[time-nuts] Allan Variance vs Allan Deviation

Magnus Danielson magnus at rubidium.dyndns.org
Tue Mar 29 13:17:57 EDT 2011

On 03/29/2011 04:01 PM, Martyn Smith wrote:
> Hello,
> Sorry that I'm harping on about this subject but I want to make sure my
> brochures are correct.
> I use the SR620 to measure Allan Variance. When I look at the SR620
> manual the formulae they give for Allan Variance is actually root Allan
> Variance.
> So root Allan Variance is the same as Allan Deviation.
> So everyone (including me), when we quote Allan Variance, we really mean
> root Allan Variance or Allan Deviation.
> Does that make sense.?

Not quite. Some people correctly use Allan Variance and Allan Deviation.

You are correct in that people say Allan Variance and report Allan 
Deviation numbers.

Knowing this, use your good judgement to determine what is Allan 
Variance and Allan Deviation. Usually you have values so far away from 
each other, that it should not be too hard to figure out which is which.

For training, look at Allan Deviation numbers of some similar device, 
consider the tau should be the same or near each other.


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