[time-nuts] ADEV vs MDEV
bruce.griffiths at xtra.co.nz
Sat Feb 6 04:53:17 UTC 2010
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
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.
Bob Camp wrote:
> 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.
> 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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