[time-nuts] Ublox 6T receiver, noisy PPS.
dan at irtelemetrics.com
dan at irtelemetrics.com
Thu Jan 22 03:02:59 UTC 2015
Hi,
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! :)
Thanks,
Dan
> 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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