[time-nuts] Question about frequency counter testing
Oleg Skydan
olegskydan at gmail.com
Fri May 11 10:42:40 EDT 2018
Hi
--------------------------------------------------
From: "Bob kb8tq" <kb8tq at n1k.org>
> The most accurate answer is always “that depends”. The simple answer is
> no.
I have spent the yesterday evening and quite a bit of the night :) reading
many interesting papers and several related discussions in the time-nuts
archive (the Magnus Danielson posts in "Modified Allan Deviation and counter
averaging" and "Omega counters and Parabolic Variance (PVAR)" topics were
very informative and helpful, thanks!).
It looks like the trick to combine averaging with the possibility of correct
ADEV calculation in the post processing exists. There is a nice presentation
made by prof. Rubiola [1]. There is a suitable solution on page 54 (at least
I understood it so, maybe I am wrong). I can switch to usual averaging
(Lambda/Delta counter) instead of LR calculation (Omega counter), the losses
should be very small I my case. With such averaging the MDEV can be
correctly computed. If ADEV is needed, the averaging interval can be reduced
and several measurements (more then eight) can be combined into one point
(creating the new weighting function which resembles the usual Pi one, as
shown in the [1] p.54), it should be possible to calculate usual ADEV using
such data. As far as I understand, the filter which is formed by the
resulting weighting function will have wider bandwidth, so the impact on
ADEV will be smaller and it can be computed correctly. Am I missing
something?
I have made the necessary changes in code, now firmware computes the Delta
averaging, also it computes combined Delta averaged measurements (resulting
in trapezoidal weighting function), both numbers are computed with
continuous stamping and optimal overlapping. Everything is done in real
time. I did some tests. The results are very similar to the ones made with
LR counting.
[1] http://www.rubiola.org/pdf-slides/2012T-IFCS-Counters.pdf
E. Rubiola, High resolution time and frequency counters, updated
version.
All the best!
Oleg UR3IQO
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