[time-nuts] ADEV

[Bob Camp]

Hi

I see ADEV not as a solution to a system design problem, but an oscillator measurement issue. If you look at the measures being used in the 60's, most of them had serious statistical flaws. You could measure them several ways and get multiple results. What ADEV gave us is a measure that could be done repeatably. You still have to do it right, but if you do it repeats.  If there is a systems rationale in the development of the measurement it's awfully well hidden in the early papers.

Bob


On Nov 13, 2010, at 8:53 AM, Magnus Danielson wrote:

> On 11/13/2010 02:20 PM, Mike Feher wrote:
>> I sure do agree, that with very low data rate systems it is significant. In
>> fact, when David Allan&  Fred Walls came up with the proposal of using this
>> measurement as an FOM for oscillators over 30 years ago, digital
>> communication rates were slow, and, the measurement was a good one.
> 
> ADEV can be trace back to 1966, but even prior to that similar estimates was being used in the scientific research, but the Allan article of 1966 provides part of the critical analysis which makes the M-sample analysis into the Allan-variance as we know it.
> 
> Their concerns where with oscillators and not data-communication which has its own set of problems and measures.
> 
>> Due to the filtering process within ADEV by collecting and integrating a large
>> number of samples, has a filtering effect of its own. Therefore, it can, and
>> will, miss the fact that there may be instantaneous phase transitions that
>> could cause havoc with high data rates and higher order PSK modulation
>> schemes.  So, again, I apologize, as I should have mentioned higher data
>> rates.
> 
> Data-rates is actually not particularly interesting, it is the dynamic properties regardless of rate, which is only a scale-factor. The property of phase-jumps is well covered in the MTIE measurement which is used along-side the TDEV measurement for telecommunication systems.
> MTIE provides the Maximum Time Interval Error... so for a window of length tau, what is the maximum difference between high and low? This is measured by taking the difference between max and min in a window, slide it over the data and take the maximum difference. This relates very well to buffer-size action and if converted over to a sine-tolerance curve (using f=1/(pi*tau) ) also can be made to match up with PLL responses.
> 
> Come to think of it, I have not seen any good Wikipedia article on it.
> 
> Anyway, one really has to understand what kind of measurement is adequate for the technical problem one is trying to address.
> 
> Cheers,
> Magnus
> 
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