[time-nuts] Question about frequency counter testing

Oleg Skydan olegskydan at gmail.com
Thu May 10 13:44:42 EDT 2018


Bob, thanks for clarification!

From: "Bob kb8tq" <kb8tq at n1k.org>
> If you collect data over the entire second and average that down for a 
> single point, then no, your ADEV will not be correct.

That probably explains why I got so nice (and suspicious) plots :)

> There are a number of papers on this. What ADEV wants to see is a single 
> phase “sample” at one second spacing.

After I read your answer I remembered some nice papers from prof. Rubiola, 
me bad - I was able to answer my question by myself. When we take a single 
phase "sample" at start and at end time of the "each tau" it is equivalent 
to summing all timestamps intervals I collect during that "tau", but by 
doing LR processing I calculate *weighted* sum, so the results will differ. 
So, it appears "ADEV" calculations is PDEV (parabolic) in reality, because 
of the current firmware processing.

I made a test with two plots for illustration - one is the classical ADEV 
(with 2.5ns time resolution), the second one with LR processed data (5e6 
timestamps per second). Both plots are made from the same data. It is 
obvious the classical ADEV is limited by the counter resolution in the left 
part of the plot. It is interesting is it possible to use the 4999998 extra 
points per each second to improve counter resolution in ADEV measurements 
without affecting ADEV?

Thanks!
Oleg UR3IQO 
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