[time-nuts] ADEV vs MDEV

Bruce Griffiths bruce.griffiths at xtra.co.nz
Sat Feb 6 06:12:14 UTC 2010


The paper on the filter approach to ADEV etc is:


Bruce Griffiths wrote:
> _Correction_
> Oops, Its the CNT91 not the CNT81 that actually does the regression fit.
> How did you achieve MDEV ~1E-13 @10s with a counter rated at 50ps 
> single shot resolution?
> Bruce
> Bruce Griffiths wrote:
>> Read the data sheet and the various application notes/white papers on 
>> the Pendulum site.
>> The intrinsic resolution of the Pendulum counter (50ps) is slightly 
>> inferior to that of the SR620 and HP5370A/B.
>> What they do is statistically process the results of a series of 
>> measurements of the input phase taken at short intervals.
>> They actually fit a regression line to the resultant series of phase 
>> measurements.
>> This process inherently filters out some of the noise of the counter.
>> If it were possible to do the same thing using an SR620 or HP5370 the 
>> noise in the output resolution would be even lower.
>> If one is building a conventional DMTD one doesn't actually need 
>> resolution for the timestamping device/counter much better than 10ns 
>> or so to achieve a resolution of around 1E-13/Tau with say a 100Hz 
>> beat frequency and 10MHz inputs to the mixer/phase detector.
>> Bruce
>> Bob Camp wrote:
>>> Hi
>>> At least from what I've seen, the Pendulum's seem to work a bit 
>>> better than the other counters you mention. That may simply be a 
>>> function of their being designed much more recently.  It could also 
>>> be the issue of comparing beat up stuff on the bench to brand new 
>>> stuff on the bench. The CNT-81 is rated to have a much better single 
>>> shot time resolution than the others.
>>> Yes I realize that in no way addresses the question you asked.
>>> MDEV and ADEV measure slightly different things. Depending on what 
>>> you are looking for MDEV may give you better insight.
>>> Bob
>>> On Feb 5, 2010, at 10:59 PM, 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
>>>> _______________________________________________

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