[time-nuts] Newbie question: GPSDO filtering principles
Peter Vince
pvince at theiet.org
Tue Apr 24 14:54:44 EDT 2007
Tom Van Baak <tvb at LeapSecond.com> wrote:
>You guessed the step size by manually setting the DAC
>to a couple of extreme values, right?
Yes, although not too extreme. I first used +/- 100 (hex) steps
around the currently used on-frequency value, and then +/- 10 (hex)
steps. The latter proved a little inconclusive due to the wander of
the various oscillators, but wasn't a million miles away the figures
obtained from the earlier larger frequency steps.
>Can you tell from the insides what GPS engine is used?
It's a tiny little Trimble board, but I couldn't see any obvious
designation on it.
>One other thing you could try, since you have other GPS
>references and counters available is see if you can find
>the 1PPS from inside. This will tell you if the GPS engine
>is working right or not. Did it auto-survey correctly? The
>lat/lon/alt look OK?
Yes, that is buffered and fed out. Interestingly it has a
double-sawtooth effect: as well as the ordinary sawtooth discussed on
the forum at length recently, there is a "tick-tock" effect every
pair of seconds, with a swing between adjacent readings of about 220
nanoseconds!
While the antennas don't have a perfectly clear view of the sky (some
tall buildings and satellite dishes mask the eastern horizon), they
are high up, and yes, the lat/long/alt seems OK. I enquired about
putting them into position-hold mode, but was told they always are
(presumably after an initial survey).
>Also, another trick to isolate GPSDO problems is to pull
>the antenna and see if the output gets a lot more stable.
It is possible to lock the DAC at a specific value, which I've done,
and that gave a much more stable output: after a few days it turned a
corner and drifted off by about 3 parts in 10^13, and about a week
later it came back flat and level - but I ain't gonna complain about
that! I've not tried pulling the antenna out, could be worth a go,
just so long as the power surge doesn't blow it up!
>By looking at the raw GPS 1PPS, the locked performance,
>and the holdover performance you get a clue if the problem
>is with the GPS engine, the phase comparator system, or
>the LO itself.
The GPS timing has quite a lot of outliers, and I wonder if they are
not being filtered out?
Regards,
Peter
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