[time-nuts] Modified Allan Deviation and counter averaging

[Poul-Henning Kamp]

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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
wrong.

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

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