Hi, Edgardo --

Your counter's noise near t=1s will dominate most measurements you're likely
to make, so there's nothing to be gained by acquiring data at even faster
rates.  This will probably be true out to at least 10-100 seconds or even
longer, depending on what you're measuring.

For best ADEV fidelity you'd want to use TI measurements rather than
frequency measurements, but frequency measurements are obviously much easier
to set up.  They can give you a good overall idea of what's going on.

-- john
Miles Design LLC

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Edgardo, the minimum sample rate in practice is determined by the noise floor of the measurement system.  I would suggest that you do a baseline test by measuring the counter reference against itself with a tau0 (sample rate) of 1 second.  You'll see a best-case result and from it you can determine at which sample rate your noise floor is better than the predicted performance of the oscillator under test.  You'll probably find that you are measurement-noise limited below several hundred seconds.

Also, you'll generally get a lower noise floor doing time interval rather than frequency measurements, so you might experiment with that, which normally involves using a low-rate signal such as PPS for "start" and "stop" channels.

John

On Nov 14, 2012, at 8:43 PM, Edgardo Molina xe1xus@amsat.org 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.

In TI mode, the noise floor is determined by the resolution of your
counter.
If the resolution is 500ps, you are looking into 5E-10 at t=1 second, if
it is 200ps then 2E-10 etc.
When you increase t by a factor of 10, your noise floor goes down by the
same factor of 10.
That is, with the same 200ps counter, you get 2E-11 at t=10 sec, 2E-12
at t=100 sec etc.
Just don't expect that to continue endlessly. There are other limiting
factors to the ADEV noise floor that come into play like thermal drift,
including the cables.

A very interesting option with TIMELAB is that you can use your 53132A
counter in continuous printing mode via RS232. This mode is faster than
what GPIB gets you. And, you don't even need a GPIB interface... that's
really cool, isn't it?

Last but not least, somehow you have to figure out what you are actually
measuring, the DUT and / or your reference, or simply the noise floor of
your test setup. This can be the hardest part. You might want to compare
your test results with some trustworthy reference measurements like
this: http://www.leapsecond.com/museum/manyadev.gif

Adrian

John Miles schrieb:

Adrian,

Part of the reason for your "astoundingly low" results is because the 53132A is rapidly averaging samples internally in order to give a more precise frequency measurement. While this trick is good for determining frequency accuracy it is bad for determining stability. What I mean is that averaging, by definition, partly removes noise (removes "instability") in the samples being collected.

In much of science reducing noise is a good thing. But when making instability measurements of clocks and oscillators the noise is usually what you are trying to measure! So getting rid of it is not always a good thing. This is especially true when the device is removing noise without telling you; it can lead to false conclusions.

As you saw, when you use heavily smoothed frequency data as input to an ADEV calculation you will get unusually low stability numbers. This is not to say the numbers are completely meaningless. But at least it's not correct to compare heavily averaged frequency measurements with actual instantaneous phase measurements.

/tvb

Hi

Since many "interesting sources" are at 5 or 10 MHz, the magic of the 5313x in frequency mode goes away. They don't get much processing gain when the source and reference have edges at the same place. Stick with time measurements for ADEV.

Bob

On Nov 14, 2012, at 9:59 PM, Adrian rfnuts@arcor.de wrote:

Adrian,

On 15/11/12 03:59, Adrian wrote:

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

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:

Dear John Miles, John Ackermann, Adrian, Tom and Bob,

Thank you all for your wisdom on this topic. I have enjoyed the weekend getting graphs from my frequency standards taking care of your advise and comments. I will soon be able to post them to my newborn web page. Details coming soon.

Best regards,

Edgardo Molina
Dirección IPTEL

www.iptel.net.mx

T : 55 55 55202444
M : 04455 10045822

Piensa en Bits SA de CV

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On Nov 14, 2012, at 7:43 PM, Edgardo Molina xe1xus@amsat.org wrote:

Dear John Ackermann,

I find your reply to my request to be very interesting. Without fear of asking. May I know how do I achieve a Tau 0 measurement in a 1 second interval with my setup? (HP-53132A + TimeLab).

Please tell me if I am wrong.  I assume I would take the 10 MHz output from the 53132A and plug it into the Channel 1 input, while the external reference of 10 MHz is disconnected.

As far as I am concerned, TimeLab does not allow a value of 0 for Tau. If I enter zero as the measurement time duration I get an error and actual acceptable Tau range. The smaller value is in the order of 1E-6s. Am I getting it right? I am sorry for my confusion.

Regards,

Edgardo Molina
Dirección IPTEL

www.iptel.net.mx

T : 55 55 55202444
M : 04455 10045822

Piensa en Bits SA de CV

Información anexa:

CONFIDENCIALIDAD DE INFORMACION

Este mensaje tiene carácter confidencial. Si usted no es el destinarario de este mensaje, le suplicamos se lo notifique al remitente mediante un correo electrónico y que borre el presente mensaje y sus anexos de su computadora sin retener una copia de los mismos. Queda estrictamente prohibido copiar este mensaje o hacer usode el para cualquier propósito o divulgar su en forma parcial o total su contenido. Gracias.

NON-DISCLOSURE OF INFORMATION

This email is strictly confidential and may also be privileged. If you are not the intended recipient please immediately advise the sender by replying to this e-mail and then deleting the message and its attachments from your computer without keeping a copy. It is strictly forbidden to copy it or use it for any purpose or disclose its contents to any third party. Thank you.

On Nov 14, 2012, at 8:13 PM, John Ackermann N8UR jra@febo.com wrote: