[time-nuts] Fluke/Pendulum PM6680/CNT-80 measurement instability/outliers

[Attila Kinali]

Hi,

Just for fun, I've set up a PM6680 to measure a 10MHz OCXO (Trimble 34310).
First I used the 10MHz output of an Trimble UCCM as reference (green trace
in the attached file, drift removed, offset to match second trace), measuring
with a gate time of 1s, one measurment every approximately 1.1s.
As you can see, there are a couple of outliers, evenly spaced in 1Hz steps.
To make sure, these weren't comming from the GPSDO (it's more noisy than
I thought it would be), I switched to the PM6680's internal reference
(violet trace) and got basically the same outliers.

If these outliers would have been only at 1Hz, I would have said they
are metastability issues in the coarse counter (which runs at 10MHz).
But getting metastability to last more 100ns is exteremly unlikely,
yet I get outliers that are 6Hz(=600ns) out. The probability for metastability
lasting that long is lower than being struck by a meteorite....much lower!

So, the question is, what am I looking at? What causes those outliers?
Does someone have a good idea?

				Attila Kinali
-- 
It is upon moral qualities that a society is ultimately founded. All 
the prosperity and technological sophistication in the world is of no 
use without that foundation.
                 -- Miss Matheson, The Diamond Age, Neil Stephenson
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