Hi Magnus,
when I noticed the difference between frequency and time interval mode
on my 53131A, I compared some sources in both modes.
There was little difference between both modes where the signal was
above the TI mode noise floor.
Below that t-range (that is, below the point where in TI mode the signal
is buried by the 1/f slope noise floor), the frequency mode showed
significantly lower values, but I had nothing at hands to compare them
with. At least they looked plausible.
In the meantime I could replace the 53131A with a Quartzlock A7 / HP
5370B combination.
You might understand that I was no longer motivated to investigate
further, especially because the '131 showed much less 'improvement' as a
'132 which I saw a plot of but don't have access to.
Cheers,
Adrian
Magnus Danielson schrieb:
Adrian,
On 15/11/12 03:59, Adrian wrote:
Hi Edgardo,
however, the 53132A is considerably more sensitive in frequency mode as
we have recently discovered.
I saw an astoundingly low ADEV noise floor of some 6E-13 at t=10 sec as
opposed to only 2.5E-11 in TI mode.
I don't have a 53132A, only a 53131A that I don't use anymore for ADEV,
so I can't be more specific.
No. The filtering that the 53132A does on frequency data does not make
it more sensitive in the ADEV context, it pre-filters the data to
improve frequency reading, yes, but that pre filtering cause bias in
the ADEV measurement and when you compensate for that bias you are
back where you started. We have been over this many times in the
history of the list, there is papers explaining it, so let's not again
spread this misconception. The 53132A is still just a 150 ps
resolution counter, which forms the 1/f limit slope on the ADEV plot
(1/f^2 power slope).
Doing a ADEV with the 53132A frequency mode gives you the stability
measure of that measurement mode, true, the trouble is that the
filtering will also applied to the source, so we do not get closer to
the source while we see an lower value in the plot and fools ourselves
that we got closer. The only thing we have achieved is a lower value,
but the relative distance between source and counter noise limit
remains the same.
The improvement you may do is to combat trigger jitter, so squaring
the signal up could get you a bit closer to the counters abilities.
If you want to break the trigger jitter and resolution noise limit of
the pure counter, besides squaring up you got to look into mixer
enhancements, and you end up doing the Dual Mixer Time Difference
(DMTD) game, but filterings such as that in the 53132A isn't it.
This filtering, which also applies to the later Pendulum counters such
as CNT-90, isn't a bad thing when you want to provide a higher
frequency resolution while still maintaining relatively high reporting
rate. It's actually a very good method. It's just that you don't get
"pure" ADEV that way, and we already have a systematic method of doing
something similar called the modified Allan Deviation (MDEV) which
actually builds on such filtering, but applied in a more systematic
way. The MDEV has known different behaviour to the noises compared to
ADEV, and this fact is used to separate noise-variants better. Dr.
Allan even is eager to point out that MDEV is actually fixing what is
broken with ADEV, and we should be using MDEV. I tend to agree.
This type of prefiltering of 53132A frequency readings will not
improve MDEV measures either.
So, that is the wrong tool for improving our measures.
The only thing is that it for most of the quality stuff we measure
won't do much harm, since it only dominates the part of the curve
where we are we are usually counter limited, so we just got another
shape of that, but it adds nothing and we gain nothing. Hence, it's
not helping us one bit. Only subtle benefit would be that auto-scaling
could work a little better, but I doubt it's a strong enough reason.
Please enlight me if I missed something important.
Cheers,
Magnus
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