On 15/08/11 09:42, WB6BNQ wrote:
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.
You can extend the graph downwards if you identified the noise sources
and their levels properly in the graph. This should always be done with
due care, and many times it is possible to measure, so that is preferred.
ADEV in itself doesn't give you a promise, but rather you need it as a
tool to separate noise-sources (together with MADEV for some noises) in
order to identify the levels of these noise sources and only then you
can go back to the graph and predict it.
So, you should not give general recommendations like that "extend freely
below 1 s" as it can fool you. Also, for shorter taus your measurement
rigg may be setting the limit, and not the oscillator... if you have a
quality oscillator. So, be careful there.
Cheers,
Magnus
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