[USRP-users] GPSDO loss of lock

Marcus D. Leech mleech at ripnet.com
Thu Apr 11 11:14:55 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 seconds?
> 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
I can't tell you why the gps_locked sensor is coming and going.

But the oscillator on board the GPSDO is of high quality, and the servo 
in the GPSDO takes a "long view", so loss-of-lock won't necessarily cause
   any really bad issues with clock stability, as long as it's getting 
good timing data on a regular basis.

Which version of UHD and firmware are you using?


-- 
Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
http://www.sbrac.org






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