[USRP-users] Sychnronize two USRP X310 with GPS

Peter Witkowski pwitkowski at gmail.com
Fri Apr 10 11:34:41 EDT 2015

I am currently working with Ettus to remedy a similar (perhaps related)
issue.  I purchased four X300s and the GPSDO for each and found that the
PPS tends to drift wildly when I started a recording.  Note that prior to a
recording, everything is locked and the PPS signals (observing the PPS Out
line on a scope) are within 10ns of each other.  I am using a GPS splitter
to pass the same signal to all four units, so I know the only source of
deviation between units are the units themselves.

It appears that the issue is thermal.  Namely when a recording starts, RF
hardware is powered on for the first time and this spike in temperature
causes issues with the GPSDO (which is very temperature sensitive).  I saw
PPS drifts between two units as high as 300ns.  Removing the top cover and
having a fan circulate air on the GPSDO reduced this number to about 45ns.
Simply touching the GPSDO (I wouldn't recommend this) causes the PPS to
skew by hundreds of nanoseconds due to the temperature of your finger.

As a result of this, I'm currently looking at solving the issue by
procuring four external clocks that produce a 10Mhz reference and PPS that
are synched to GPS.  I currently don't see a path forward where the X300
series can be used usefully with the GPSDO unless you can get away with a
set-up where you provide the thermal solution to the X300 (and can accept a
fairly large PPS drift when your recording starts).

On Fri, Apr 10, 2015 at 7:15 AM, Sanjoy via USRP-users <
usrp-users at lists.ettus.com> wrote:

> Hi all,
> I am working on the same kind of setup and facing the same problem with
> time
> sync. If anyone find the solution please post it or let me know the way.
> I am taking the last pps time and using set_start_time for future
> streaming.
> Is there any way to know the exact streaming time of the frontend so both
> tx
> and rx frontend could be synchronized.
> regards
> Sanjoy
> _______________________________________________
> USRP-users mailing list
> USRP-users at lists.ettus.com
> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com

Peter Witkowski
pwitkowski at gmail.com
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