[time-nuts] Allan Variance versus Allan Deviation

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 28 08:36:02 EDT 2011

Martyn Smith wrote:
> Hello,
> I'm trying to compare the stability specifications of two oscillators.
> The first spec is 2 x 10E-11 Allan Variance.
> The second spec is 3 x 10E-11 Allan Deviation.
> As Allan deviation (ADEV), is the square root of Allan variance, I 
> thought the first spec is the better of the two, but a collegue 
> disagrees, saying 3x10E-11 Allan Deviation is a better spec than 2 x 
> 10E-11 Allan Variance.
> I'm sure the million experts at there will give me the correct answer
> Regards
> Steve Jones
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Strictly you are correct, however some early datasheets (eg Crystek, ) 
say Allan variance whilst they actually tabulated its square root or 
Allan deviation.


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