[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"Â  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 <km5es at virginia.edu> 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
>> <https://www.jackson-labs.com/index.php/products/fury> and a Mini-JLT
>> <https://www.jackson-labs.com/index.php/products/fury>. 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
>> <image.png><image.png><image.png>_______________________________________________
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