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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