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

[Krishna Makhija]

Thanks Mattia. What did you use for your Layer 1? I need to place one of
the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
could use WLAN but can you get sub-nanosecond performance over wi-fi? My
initial guess would be no but I am not certain.

Michael: I've had the same question but I can't see how it could
possibly "know" its own frequency error/uncertainty. What would it
reference to? I'll try asking the manufacturer anyway.

Regards,
Krishna

On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi <mattia.rizzi at gmail.com> wrote:

> 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
> > > >>>
> > > >>
> > >
> >
> <image.png><image.png><image.png>_______________________________________________
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