[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
<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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