[time-nuts] Correcting jitter on the 1 PPS signal from aGPSreceiver.
Tom Van Baak
tvb at LeapSecond.com
Sat Sep 13 22:33:55 EDT 2014
> The cables are not exactly the same lengths. Differences in length
> will result in a fixed offset. I am not concerned about such fixed
> errors, only jitter.
> I am comparing the rising edges which is what the spec defines as the
> reference edge.
Correct, the survey position is determined only by the phase center of the antenna, not by cable length. And cable length mismatches should make no difference in your jitter measurements.
But one thing to check is how sharp the 1PPS rising edge is -- right at the input to your TI counter. I use a BNC tee with one leg open allowing a 'scope check (set to 1M input). If your risetime is a couple of ns like mine is, then all is well. Slow risetime can be a huge source of timing jitter. Check both 50R and 1M at the counter input. Use DC, not AC coupling. Use fixed trigger, never auto-trigger. Pick a trigger level that matches the maximum slope.
Some examples of good/bad GPS 1PPS risetimes: http://leapsecond.com/pages/gpsdo-rise/
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