[time-nuts] Question about frequency counter testing

Bob kb8tq kb8tq at n1k.org
Thu May 10 14:12:39 EDT 2018


Hi

> On May 10, 2018, at 1:44 PM, Oleg Skydan <olegskydan at gmail.com> wrote:
> 
> 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?

The most accurate answer is always “that depends”. The simple answer is no. If you take a look at some of the papers from 
the 90’s you can find suggestions on doing filtering on the first point in the series. The gotcha is that it does impact the first
point. The claim is that if you do it right, it does not impact the rest of the points in the series. 

Bob


> 
> Thanks!
> Oleg UR3IQO 
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