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

[Michael Wouters]

Dear Krishna

You should probably check with the manufacturer exactly how one should
interpret  eg SYNC:FEE?
The manual says:
"This query returns the Frequency Error Estimate."
but is this a offset due to eg a DAC resolution limitation, or is it
the uncertainty in the frequency?
I am guessing it is the latter.

Regards

On Mon, Mar 7, 2022 at 5:01 AM Krishna Makhija  wrote:
>
> Hello,
>
> I am new to the whole precision time-keeping game (and to this mailing
> list) so I apologize in advance if my question is too naive or has been
> answered already in your mailing list.
>
> Is it possible to have two separate GPSDOs, each with their own antennas,
> be phase coherent to each other? I have a Jackson-Labs Fury
>  and a Mini-JLT
> . I am using each to
> provide a 10 MHz reference to two separate software-defined radios (SDRs).
> In my tests I find that the phase offset between said SDRs has a slow
> time-varying behavior. I know the frequency errors of the GPSDOs are of the
> order of parts per trillion which will show up as slow time-varying phase
> offsets but I was hoping to use the PPS offsets and instantaneous frequency
> errors that I get from these modules (using SCPI commands) to be able to
> "back out" or predict what that time-varying phase offset would be. Is such
> a thing possible? Currently, the time-varying phase change does not seem to
> follow any discernible pattern and my attempts at backing out the phase
> change do not match my measurements.
>
> Here is the math I am using for calculating what I *think *the phase
> *should* be:
> [image: image.png]
> [image: image.png]
>
> [image: image.png]
> Does any of this seem sensible? Any input is appreciated.
>
> TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
> possible but can you use PPS offsets and frequency errors metadata to
> correct for it in post?
>
> Regards,
> Krishna
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[time-nuts] Re: Phase coherence with 2X GPSDO

[ed breya]

I actually have this kind of setup, with two "identical" GPSDO boards I 
salvaged from some telecom gear long ago. I've yet to be able to 
identify them by make or model. All I know is they use Oncore UT or GT 
RX units, and seem to work, according to the LED indicators and the 10 
MHz outputs.


I built them into one box with a common supply, and common RF in from a 
Wilkinson splitter, fed from an active antenna. When I first got it all 
fired up and working, I was disappointed that the outputs drift in 
relative phase. Sometimes they happen to remain close for long times, 
and  sometimes they obviously slide quickly enough to watch the shift 
over say a half hour or so, observed on a scope. The direction can go 
either way at any time. After I thought about it some, I realized that 
they can't be perfectly aligned, even if there were no sawtooth errors 
from each one's independent situation and activity.


I think the basic limit besides the GPS processing and noise, is the 
discreteness of the frequency control. They use 20 bit DACs for tuning, 
which seems pretty good, but the overall effect of two separate systems, 
each trying to perfect its own deal according to the GPS process, to 
compensate for temperature and their individual characteristics, just 
leads to a certain amount of relative drift. I think the times when they 
seem very stable are when everything happens to be just right, but 
mostly that's not the case. Presuming they actually are working right, 
over the long term, they should both agree on the average frequency, but 
not moment by moment.


So, this is all with two same-type units in the same environment 
conditions, more or less, and getting the same GPS signal, more or less. 
Imagine different units in different locations and conditions.


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

[Lux, Jim]

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

Hello Tom,

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

Bob,

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

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

Thanks for your inputs so far.

Krishna


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


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


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


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


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


You sort of have two choices in front of you:

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


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




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

[Krishna Makhija]

Hello Tom,

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

Bob,

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

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

Thanks for your inputs so far.

Krishna

On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq  wrote:

> Hi
>
> They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> can get them to *maybe* ten degrees with this and that done here or there.
>
> If you want them within a degree, no you can’t do that directly with GPS.
> If your definition of phase coherent is zero degrees, a pair of  SDR’s off
> the
> same buffered clock will have issues with that definition in the real
> world
> of temperature wandering around …..
>
> Bob
>
> > On Mar 6, 2022, at 2:04 PM, Krishna Makhija  wrote:
> >
> > Hi Bob,
> >
> > I am currently getting +- 10 ns nominal. The antennas are currently
> almost
> > next to each other (roughly 1-2 inches apart). Yes, they should be
> outside
> > of each of their farfield zones. Here is the PPS offset I am seeing
> during
> > measurement:
> > [image: image.png]
> > And this is the frequency error I am seeing:
> > [image: image.png]
> > Overall, the GPSDOs seem to work pretty well. But the question still
> > remains if one can hope to get them to be phase coherent, either in
> > real-time or in post-processing.
> >
> > Jeremy: I bought these items by writing to them. I chose to place a
> > purchase order (since I did it through my organization), but you might be
> > able to order by talking to them directly and paying using a credit card.
> >
> > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> >
> >> Hi
> >>
> >> How close are you trying to get?
> >>
> >> How far apart are the GPSDO’s?
> >>
> >> A “run of the mill” number would be out around 100 ns. A “pretty good”
> >> number is in the 20 ns range. A “crazy good” number would be 2 ns. To
> >> do better than this, you likely would need to go to a more exotic
> >> configuration
> >> on the GPSDO.
> >>
> >> Bob
> >>
> >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija 
> wrote:
> >>>
> >>> Hello,
> >>>
> >>> I am new to the whole precision time-keeping game (and to this mailing
> >>> list) so I apologize in advance if my question is too naive or has been
> >>> answered already in your mailing list.
> >>>
> >>> Is it possible to have two separate GPSDOs, each with their own
> antennas,
> >>> be phase coherent to each other? I have a Jackson-Labs Fury
> >>>  and a Mini-JLT
> >>> . I am using
> each
> >> to
> >>> provide a 10 MHz reference to two separate software-defined radios
> >> (SDRs).
> >>> In my tests I find that the phase offset between said SDRs has a slow
> >>> time-varying behavior. I know the frequency errors of the GPSDOs are of
> >> the
> >>> order of parts per trillion which will show up as slow time-varying
> phase
> >>> offsets but I was hoping to use the PPS offsets and instantaneous
> >> frequency
> >>> errors that I get from these modules (using SCPI commands) to be able
> to
> >>> "back out" or predict what that time-varying phase offset would be. Is
> >> such
> >>> a thing possible? Currently, the time-varying phase change does not
> seem
> >> to
> >>> follow any discernible pattern and my attempts at backing out the phase
> >>> change do not match my measurements.
> >>>
> >>> Here is the math I am using for calculating what I *think *the phase
> >>> *should* be:
> >>> [image: image.png]
> >>> [image: image.png]
> >>>
> >>> [image: image.png]
> >>> Does any of this seem sensible? Any input is appreciated.
> >>>
> >>> TL;DR: Trying to get phase coherence between two separate GPSDOs may
> not
> >> be
> >>> possible but can you use PPS offsets and frequency errors metadata to
> >>> correct for it in post?
> >>>
> >>> Regards,
> >>> Krishna
> >>>
> >>
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> >> send an email to time-nuts-le...@lists.febo.com
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
can get them to *maybe* ten degrees with this and that done here or there. 

If you want them within a degree, no you can’t do that directly with GPS. 
If your definition of phase coherent is zero degrees, a pair of  SDR’s off the 
same buffered clock will have issues with that definition in the real world 
of temperature wandering around …..

Bob

> On Mar 6, 2022, at 2:04 PM, Krishna Makhija  wrote:
> 
> Hi Bob,
> 
> I am currently getting +- 10 ns nominal. The antennas are currently almost
> next to each other (roughly 1-2 inches apart). Yes, they should be outside
> of each of their farfield zones. Here is the PPS offset I am seeing during
> measurement:
> [image: image.png]
> And this is the frequency error I am seeing:
> [image: image.png]
> Overall, the GPSDOs seem to work pretty well. But the question still
> remains if one can hope to get them to be phase coherent, either in
> real-time or in post-processing.
> 
> Jeremy: I bought these items by writing to them. I chose to place a
> purchase order (since I did it through my organization), but you might be
> able to order by talking to them directly and paying using a credit card.
> 
> On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq  wrote:
> 
>> Hi
>> 
>> How close are you trying to get?
>> 
>> How far apart are the GPSDO’s?
>> 
>> A “run of the mill” number would be out around 100 ns. A “pretty good”
>> number is in the 20 ns range. A “crazy good” number would be 2 ns. To
>> do better than this, you likely would need to go to a more exotic
>> configuration
>> on the GPSDO.
>> 
>> Bob
>> 
>>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:
>>> 
>>> Hello,
>>> 
>>> I am new to the whole precision time-keeping game (and to this mailing
>>> list) so I apologize in advance if my question is too naive or has been
>>> answered already in your mailing list.
>>> 
>>> Is it possible to have two separate GPSDOs, each with their own antennas,
>>> be phase coherent to each other? I have a Jackson-Labs Fury
>>>  and a Mini-JLT
>>> . I am using each
>> to
>>> provide a 10 MHz reference to two separate software-defined radios
>> (SDRs).
>>> In my tests I find that the phase offset between said SDRs has a slow
>>> time-varying behavior. I know the frequency errors of the GPSDOs are of
>> the
>>> order of parts per trillion which will show up as slow time-varying phase
>>> offsets but I was hoping to use the PPS offsets and instantaneous
>> frequency
>>> errors that I get from these modules (using SCPI commands) to be able to
>>> "back out" or predict what that time-varying phase offset would be. Is
>> such
>>> a thing possible? Currently, the time-varying phase change does not seem
>> to
>>> follow any discernible pattern and my attempts at backing out the phase
>>> change do not match my measurements.
>>> 
>>> Here is the math I am using for calculating what I *think *the phase
>>> *should* be:
>>> [image: image.png]
>>> [image: image.png]
>>> 
>>> [image: image.png]
>>> Does any of this seem sensible? Any input is appreciated.
>>> 
>>> TL;DR: Trying to get phase coherence between two separate GPSDOs may not
>> be
>>> possible but can you use PPS offsets and frequency errors metadata to
>>> correct for it in post?
>>> 
>>> Regards,
>>> Krishna
>>> 
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Lux, Jim]

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

Hi

How close are you trying to get?

How far apart are the GPSDO’s?

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



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


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


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


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


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



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


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





Bob


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

Hello,

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

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

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

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

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

Regards,
Krishna
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[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Bob kb8tq]

Hi

Looking at what you *tell* the DAC to do is only part of the process.
There are a number of issues that get into the mix that can cause
the DAC to do something other than what you tell it to do. A Vref
that changes is one example, there are an unfortunately large number
of other possibilities …. In an AC ( like a sound card ) application
a lot of these things really would not matter. In a DC application 
(like driving an oscillator) grubby details do start to get into the mix.

Bob

> On Mar 6, 2022, at 1:39 PM, Erik Kaashoek  wrote:
> 
> Hi Bob
> Good to hear.
> As the internal logging does collect the DAC settings and the frequency of 
> the TCXO versus a Rb standard I'm lucky to have both. And full scale 
> linearity testing of the DAC's hopefully will show any issues there. They 
> should be linear within 0.2 step.
> I've already build the single mixer+low noise 24 bit microphone input sound 
> card setup and locked a 4GHz PLL to a Rb and used the 400th overtone of 10MHz 
> from the TCXO so the lowest audio frequencies (20Hz) are in reality 0.05Hz, 
> low enough to connect to the 1Hz gate time measurement with the counter.
> At first impression the phase noise is not terrible, now the calibration 
> needs to be done by comparing with a 10MHz source with known (bad) phase 
> noise.
> The audio analysis SW used can do dB/log Hz.
> Will be interesting to share the results here and hear your feedback.
> Erik.
> 
> On 6-3-2022 18:49, Bob kb8tq wrote:
>> Hi
>> 
>> DVM’s show up from a lot of places. Indeed some are nutty when
>> shipped from here to there. The shopping process is always going
>> to be “delivered price” based.
>> 
>> The reason for looking at the dac output is: The frequency just jumped
>> 2x10^-8 … was it from the dac or internal to the TCXO ? The oscillator
>> drifted 5x10^-9 it one minute, was it the DAC / Vref? …
>> 
>> Phase Noise:
>> 
>> The “quick / simple” way to do phase noise is with a single mixer setup.
>> You run both ports at “max” ( so 7 dbm on a 7 dbm mixer) and lock them
>> in quadrature. A fairly simple audio amp based on any of a number of
>> op-amps boosts the output to something an audio spectrum analyzer
>> or sound card can “see”. You would like an opamp with something in the
>> 1 nv/Hz vicinity in terms of noise.
>> 
>> Once you get the setup going, it’s just a matter of calibrating things. There
>> are a variety of app notes and papers on that part. It normally involves
>> unlocking the loop and measuring the phase slope of the beat note with
>> whatever you are using to look at noise. ( The op amp “preamp” normally
>> gets switched out for this step).
>> 
>> Not crazy expensive or hard to wire up. I’ve built the circuit a lot of times
>> using “dead bug” sort of construction. They all seem to work.
>> 
>> On a typical GPSDO design, you are looking at noise inside about 10 KHz.
>> Sure there could be issues anywhere, but the most likely stuff is below the
>> max limit on the typical sound card. Low end wise, it would be nice to get
>> to 1 Hz. That isn’t going to be easy with the typical sound card.
>> 
>> Yes that glosses over all the “joy” of tracking down ground loops and other
>> local noise sources. You are measuring a *very* low level signal. Quiet
>> supplies and good grounding are part of this.
>> 
>> Bob
>> 
>> 
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[time-nuts] Re: Phase coherence with 2x GPSDO

[Bob kb8tq]

Hi

How close are you trying to get?

How far apart are the GPSDO’s?

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

Bob

> On Mar 6, 2022, at 12:55 PM, Krishna Makhija  wrote:
> 
> Hello,
> 
> I am new to the whole precision time-keeping game (and to this mailing
> list) so I apologize in advance if my question is too naive or has been
> answered already in your mailing list.
> 
> Is it possible to have two separate GPSDOs, each with their own antennas,
> be phase coherent to each other? I have a Jackson-Labs Fury
>  and a Mini-JLT
> . I am using each to
> provide a 10 MHz reference to two separate software-defined radios (SDRs).
> In my tests I find that the phase offset between said SDRs has a slow
> time-varying behavior. I know the frequency errors of the GPSDOs are of the
> order of parts per trillion which will show up as slow time-varying phase
> offsets but I was hoping to use the PPS offsets and instantaneous frequency
> errors that I get from these modules (using SCPI commands) to be able to
> "back out" or predict what that time-varying phase offset would be. Is such
> a thing possible? Currently, the time-varying phase change does not seem to
> follow any discernible pattern and my attempts at backing out the phase
> change do not match my measurements.
> 
> Here is the math I am using for calculating what I *think *the phase
> *should* be:
> [image: image.png]
> [image: image.png]
> 
> [image: image.png]
> Does any of this seem sensible? Any input is appreciated.
> 
> TL;DR: Trying to get phase coherence between two separate GPSDOs may not be
> possible but can you use PPS offsets and frequency errors metadata to
> correct for it in post?
> 
> Regards,
> Krishna
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[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Erik Kaashoek]

Hi Bob
Good to hear.
As the internal logging does collect the DAC settings and the frequency 
of the TCXO versus a Rb standard I'm lucky to have both. And full scale 
linearity testing of the DAC's hopefully will show any issues there. 
They should be linear within 0.2 step.
I've already build the single mixer+low noise 24 bit microphone input 
sound card setup and locked a 4GHz PLL to a Rb and used the 400th 
overtone of 10MHz from the TCXO so the lowest audio frequencies (20Hz) 
are in reality 0.05Hz, low enough to connect to the 1Hz gate time 
measurement with the counter.
At first impression the phase noise is not terrible, now the calibration 
needs to be done by comparing with a 10MHz source with known (bad) phase 
noise.

The audio analysis SW used can do dB/log Hz.
Will be interesting to share the results here and hear your feedback.
Erik.

On 6-3-2022 18:49, Bob kb8tq wrote:

Hi

DVM’s show up from a lot of places. Indeed some are nutty when
shipped from here to there. The shopping process is always going
to be “delivered price” based.

The reason for looking at the dac output is: The frequency just jumped
2x10^-8 … was it from the dac or internal to the TCXO ? The oscillator
drifted 5x10^-9 it one minute, was it the DAC / Vref? …

Phase Noise:

The “quick / simple” way to do phase noise is with a single mixer setup.
You run both ports at “max” ( so 7 dbm on a 7 dbm mixer) and lock them
in quadrature. A fairly simple audio amp based on any of a number of
op-amps boosts the output to something an audio spectrum analyzer
or sound card can “see”. You would like an opamp with something in the
1 nv/Hz vicinity in terms of noise.

Once you get the setup going, it’s just a matter of calibrating things. There
are a variety of app notes and papers on that part. It normally involves
unlocking the loop and measuring the phase slope of the beat note with
whatever you are using to look at noise. ( The op amp “preamp” normally
gets switched out for this step).

Not crazy expensive or hard to wire up. I’ve built the circuit a lot of times
using “dead bug” sort of construction. They all seem to work.

On a typical GPSDO design, you are looking at noise inside about 10 KHz.
Sure there could be issues anywhere, but the most likely stuff is below the
max limit on the typical sound card. Low end wise, it would be nice to get
to 1 Hz. That isn’t going to be easy with the typical sound card.

Yes that glosses over all the “joy” of tracking down ground loops and other
local noise sources. You are measuring a *very* low level signal. Quiet
supplies and good grounding are part of this.

Bob



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[time-nuts] Phase coherence with 2x GPSDO

[Krishna Makhija]

Hello,

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

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

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

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

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

Regards,
Krishna
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[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Bob kb8tq]

Hi

DVM’s show up from a lot of places. Indeed some are nutty when 
shipped from here to there. The shopping process is always going
to be “delivered price” based. 

The reason for looking at the dac output is: The frequency just jumped
2x10^-8 … was it from the dac or internal to the TCXO ? The oscillator 
drifted 5x10^-9 it one minute, was it the DAC / Vref? …

Phase Noise:

The “quick / simple” way to do phase noise is with a single mixer setup.
You run both ports at “max” ( so 7 dbm on a 7 dbm mixer) and lock them
in quadrature. A fairly simple audio amp based on any of a number of 
op-amps boosts the output to something an audio spectrum analyzer 
or sound card can “see”. You would like an opamp with something in the
1 nv/Hz vicinity in terms of noise. 

Once you get the setup going, it’s just a matter of calibrating things. There
are a variety of app notes and papers on that part. It normally involves
unlocking the loop and measuring the phase slope of the beat note with
whatever you are using to look at noise. ( The op amp “preamp” normally 
gets switched out for this step).

Not crazy expensive or hard to wire up. I’ve built the circuit a lot of times
using “dead bug” sort of construction. They all seem to work. 

On a typical GPSDO design, you are looking at noise inside about 10 KHz. 
Sure there could be issues anywhere, but the most likely stuff is below the 
max limit on the typical sound card. Low end wise, it would be nice to get 
to 1 Hz. That isn’t going to be easy with the typical sound card. 

Yes that glosses over all the “joy” of tracking down ground loops and other
local noise sources. You are measuring a *very* low level signal. Quiet 
supplies and good grounding are part of this. 

Bob


> On Mar 6, 2022, at 11:37 AM, Erik Kaashoek  wrote:
> 
> Bob,
> Shipping will be a bit of an issue as I'm not in the USA.
> But the most sensitive voltmeter may be the VC-TCXO so some SW was added to 
> the GPSDO/Timer/Counter to sweep the dac's while measuring a 10 MHz signal 
> from a Rb.
> This showed the high dac steps are all monotonic and almost equal size.
> For the low dac a single step is to small too accurately measure the 
> frequency difference but again the steps seem to be monotonic.
> 
> I'm aware of different way's to measure phase noise. Is there a method 
> particularly good for GPSDO phase noise measurement?
> Erik.
> 
> On 5-3-2022 20:50, Bob kb8tq wrote:
>> Hi
>> 
>> One suggestion on the frequency plot:
>> 
>> TimeLab as you have it set does a “per pixel” process on its charts. You have
>> a limited screen resolution and a ton of data. You have to do something. 
>> There
>> is no way to display it all.
>> 
>> It’s a good idea to zoom in to some of the “problem regions” and see
>> what is really going on. This reduces the number of data points that
>> map to one pixel and potentially changes what you see a bit. The closer you
>> get to one data point per pixel, the closer you are to “reality” without any
>> processing potentially getting in the way.
>> 
>> On the DVM, eBay (with some time spent shopping) will sell you a working
>> 6 1/2 digit device for < $100 if you are patient. If you need it now, it’s 
>> still
>> likely to be below $200. Yes it will be a big clunky box that takes up bench
>> space. It will be HPIB and not serial i/o. The display might be a bit fun in
>> bright lighting.
>> 
>> While you are shopping, I would get set up for phase noise as well. Often
>> it can spot issues that are tough to find with a second to second data 
>> stream.
>> The setup does not have to be very fancy or terribly expensive.
>> 
>> Bob
>> 
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[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[Erik Kaashoek]

Bob,
Shipping will be a bit of an issue as I'm not in the USA.
But the most sensitive voltmeter may be the VC-TCXO so some SW was added 
to the GPSDO/Timer/Counter to sweep the dac's while measuring a 10 MHz 
signal from a Rb.

This showed the high dac steps are all monotonic and almost equal size.
For the low dac a single step is to small too accurately measure the 
frequency difference but again the steps seem to be monotonic.


I'm aware of different way's to measure phase noise. Is there a method 
particularly good for GPSDO phase noise measurement?

Erik.

On 5-3-2022 20:50, Bob kb8tq wrote:

Hi

One suggestion on the frequency plot:

TimeLab as you have it set does a “per pixel” process on its charts. You have
a limited screen resolution and a ton of data. You have to do something. There
is no way to display it all.

It’s a good idea to zoom in to some of the “problem regions” and see
what is really going on. This reduces the number of data points that
map to one pixel and potentially changes what you see a bit. The closer you
get to one data point per pixel, the closer you are to “reality” without any
processing potentially getting in the way.

On the DVM, eBay (with some time spent shopping) will sell you a working
6 1/2 digit device for < $100 if you are patient. If you need it now, it’s still
likely to be below $200. Yes it will be a big clunky box that takes up bench
space. It will be HPIB and not serial i/o. The display might be a bit fun in
bright lighting.

While you are shopping, I would get set up for phase noise as well. Often
it can spot issues that are tough to find with a second to second data stream.
The setup does not have to be very fancy or terribly expensive.

Bob


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[time-nuts] Re: Tuning a GPSDO loop for optimal disturbance handling

[glen english LIST]

If you are looking for a reasonably affordable   5 digit high count 
(8) DMM, I can recommend the VICHY VC8145


something like about USD160 new.

has RS232

absolute accuracy is surprisingly good, close to my old Keithley. I used 
to cal with a mercury cell.


good for looking at up to 8 volts with 4 digits after the decimal...

On 6/03/2022 6:50 am, Bob kb8tq wrote:

On the DVM, eBay (with some time spent shopping) will sell you a working
6 1/2 digit device for < $100 if you are patient. If you need it now, it’s still
likely to be below $200. Yes it will be a big clunky box that takes up bench
space. It will be HPIB and not serial i/o. The display might be a bit fun in
bright lighting.

While you are shopping, I would get set up for phase noise as well. Often
it can spot issues that are

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