[time-nuts] Allan Deviation

Bruce Griffiths bruce.griffiths at xtra.co.nz
Mon Mar 28 21:59:26 UTC 2011


Martyn Smith wrote:
> Hi,
>
> Thanks for the answers but I'm still confused.
>
> The Stanford PRS10 has an Allan Variance of 2E-11 but a Symmetricomm 
> unit has a Allan Deviation of 3E-11.
>
> So according to the answers you gave, the Symmetricomm unit has a 
> stability thousands of times better than the PRS10.
>
> This, dispite the Symmetricon unit having 25 to 40 dB worse phase 
> phase noise than the PRS10 (at 1 and 10 Hz offsets).
>
> Surely that's not possible since the PRS10 is pretty good in the first 
> place.
>
> Steve
>
The PRS10 specs say Allan Variance but the actual figures given are for 
its Allan Deviation.
No rubidium commercial standard is as noisy as their specs would 
otherwise imply.
This error is still surprisingly common.

Bruce




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