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

[Mattia Rizzi]

Hello Krishna,
what is your end application? How far away are those boards?
If each SDR can communicate to each other, you can run PTP over an ad-hoc
Layer 1.
I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
less than 0.5ns RMS noise and two-way ranging error down to 10-20
centimeters, using two SDR.


cheers

Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <km5es at virginia.edu>
ha scritto:

> 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
> > >>>
> > >>
> >
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