[time-nuts] PIC Divider Performance (was Re: time-nuts Digest, Vol 56, Issue 71)

Tom Van Baak tvb at LeapSecond.com
Tue Mar 31 01:48:43 UTC 2009

> Over 1000 measurements at 1 PPS, I got a standard deviation of 46.9 ps.
> The delta between minimum and maximum readings was 330 ps.  Since we
> were testing two independent dividers, I suppose you could divide the
> standard deviation by the square root of 2, which gives about 33 ps.
> I then did a test with a common PPS signal driving both inputs to the
> 5370B, with the stop signal run through a 4 meter cable for delay.  This
> should show the noise floor of the TIC.  The results for 1000 samples
> there were 23.0 ps standard deviation, and delta between min and max
> readings was 140 ps.
> Therefore, the PIC divider is a bit above the 5370B noise floor, but not
> much.


Thanks for doing this. That technique, and your results,
sound similar to what I did when I tested the divider. Cool.

I suspect the 5370 standard deviation will even decrease
further when the two dividers are sync'ed closer. Part of
what you're seeing is jitter in the 5370 timebase, I think.

One thing I'd like to point out -- if one is using a divider in
order to produce a 1PPS in order to compare against a
reference 1PPS using a ns or sub-ns TIC then the divider
should be better than the TIC or at least on par with it, but
it doesn't have to be perfect (if such a thing exists in the
world of timing).

Since I do most of my 1PPS timing with TICs like a sr620
(25 ps) or hp 53131 (750 ps) or hp 53132 (150 ps), the
divider noise is minimal to nothing compared to the jitter or
resolution of the counter itself. This is even more so when
the reference 1PPS comes from a GPS engine (as is the
case when a PIC divider is used in a GPSDO).


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