[time-nuts] Metastability in a 100 MHz TIC

Richard H McCorkle mccorkle at ptialaska.net
Fri Jul 20 17:38:06 EDT 2007

Thanks guys for the input,
I wanted to clarify that the time interval being measured at each
GPS 1PPS is typically 2/5 of a 1.6us window (Rb/16) or 640ns at
setpoint. The filter input value is built by accumulating (not
averaging) 120 of these samples and sawtooth correcting the result
before filtering. This gives a filter input value with 10ns per
sample / 120 samples or 83ps per count resolution per 120-second
update. The original 16-bit filter was extended to 24-bits to
improve the filter resolution with the 23-bit DAC. The disciplined
oscillator stability using this inexpensive design is orders of
magnitude better than the 1e-11 per day specification of the
original Shera design.
  I am just powering up a similar version with a variable update
rate on an MTI 260 double oven OCXO to see what the stability looks
like with a respectable OCXO. (I finally found one MTI 260 out of
six I tested that didn’t jump in phase every few weeks!) Once I
have this unit up and stable and have some baseline data I will
pull out my Vectron and Bliley 100 MHz OCXOs and try Ulrich’s
suggestion of using one of them for the TIC clock to see if that
improves the disciplined oscillator stability. They both have
around 1e-9 stability and should be enough better than an XO to
see if further improvement in disciplined oscillator stability
is possible using this design.
  The reason I went this route was to see what improvements could
be made to create a high performance $50 controller using readily
available DIP packaged parts that could be assembled on a perf
card over a weekend by the average hobbyist. Age can do bad things
to the eyes and hands that make surface mount components and high
density IC pinouts hard to deal with for some older hobbyists.

Thanks again,

-------------------------------- Original Message ---------------------------------
Subject: Re: [time-nuts] Metastability in a 100 MHz TIC
From:    "Tom Van Baak" <tvb at LeapSecond.com>
Date:    Fri, July 20, 2007 6:57 am
To:      "Discussion of precise time and frequency measurement" <time-nuts at febo.com>

> 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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