[time-nuts] Metastability in a 100 MHz TIC

Tom Van Baak tvb at LeapSecond.com
Fri Jul 20 10:57:21 EDT 2007

> Question: Since you are comparing
> TWO oscillators by means of an THIRD oscillator (the tic's time base),
> does the tic's time base stability influence your measurement results or
> not?

Partly yes, for tau < 1 second.
Mostly no, for tau > 1 second.

> Clearly so, if you think about it for a while. With this arrangement it
> is not possible to decide whether 1pps a or 1pps b or the tic's time
> base are responsible if you notice statistical fluctuations in the
> measurement results. The measured results will be an statistical average

Perhaps I misunderstand your setup, but it seems to the
third timebase is only used for a time interval measurement,
not the time measurement. Thus the requirements on its
stability are much less than the two 1pps sources. Think of
it not as a "timebase", but a "time interval base".

For example, suppose you want to measure 1pps sources
which are within 10 microseconds in phase, to a resolution
of 100 ps. To make an ADEV plot for tau 1 second to 1 day,
you need to collect days of data, but you only need a
TIC timebase that is accurate and stable to one part in ten to
the 5th, at tau of 10 us. Any cheap XO will do that, no?

You don't need cesium timebases for a 1pps TIC.


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