[time-nuts] Brooks Shera
magnus at rubidium.dyndns.org
Thu Mar 28 12:30:34 EDT 2013
On 03/26/2013 01:35 AM, Bruce Griffiths wrote:
> Richard H McCorkle wrote:
>> Hi Tom,
>> In the Shera design the instability of the XO timebase is
>> a key factor in improving the 30-second update resolution.
>> With the XO drift varying the sample point across the 1PPS
>> and 312.5 KHz edges the samples are constantly varying and
>> the average of the samples has a resolution much closer to
>> (1/CLK)/30 or 1.4ns. If both signals were synchronized
>> with the clock the start and stop edges of every sample
>> would fall exactly on a clock. In lock the odds of the
>> sources having a sample-to-sample variation greater than
>> 1 clock period (+/- 41.7ns) over the 30 second sample
>> period is low. So the probability is high that you would
>> get 30 identical samples with an average resolution much
>> closer to 41.7ns (+/- 1 clock).
> The above assertion is not correct if the inputs to the synchroniser(s)
> are asynchronous to the synchroniser clock.
> The gate period will have a bounded (neglecting the effect of
> metastability at the output of the synchronisers) variation of +- 1 count.
> Provided that the synchroniser (and counter) clock and the divided down
> disciplined oscillator clock meet the conditions outlined by David Chu in:
> In practice these conditions may be difficult to meet without adequate
> random phase modulation of PPS (and the divided down disciplined
> oscillator signal).
> Alternatively phase modulating the synchroniser/counter clock is perhaps
> The sawtooth phase modulation of the PPS signal output produced by many
> timing GPS receivers isnt sufficiently random to be particularly useful.
A variant of the modulation was used in the HP5328A with 040 option when
doing T.I. averaging. As the 10 MHz time-base was PLLed to 100 MHz for
increased singel-shot resolution, a noise source was added to the loop
for averaging to make sure that the phase modulation smoothed the
various phase-relationships between the clocks to produce meaningful
As for PPS sawtooth, the trouble with that approach is that the clock
generating the PPS isn't tuned to a suitable frequency for this form of
beat-note averaging to always achieve the needed resolution, and the
hanging bridge is when that beating has a "blind spot" and does not
produce useful delta-information at sufficient rate. You can also have
beating conditions causing many transitions, but few of these
transitions carry any useful information, because it sweeps a too low
set of phase-states to do any meaningfull averaging.
I have considered writing something up on this topic, but have not come
around to do it.
Thanks for the reference Bruce! I've seen it, so it was a good reminder.
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