[time-nuts] Allan Variance versus Allan Deviation

[Magnus Danielson]

On 03/28/2011 02:36 PM, Bruce Griffiths wrote:
> 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.

It's also common to have plots saying "Allan Variance" and plotting the 
Allan Deviation.

Since Allan Deviation is the square root of Allan Variance, 2E-11 Allan 
Variance is about 4.5E-6 in Allan Deviation so an Allan Deviation of 
3E-11 is better than an Allan Variance of 2E-11... by far.

Use Allan Deviation through-out.

Cheers,
Magnus