[time-nuts] Metastability in a 100 MHz TIC

Tom Van Baak tvb at LeapSecond.com
Fri Jul 20 20:10:45 EDT 2007


I like your point about the random quantization error in the
sawtooth. Yes, that would help the noise by a few dB.

On the other hand it would also seem the 10 ns resolution
of the TIC is the limiting factor (by an order of magnitude)
over the 1 ns resolution limit of the sawtooth corrections,
so improving the quality of the sawtooth corrections has
limited gain.

Now, I'd still like to pursue the issue of noise in the 100 MHz
oscillator. Do we agree one doesn't need a cesium for this?
Or even an XO?

True, you want some accuracy in the 100 MHz. But the counts
are only integers from 0 to 160 so the accuracy requirement
is just 3 digits, 0.1% (so cheap quartz, at 1e-6, or cesium, at
1e-13, is extreme overkill). I mean, almost anything wiggling at
100.0 MHz will serve as an adequate timebase.

Also, as you point out, instability or jitter is your friend, not
your enemy in this case. Would it be possible to introduce
the +/- 5 ns jitter deliberately in the 1pps trigger level instead
of in the timebase? I.e., slow down the rising edge enough
so that you get jitter for free?

Another solution might be to deliberately choose an inaccurate
and unstable oscillator; use the 1pps to count oscillator cycles
per second, as well as to count the time interval. The larger
count can be used to calibrate the smaller count on every count.
This gives all the jitter you need and avoids any injection issues.

While you're at it, how about N of these oscillators, each making
its own out of phase measurement of the same OCXO-GPS 1pps.
Another couple of dB of resolution...


----- Original Message ----- 
From: "Dr Bruce Griffiths" <bruce.griffiths at xtra.co.nz>

> Tom
> 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.
> Bruce

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