[time-nuts] Ublox 6T receiver, noisy PPS.

dan at irtelemetrics.com dan at irtelemetrics.com
Wed Jan 21 22:02:59 EST 2015

  Yes, the log has Raw TIC phase, saw tooth correction, and corrected 
TIC phase. Unfortunately I do not have another TIC available at the 
moment (It's on the wish list!) Also, with the GPS and OCXO, it's the 
classic two clock problem! ;) 
  It took a bit to pull the data together for graphing. The two graphs 
show 9000 seconds of recorded data, few days apart. One from the 
'noisy' period of time and the other from after the last survey. In the 
graphs the corrected 'noisy' data has a peak to peak wander of about 
twice that (26nS) of the corrected 'normal' wander (13nS). Other 
periods of time show wander a bit worse. However I tried to pick some 
samples that had similar EFC DAC movement, to try to keep the 
comparison even. 
  The magenta trace is saw tooth correction, in ns. The blue trace is 
uncorrected phase between the PPS and the HP10811. The yellow trace is 
corrected phase. You will note that the GPSDO is setup for a 50nS phase 
shift between the PPS and OXCO. 
  Additional things to note: I intentionally did not touch the system 
when the noise started (Trying not to change anything). The only thing 
that was done was the survey was started again. The HVAC system cycles 
here about every 30 to 40 minutes, so this data includes several temp 
swings. It's been about a month since the GPS was last power cycled. 
The 'noisy' period of data lasted a few days, and ended with the 
survey. Until the noisy period of data things have been running nicely 
since about last September. The antenna has not been moved, but some 
snow was removed from the roof around the antenna about a week before 
the 'noisy' period started. The HP10811 has been aging at about 5e-12 
per day... 
  My gut tells me something happened in the GPS, although I'm not sure 
what. Maybe this is even normal for a GPS? My feeling is not, but you 
all know this stuff better I do! :) 

  > So, your PPS wander is measured by the GPSDO internal TIC and the
  > result is sawtooth corrected and usually gives 6ns, suddenly it went
  > to 25ns. Well, the first move, IMO, is to connect an external TIC and
  > track the GPS PPS against the GPSDO PPS and see whether, when you have
  > the jump, it is recorded only by the internal TIC or not. You have a
  > log file: what's in it? You said that the PPS was still getting
  > corrections but what about the uncorrected PPS wander? Was it still
  > the same after the jump? Can you share your log file with the data
  > before and after the jump? I'm assuming the log file has the raw PPS
  > measurements and the applied corrections listed every second. 

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