[time-nuts] Cheap jitter measurements

Hal Murray hmurray at megapathdsl.net
Thu Apr 5 22:09:51 EDT 2018


>> What do you mean by "jitter" and what do you really want to do?
> I mean jitter as NTP defines jitter.  Whatever that is.

I think you need to figure out what you want to do so you don't fool yourself.

ntpd is a PLL.  There is a low pass filter in the control loop.  It will 
track the low frequency wander of the source.

Assume you have a stable Rb reference.

Assume your GPSDO has a (small) day/night shift.

If you collect data with a TICC referenced to that Rb, that shift will show 
up.  If you collect data with ntpd, you will get the filtered version of that 
shift.  I'd expect small bumps at the day/night changes.

Ballpark for the time constant on the filter is the polling interval.  You 
can see the results of the filter by introducing a "bump" with ntpfrob.

-- 
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