[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 <km5es at virginia.edu> 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 <kb8tq at n1k.org> 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 <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
>>> 
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