On 10/01/2012 11:13 PM, Tom Van Baak wrote:
I was wondering why the resolution in TI mode is so much limited, since
I never had any problems measuring 5 or 10 MHz frequencies with up to 12
figures on that counter.
Limited? The question is not so much counting the figures but asking if
the how much the figures count.
As a quic 'n dirty test, I tried to measure ADEV with my 53131A in
frequency mode using a gate time of 5.1 s for max resolution, and found
the noise floor had shifted down to 3...4E-12 at 10 sec, and going
further down to 5E-13 at 1000 sec from where it was equal to the TI mode
noise floor.
In frequency mode, the hp 53131A/53132A counters use a clever internal
averaging mode. It's mentioned in the manual. The readings it reports are
a highly oversampled mean frequency. This is nothing to complain about,
really. Typically, with a frequency counter you are only interested in a
smoothed averaged result. And these hp and other modern frequency counters
do this quite well.
So, what is the best method to use for ADEV? What instruments and setups
are you using, and what works best for you? How to get the max out of
the given instruments?
An ADEV measurement is rather different from average frequency measurement.
ADEV tries to tell you the variance, the deviations from the average
frequency, as a function of tau. It's purpose is to measure the noise, not
make the noise go away.
True, but the measure is also there to show you how credible your
frequency measures are, so what you wrote above does not add up with
what you then later say.
Turns out that ADEV fills two purposes, both giving a reference curve
for oscillators and estimating the precision of it's frequency measure.
If you change your frequency measure, you will need another estimator
for the precision since it needs to include the averaging effect that
your frequency stability method applies. That number would however not
give you the proper ADEV curve.
It's tricky business.
The difference between these ADEV and the deviation of the estimator is
a bias function, which is usually expressed as a multiplicative bias
function for these measures.
So the more a frequency counter averages (in order
to give you a smooth average frequency) the more it is actually suppressing
the very variations that you are trying to measure. In order to gain
"precision" these frequency counters are removing part of the "variance";
ADEV is completely ignores precision and is only concerned with the variance.
Which is true when we talk about the AVAR/ADEV reference. You are
confusing things a little when you talk about ADEV and then say
variance, it's like saying AVAR and deviation.
This is why one-shot phase meters or time interval counters give a more pure
view of oscillator performance. Yes, they tend to report a little to a lot
more noise -- but that's because there *is* more noise. An oversampling
frequency counter takes the liberty to average away as much noise as it
possibly can, to suppress the short-term variations, and present just a
single value as the one true answer.
It's a matter of learning what the tool does for you and what it doesn't
do for you.
I rarely see the frequency measures with the ADEV next to it or even
better... under it.
Cheers,
Magnus
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