[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 5 23:12:10 EST 2010

Pete Rawson wrote:
> Efforts are underway to develop a low cost DMTD apparatus with
> demonstrated stability measurements of 1E-13 in 1s. It seems that
> existing TI counters can reach this goal in 10s. (using MDEV estimate
> or 100+s. using ADEV estimate). The question is; does the MDEV tool
> provide an appropriate measure of stability in this time range, or is
> the ADEV estimate a more correct answer?
> The TI performance I'm referring to is the 20-25 ps, single shot TI,
> typical for theHP5370A/B, the SR620 or the CNT81/91. I have data
> from my CNT81showing MDEV<  1E-13 in 10s. and I believe the
> other counters behave similarly.
> I would appreciate any comments or observations on this topic.
> My motivation is to discover the simplest scheme for making
> stability measurements at this performance level; this is NOT
> even close to the state-of-the-art, but can still be useful.
> Pete Rawson

You cant use the CNT81 (or that of similar counters) output to calculate 
MDEV or ADEV, the internal averaging process within the counting 
interval negates this.
Enrico wrote a paper on why this is so which was later corrected by the 
Australians (Enrico got a weighting function wrong).
Conventional time interval counters like the SR620 and HP5370A/B are OK.

MDEV and ADEV can be viewed as measuring the ODEV at the output of 
bandpass filters with different bandpass shapes where the parameters of 
the bandpass filter used vary with Tau.


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