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

[Jim Lux]

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)