[time-nuts] Metastability in a 100 MHz TIC

Richard H McCorkle mccorkle at ptialaska.net
Fri Jul 20 18:47:08 EDT 2007


Tom,
The GPS 1ns sawtooth corrections are accumulated in a 16F688 during the
120 second sample period with care that they match the same 120 1-second
10ns resolution phase samples collected. The accumulated sawtooth
correction is read at the end of the sample period before the sawtooth
correction for the next sample is sent by the GPS, scaled to match the
accumulated phase count resolution and added to the phase count before
the value is sent to the filter. This simplifies the design and has the
same effect as adding a scaled sawtooth correction matching the counter
resolution from each sample once per second. I do indeed make sure both
numbers cover the same samples and have the same LSB resolution before
adding so the results are valid.

Richard


> 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
>
> ----- Original Message -----
> From: "Richard H McCorkle" <mccorkle at ptialaska.net>
>
> 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,
> Richard
>
> -------------------------------- 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.
>
> /tvb
>
>
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