On 8/15/11 8:07 AM, Ulrich Bangert wrote:
Jim,
pardon to correct you but
no apologies needed..
It's if you measured the frequency (instantaneously) at one second
intervals, and calculated the standard deviation, that would be the
ADEV for tau=1 second.
is simply wrong in at least two aspects.
First: The measurements need to be the AVERAGE frequency measured
over a time interval of one second, which most easily is done by
setting the counter's gate time to one second.
Note that the misunderstanding about the difference between an
instantaneous measurement of frequency (which is possible too, but
not with counter based methods) and an averaging measurement of
frequency over a given time interval was one of the sources of the
upsetting discussion between Bruce and WarrenS in this group about
the tight PLL method. For that reason I think we need to be very
carefull in not to use terms as "instantaneous" in the wrong sense.
Second: It may be due to that you wanted to explain something as easy
as possible but I hope that it is clear to you that Allan's fame
relates to the fact that he found out that the standard deviation is
exactly the WRONG tool to use in this case and that he needed to
formulate a new species of deviation that is today called after him.
Corrections gratefully acknowledged..
(and, as well, the correction that it's actually first differences that
are being used, not difference against the nominal)
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