[time-nuts] frequency stabilty question
WB6BNQ
wb6bnq at cox.net
Mon Aug 15 07:42:25 UTC 2011
Tom Van Baak wrote:
> > Does this mean the observations made were at the very begining
> > and the very end of the 1 second time.
>
> Correct.
>
> > If so what value about all the values in between? What happens
> > if the oscillator deviated far worse than this during the interrim.
>
> Oscillator behavior in between 1 second intervals is unknown. As
> you say it could be much worse, or it could be much better. If you
> need to know for sure, then you must re-run the measurement
> and collect data at shorter intervals this time.
>
> Specifically, if you make only one measurement per second you
> can compute ADEV(1 second), or actually ADEV(N seconds).
>
> By contrast, if you collect 100 measurements per second you
> can compute ADEV(0.01 x N seconds). This allows you to see
> how well the oscillator performs for intervals less than a second.
>
> > Or does the measurement consist of making measurements
> > every cycle during that 1 second and then entering all those
> > values into a formula that accounts for them all??
>
> If you had measurements every cycle it would be great. But not
> all measurement systems can measure every cycle. In the real
> world there are limitations on the rate of measurement. There
> are also limitations on the resolution of each measurement.
> With some instruments you can increase the rate but you then
> loose resolution, or visa-versa.
>
> These two limits dictate how short a tau you can plot and how low
> an ADEV you can observe. I can provide examples if you wish.
>
> /tvb
>
Hi Tom,
I was under the impression that the ADEV curves predict a confidence level
between measurement points based on the averaging of the noise over the time
between measurement points ?
For example, a quality oscillator would have more noise shown at times shorter
then 1 second, but the curve goes towards less noise as the time frame lengthens
out. The point being that the 1 second ADEV value would indicate that over a 1
second time frame the reliability of a measurement would be the ADEV of the 1
second time frame.
If the above is true and in relation to a frequency counter using a 1 second
gating period, then the issue of what is going on during the gating period is of
little concern or consequence if the time base is behaving as described by the
ADEV curve. Is that a correct observation ?
If Paul was going to make shorter measurement times, then the shorter time frame
ADEV values would, obviously, be important. I was not sure his question came
from the perspective you saw and that is why I was bringing up the above
questions.
Bill....WB6BNQ
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