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

[Krishna Makhija]

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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