[USRP-users] GPSDO loss of lock

Marcus D. Leech mleech at ripnet.com
Sun Apr 14 13:55:15 EDT 2013

On 04/14/2013 01:36 PM, Sivan Toledo wrote:
> 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?
Testing those values so frequently is going to cause unnecessarily-high 
control traffic.  Unless things are *broken*, there's absolutely no reason
   to test them so frequently.  Every few seconds perhaps, but every 64 
packets?  That's a very high rate.  And the GPS_LOCK sensor mechanism
   within the GPSDO probably doesn't even operate over such short time 
intervals--like I said, it's taking a "long view".

The VCTCXO is your typical GPSDO is designed to provide fairly-long 
"holdover" times, and the servo mechanism tends to be able to cope with
   only-occasional "good" fixes from the satellite constellation.  
Again, I can't comment on why your GPSDO is reporting loss-of-lock so 
   but it likely won't have a huge impact.

> On Thu, Apr 11, 2013 at 6:14 PM, Marcus D. Leech <mleech at ripnet.com 
> <mailto: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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Marcus Leech
Principal Investigator
Shirleys Bay Radio Astronomy Consortium

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