[USRP-users] GPSDO loss of lock

Sivan Toledo stoledo at tau.ac.il
Sun Apr 14 13:36:23 EDT 2013


The UHD version is UHD_003.005.001. Not sure about firmware version because
we did not write down which version we uploaded to the N200. Is there a way
to tell? Should we upgrade to the latest firmware/FPGA image?


On Thu, Apr 11, 2013 at 6:14 PM, Marcus D. Leech <mleech at ripnet.com> wrote:

> 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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