[time-nuts] [?? Probable Spam] Re: frequency stabilty question

Jim Lux jimlux at earthlink.net
Tue Aug 16 03:39:28 UTC 2011

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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