and a well surveyed GPSDO.

The assumption is that your GPSDOs are not moving.  The "well surveyed" means 
you know where the antenna is located.



-- 
These are my opinions.  I hate spam.



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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi

A far more common approach is to let the two oscillators free run and to record 
something like a local broadcast station. 
You then post process all of the data to give you the phase accuracy. One of 
several gotcha’s is the stability of any 
radio link at the level you are looking for.

Bob

> On Jun 16, 2017, at 5:01 PM, life speed  wrote:
> 
> I was afraid of that, I guess it doesn't hurt to ask.  Perhaps I could 
> implement an ISM band radio link for the purpose of locking the two 
> oscillators.  Of course that wouldn't reach a couple miles either.
>  
> - Lifespeed
> 
> 
> 
> Hi
> 
> A *lot* depends on your definition of “phase locked”.  If indeed you are 
> after 0.1 degree at 100 MHz, that gets into the “no can do” range. To put 
> some numbers on it, 0.1 degree at 100 MHz is 2.7 ps. GPS time as received 
> simply is not stable to that level … If you drop back to about 20 degrees, 
> you start to get into the “maybe can do” range. 
> 
> Bob
> 
> 

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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Bob kb8tq]

Hi

A *lot* depends on your definition of “phase locked”.  If indeed you are after 
0.1 degree at 100 MHz, that gets into the “no can do” range. To put some 
numbers on it, 0.1 degree at 100 MHz is 2.7 ps. GPS time as received simply is 
not stable to that level … If you drop back to about 20 degrees, you start to 
get into the “maybe can do” range. 

Bob

> On Jun 16, 2017, at 2:40 PM, life speed via time-nuts  
> wrote:
> 
> I may already know the answer to this, but I figured I would ask the time 
> nuts anyway.  I have an application where I would like to phase-lock two 
> oscillators together, probably 10MHz OCXOs as they have particularly good 
> Allen Deviation compared to what I would ultimately like to use, a 100MHz 
> crystal oscillator locally PLL'd to the 10MHz oscillators.  These oscillators 
> will be separated by a distance of a few yards up to a few miles.  The 
> requirement is not that their phases align perfectly, as in the conventional 
> locally-connected phase-locked loop sense, but rather that any phase 
> difference between the two oscillators resulting from arrival times of GPS 
> signals are held constant.  Perhaps this shows my lack of understanding of 
> GPS time, I don't know if travel time is accounted for in commercial GPS 1Hz 
> outputs, it may well be corrected.
> 
> I suspect the result of a GPSDO is not the same as phase-locking two 
> oscillators together.  Perhaps it is frequency locking?  Which, if the phase 
> difference were held constant to within 0.1 degree, would be acceptable.  Not 
> sure this is the result either.
> Thanks for the info, - Lifespeed
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Re: [time-nuts] GPS discipline oscillator vs phase lock

[Attila Kinali]

Moin,

On Fri, 16 Jun 2017 18:40:26 + (UTC)
life speed via time-nuts  wrote:

> I suspect the result of a GPSDO is not the same as phase-locking two
> oscillators together.  Perhaps it is frequency locking?  Which, if the phase 
> difference were held constant to within 0.1 degree, would be acceptable.  Not 
> sure this is the result either.

Nope, GPSDOs do phase lock on the "true" GPS time. The offset between
GPS time and the GPSDO output is mainly dependent on the antenna,
the antenna cable and the receiver. The antenna delay is mostly
the temperature dependence of the antenna itself and its filter.
The cable is temperature dependence of length and dielectric constant.
The receiver is mostly temperature dependence of filters.

With a bit care, you can keep the stability of these to better than 1ns.
The offset stability between two GPSDO setups of the same kind should
be an order of magnitude smaller (given similar temperatures), maybe
even two.

The analysis above is under the assumption that you have good sky view,
with little multi-path and a well surveyed GPSDO. If you have bad sky
view or lots of multi-path or are off with the survey coordinates,
you can easily get a jitter in the order of 100's of ns.


What you have not talked about is, how well you want to keep the
phase between the two GPSDO stable.

If you look at ADEV numbers of GPSDOs, then you will see that the stability
easily goes down to 1e-10, some even reach a few parts of 1e-12 at taus < 1000s.
This should give you an indication what's possible.


HTH

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] GPS discipline oscillator vs phase lock

[life speed via time-nuts]

I may already know the answer to this, but I figured I would ask the time nuts 
anyway.  I have an application where I would like to phase-lock two oscillators 
together, probably 10MHz OCXOs as they have particularly good Allen Deviation 
compared to what I would ultimately like to use, a 100MHz crystal oscillator 
locally PLL'd to the 10MHz oscillators.  These oscillators will be separated by 
a distance of a few yards up to a few miles.  The requirement is not that their 
phases align perfectly, as in the conventional locally-connected phase-locked 
loop sense, but rather that any phase difference between the two oscillators 
resulting from arrival times of GPS signals are held constant.  Perhaps this 
shows my lack of understanding of GPS time, I don't know if travel time is 
accounted for in commercial GPS 1Hz outputs, it may well be corrected.

I suspect the result of a GPSDO is not the same as phase-locking two 
oscillators together.  Perhaps it is frequency locking?  Which, if the phase 
difference were held constant to within 0.1 degree, would be acceptable.  Not 
sure this is the result either.
Thanks for the info, - Lifespeed
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