[time-nuts] Modified Allan Deviation and counter averaging

Poul-Henning Kamp phk at phk.freebsd.dk
Sat Aug 1 16:32:52 EDT 2015

In message <49C4CCD3-09CE-48A4-82B8-9285A43814E3 at n1k.org>, Bob Camp writes:

> The approach you are using is still a discrete time sampling
> approach. As such it does not directly violate the data requirements
> for ADEV or MADEV.  As long as the sample burst is much shorter
> than the Tau you are after, this will be true. If the samples cover < 1%
> of the Tau, it is very hard to demonstrate a noise spectrum that
> this process messes up.

So this is where it gets interesting, because I suspect that your
1% "lets play it safe" threshold is overly pessimistic.

I agree that there are other error processes than white PM which
would get messed up by this and that general low-pass filtering
would be much more suspect.

But what bothers me is that as far as I can tell from real-life
measurements, as long as the dominant noise process is white PM,
even 99% Tau averaging gives me the right result.

I have tried to find a way to plug this into the MVAR definition
based on phase samples (Wikipedia's first formula under "Definition")
and as far as I can tell, it comes out the same in the end, provided
I assume only white PM noise.

But I have not found any references to this "optimization" anywhere
and either I'm doing something wrong, or I'm doing something else

I'd like to know which it is :-)

Poul-Henning Kamp       | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG         | TCP/IP since RFC 956
FreeBSD committer       | BSD since 4.3-tahoe    
Never attribute to malice what can adequately be explained by incompetence.

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