[time-nuts] Metastability in a 100 MHz TIC

Dr Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Jul 20 19:16:57 EDT 2007

Tom Van Baak wrote:
> Richard,
> A 1.6 us window mean you have almost no issues with
> the accuracy or stability of your 100 MHz sample clock.
> 10 ns out of 1.6 us is 1/2 percent; clock counts won't
> exceed 160; a quartz timebase is overkill.
> Do I understand correctly: you make each raw 1pps
> time interval measurement down to 10 ns resolution,
> then (in software, I presume, one second later) apply
> a negative sawtooth correction with 1 ns resolution, then
> average 120 of those sums, and then expect a 84 ps
> resolution result? Something doesn't sound quite right.
> /tvb

If the quantisation error of the sawtooth correction is random then 
averaging will reduce the noise in the average sawtooth correction to 
somewhat below 1ns.
If the 100MHz oscillator phase drifts randomly with respect to the phase 
of the OCXO being disciplined then averaging will indeed reduce the 
noise of the phase measurement to below 10ns, however although the 
calculated resolution is 83ps the noise in the result will be somewhat 
higher than this.

Indeed a quartz timebase is perhaps too stable for effective averaging 
unless the PPS signal has around 10ns or so rms jitter on its leading edge.
However if a less stable timebase is used to ensure it has sufficient 
short term instability to ensure accurate averaging it will be necessary 
to measure/calibrate its frequency perhaps every second to correct for 
drift in the timebase oscillator.

Relying on the 100MHz crystal oscillator phase drifting around in a 
random fashion so that the phase error averages are unbiased estimates 
of the true phase error is perhaps expecting too much unless heroic 
measures are taken to ensure that the 100MHz oscillator doesnt phase 
lock to the OCXO output via injection locking.

The phase error averaging will be improved by adding sufficient jitter 
to the PPS signal to increase its random timing jitter to around 10ns 
rms or so.
If this is done the 100MHz clock can be derived from the oscillator 
being disciplined and the averaged phase will still have a very small bias.


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