Hi Robin, Thank you for the reply. I have debugged the problem more and realized two issues.
1) The massive drift of 14 microseconds/second was due me using a 6V, 2A brick supply. This is my fault! After switching it out for a 3A supply, I'm able to get the GPSDO to lock (GPS_SERVO message returns 6 for lock status field) 2) Once you start collecting data, GPSDO PPS & 10 MHz start drifting from UTC. As mentioned, this is probably due to a voltage sag on the 6V supply for the Firefly GPSDO. For issue 2, I measured the nominal voltage to GPSDO (USRP N200 J509) to be 5.678V (+-2mV), then once I start a data collection, the minimum voltage I see is 5.532V, then after a few seconds of data collection it reaches a "steady state" of 5.545 V. This small change of (5.678-5.545 V) 133 mV is enough to give the PPS a drift of about 5-8 nsec/second. Eventually the GPSDO feedback loop catches this and steers it back to something more acceptable. For reference, the voltage at the input to N200 (J101) is 5.901V+-5mV when idle and collecting samples. This slight deviation of < 100 nanoseconds due to voltage change is fine for my application, I realized I can just read back the GPS_SERVO sensor and get the actual offset from UTC to correct any timestamps. Regards Stephan On Fri, Nov 9, 2018 at 5:09 PM Robin Coxe <robin.c...@ettus.com> wrote: > Hi Stephan. Your issue looks similar to one that has been previously > reported. The Hardware Sustaining Engineering team is currently > investigating. > > Would it be possible for you to try powering the GPSDO module from a lab > supply instead of plugging it in to the N210 motherboard and checking if > your issue still persists? > That would help us determine if you are experiencing the same problem. > Another helpful measurement would be to measure the voltage drop of the 6V > wall wart when the GPSDO IS powered by the N210 motherboard. > > We will keep you and list updated on the progress of the investigation. > > -Robin > > > On Fri, Nov 9, 2018 at 3:06 AM Stephan Esterhuizen via USRP-users < > usrp-users@lists.ettus.com> wrote: > >> Hi All, >> >> I have two N200 USRPs with Jackson-Labs Firefly 1A GPSDO. I'm doing a >> very basic test, where I have a common active GPS antenna split 2 ways >> (with DC block on one N200 GPS antenna port). I then watch the 1PPS from >> each GPSDO on a scope, and unfortunately see the PPS drift by as much as 14 >> microseconds per second relative to each other. When this offset reaches >> about 1.5-2.0 milliseconds, I see the PPS relative offset "snap" down to a >> few micro seconds error, but then they immediately start drifting apart >> again. >> >> The N200 GPSDO reports they're locked. I wrote some quick python code to >> query sensors: >> >> ---------------------- >> FIRST N200: >> ---------------------- >> [INFO] [GPS] Found an internal GPSDO: Jackson-Labs, FireFly , Firmware >> Rev 0.929 >> [INFO] [USRP2] Setting references to the internal GPSDO >> GPS lock status: locked >> GPS_GPGGA: >> $GPGGA,095011.00,xxxx.xxxxx,N,xxxx.xxxxx,E,1,10,0.9,320.7,M,46.9,M,,*6C >> GPS_SERVO: 18-11-09 3888 94089 21.81 2.08E-11 13 10 6 0x0 >> GPS epoch time: 1541757012 seconds >> Ref: locked >> >> >> ---------------------- >> SECOND N200 >> ---------------------- >> >> [INFO] [GPS] Found an internal GPSDO: Jackson-Labs, FireFly , Firmware >> Rev 0.929 >> [INFO] [USRP2] Setting references to the internal GPSDO >> GPS lock status: locked >> GPS_GPGGA: >> $GPGGA,095027.00,xxxx.xxxxx,N,xxxx.xxxx,E,1,08,1.1,317.9,M,46.9,M,,*63 >> GPS_SERVO: 18-11-09 2948 120000 796.02 9.59E-09 13 8 2 0x326 >> GPS epoch time: 1541757029 seconds >> Ref: locked >> >> ============ >> >> At this point, it might be a question for Jacksonlabs, since I'm not >> really using any N200 functions except for reading back the Ublox GPS NMEA >> strings. >> >> Has anyone seen this kind of behavior before? >> >> I'm suspecting the GPS_SERVO field might shed some light, but don't quite >> know how to read it. >> >> Thanks for any pointers >> >> Stephan >> _______________________________________________ >> USRP-users mailing list >> USRP-users@lists.ettus.com >> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com >> >
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