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

[Krishna Makhija]

Hello Tom,

Yes, the GPSDOs are working well. However, when I use each as a reference
to a separate radio, I find there is a slow phase change over time between
said radios. I imagine this is expected since there will always be some
error between two discrete oscillators. However, I am hoping to use the PPS
and FEE metadata to compute what the phase *should* have been in
post-processing. So far, it is not working out for me. I am wondering if
that is even possible or if my math is just wrong.

Bob,

The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +- 10
ns is >360 deg. With a common-mode reference I see a small phase change (+-
3-4 deg) but that is not an option for my application.

Where does the PPS offset come from? Isn't it from the positioning error?
Typical GPS receivers have 1-3 m of positioning error which should give
you +- 10 ns. Why is this a "dream" performance? It should be expected from
any modern GPS receiver.

Thanks for your inputs so far.

Krishna

On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> 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
> >>>
> >>
> <image.png><image.png><image.png>_______________________________________________
> >>> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe
> >> send an email to time-nuts-leave at lists.febo.com
> >>> To unsubscribe, go to and follow the instructions there.
> >> _______________________________________________
> >> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe
> send
> >> an email to time-nuts-leave at lists.febo.com
> >> To unsubscribe, go to and follow the instructions there.
> > <image.png><image.png>_______________________________________________
> > time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe
> send an email to time-nuts-leave at lists.febo.com
> > To unsubscribe, go to and follow the instructions there.
> _______________________________________________
> time-nuts mailing list -- time-nuts at lists.febo.com -- To unsubscribe send
> an email to time-nuts-leave at lists.febo.com
> To unsubscribe, go to and follow the instructions there.