[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 <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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