The FatPPS should be pretty stable when you trigger off the leading edge
of the output, since the delays are in silicon. But the trailing
(usually falling) edge of the output is a different matter as the pulse
width is controlled by a simple RC circuit and is likely to wander all
over the place.
John
On 3/29/22 05:10, Hal Murray wrote:
I'm putting some data collection toys back together.
I've got a TAPR TICC watching a couple of PPS signals.
The clock for the TICC comes from a HP 5334B with the good crystal option.
It's not right-on in frequency, but there is no control voltage that might be
wiggling around so it should be stable.
This setup is half backwards. I'm using the PPS to calibrate the frequency of
the TICC reference clock, then assuming the reference clock is short term
stable and using it to examine the short term characteristics of the PPS
signals.
The green line is the PPS signal from a Sure demo board with a SKG16B GPS chip.
http://www.leapsecond.com/pages/MG1613S/
The wander in the green line is a combination of warmup and temperature. The
5334B was off for several months. This graph starts on day 18. The wiggles
are easy to see on an expanded scale but that blows the FatPPS signal off
scale. The daily temperature swings are roughly the same size as the daily
drift. (Another graph for another message...)
The PPS signals are connected via clip leads to handy places. One of those
was after a FatPPS. That signal is upside down so the TICC is triggering on
the trailing edge. No problem, I thought, I can just fix that up when
graphing things. :)
After I saw the initial results, I put a temperature probe on the FatPPS. The
spike at 18 hours is the direct morning sun peeking through the houses and
trees across the street.
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