[USRP-users] GPSDO loss of lock

Sivan Toledo stoledo at tau.ac.il
Thu Apr 11 07:40:42 EDT 2013

We are trying to accurately measure time-of-arrival in two identical USRPs
(N200 with WBX), both with the built-in GPSDO. We see differences in
time-of-arrival between the two receivers, even at excellent SNRs, and we
are trying to track the reason for this. Both receivers are connected to
the same antenna through a T connector with identical cables.

One thing that we checked is the GPS lock, the LO lock, and the REF lock.
We test for all 3 every 64 packets of samples we get from UHD. We found
that the LO and REF are locked all the time, but the gps_locked sensor
reports loss of lock every 2 to 3 seconds. It then locks and unlocks a
couple of times (usually 4) in quick succession, then locks for 2-3 more
seconds, and so on.

The GPS antennas seem fine and the GGA sentence reports that the receiver
sees 11 satellites. So I don't think that flaky GPS reception is the
problem. This happens to both units.

We have 2 questions:
1. Why does the gps_locked reports no lock for a short period every few
2. How does this affect the 10MHz clock to which the sampling clock and the
LO are locked? Can GPS loss of lock cause a significant drift or increased
phase noise in the 10MHz reference and the clocks that are locked to it?

Thanks, Sivan
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