[time-nuts] Measuring ADEV with a PM6681 or CNT-81
Ulrich Bangert
df6jb at ulrich-bangert.de
Thu Oct 30 12:02:53 UTC 2008
Stefan,
> Apart from a suitable divider, I'd....
If you a dedicated follower of PIC microcontrollers you find something
suitable here on Tom's pages
http://www.leapsecond.com/pages/ppsdiv/
Otherwise if you like AVRs then you can find something on my pages
http://www.ulrich-bangert.de/html/downloads.html
Suitable sine to ttl circuits designed by Bruce Griffiths are to be
found at
http://www.ko4bb.com/~bruce/CLKSHPR.html
Best regards
Ulrich
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Heinzmann,
> Stefan (ALC NetworX GmbH)
> Gesendet: Donnerstag, 30. Oktober 2008 12:42
> An: Discussion of precise time and frequency measurement
> Betreff: [!! SPAM] Re: [time-nuts] Measuring ADEV with a
> PM6681 or CNT-81
>
>
> Thanks to all who replied, and sorry for the delay. I was
> away and unfortunately unable to follow the discussion until today.
>
> From Tom's comments and others I extract that the PM6681 can
> only be used for measuring the ADEV when employing the picket
> fence technique. A direct measurement is prevented by the
> counter's dead-time. Apart from a suitable divider, I'd
> probably need to write my own piece of software for massaging
> the data so that it can be used for an ADEV plot. Unless of
> course there is some software readily available that includes
> handling of the picket fence technique.
>
> My other idea of using two counters in lockstep, so that they
> measure alternate periods of the signal, hasn't been
> commented on. Is it a silly idea, apart from the fact that it
> needs two counters? Has anyone tried anything like it?
>
> Thanks and Cheers
> Stefan
>
>
> -----Ursprüngliche Nachricht-----
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Duckworth
> Gesendet: Samstag, 25. Oktober 2008 03:11
> An: 'Discussion of precise time and frequency measurement'
> Betreff: Re: [time-nuts] Measuring ADEV with a PM6681 or CNT-81
>
> Stefan,
>
>
>
> Well, I have been sort of staying out of the recent
> discussions regarding oscillator stability measurements,
> ADEV, etc., but here goes my two cents worth. I hope it
> doesn't muddy the waters too much and is somewhat helpful.
>
>
>
> There is a fundamental problem with trying to do ADEV (Allan
> Deviation) with a counter that has a gated measurement engine
> (all standard counters). The problem is that when the count
> engine gate closes (so the counter can compute the number of
> events, report the result as a frequency, and clear its
> registers), the counter is blind (dead time) to any signal at
> the input. Allan deviation REQUIRES than a minimum number of
> measurements be made back-to-back (no dead time between
> measurements) in order to capture nondeterministic
> fluctuations of the signal you're trying to measure. This
> requires a measuring device (counter) that continuously
> records ALL events. It does this by time-stamping a fixed
> period in a separate register in the counter along with the
> input register (the signal being measured), and a time-base
> register that records the frequency the signal is to be
> compared with. There is no 'gate' as such in a time-stamping counter.
>
>
>
> Most stability measurements, longer than say 100 seconds, are
> made in the frequency domain because energy changes, as a
> result of heat, are by far the predominate cause of stability
> (aging) issues (see below). Stability issues within shorter
> time periods, say <100 sec., are often nondeterministic, and
> cannot be accurately quantified in the frequency domain, but
> must be measured in the time domain, using statistical
> weighting. In other words, unpredictable, and often little
> understood, events (Shot and thermal noise in the active
> devices, random variations in the frequency-determining
> elements, cosmic rays, etc.) often predominate short term
> stability measurements and must be described statistically,
> as their occurrence and duration are random. Allan deviation
> is a widely accepted time-domain statistical measurement
> whose calculated results compare well with the more common
> frequency domain measurement of longer time periods.
>
>
>
> Allan deviation measurements are based on the sample variance
> of the fractional-frequency fluctuations. Without specifying
> the number of samples N, and the repetition interval T, for
> measurements of duration t, the measure of frequency
> stability is dimensionless and would converge to a
> meaningless limit. Secondly, some actual noise processes
> contain substantial fractions of the total noise power in the
> instantaneous fractional-frequency range below one cycle per
> year. In order to improve compatibility of data, it is
> important to specify a particular N and T. The Allan variance
> chooses N=2 and T=t (i.e. no dead time between measurements).
> A good estimate can be obtained by a limited number, m, of
> measurements (m=*100). Root Allen variance is expressed as a
> quantity divided by the square of the measurements of
> duration t, (i.e., 3 x 10-11/*t).
>
> .
>
> Classical variance diverges for commonly observed noise
> processes, such as random walk (i.e., the variance increases
> with an increasing number of data points). The advantage with
> the Allan variance is that it:
>
> * converges for all noise processes observed in precision
> oscillators;
>
> * has straightforward relationship to power law
> spectral density
> (spectral density of the frequency fluctuations);
>
> * is easy to compute, and;
>
> * is faster and more accurate in estimating noise
> processes than
> the Fast Fourier Transform.
>
>
>
> Aging in quartz crystal oscillators is caused by changes in
> either the quartz crystal itself or the associated components
> found in the oscillator assembly. Aging is the result of a
> combination of several factors having complex, and only
> partially understood, components that effect the aging
> specification. Effects can include the cut (orientation) of
> the crystal; vibration modes; frequency of cut size;
> temperature of operation and variations of temperature; drive
> energy; gravity; physical orientation;
> shock; electromagnetic interference; diffusion of impurities
> and the outgassing of the quartz crystal; the glass or
> ceramic base; the adhesive used to mount the quartz; metal
> migration from the electrodes into the quartz surface; stress
> relief of the crystal mounts; changes of electric component
> values over time; and, voltage regulation.
>
>
>
> This is probably more information than you really wanted, but
> oh well, enjoy.
>
>
>
> Tom
>
> Tom Duckworth
>
> 510-886-1396
>
>
>
>
>
> -----Original Message-----
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On Behalf Of Heinzmann,
> Stefan (ALC NetworX GmbH)
> Sent: Friday, October 24, 2008 2:03 AM
> To: Discussion of precise time and frequency measurement
> Subject: Re: [time-nuts] Measuring ADEV with a PM6681 or CNT-81
>
>
>
> Thanks, Tom!
>
>
>
> Apart from the specifics with TimeView, how does one go about
> measuring ADEV with a counter that does have a dead-time? Are
> there tricks that can be played, either with a special
> measurement setup or with some data postprocessing? Or with
> two counters that are somehow made to cooperate?
>
>
>
> Sorry if I'm asking the obvious, you can see that I'm a greenhorn. ;-)
>
>
>
> Cheers
>
> Stefan
>
>
>
> -----Ursprüngliche Nachricht-----
>
> Von: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] Im Auftrag von Tom Duckworth
>
> Gesendet: Donnerstag, 23. Oktober 2008 22:19
>
> An: 'Discussion of precise time and frequency measurement'
>
> Betreff: Re: [time-nuts] Measuring ADEV with a PM6681 or CNT-81
>
>
>
> Stefan,
>
>
>
> I am retired from Pendulum Instruments and could answer your
> question but I
>
> have instead refered your question to a current engineer with
> the company
>
> and you should receive an answer from them shortly. They are
> the experts for
>
> this question.
>
>
>
> Tom
>
> Tom Duckworth
>
> 510-886-1396
>
>
>
>
>
> -----Original Message-----
>
> From: time-nuts-bounces at febo.com
> [mailto:time-nuts-bounces at febo.com] On
>
> Behalf Of Heinzmann, Stefan (ALC NetworX GmbH)
>
> Sent: Thursday, October 23, 2008 10:12 AM
>
> To: time-nuts at febo.com
>
> Subject: [time-nuts] Measuring ADEV with a PM6681 or CNT-81
>
>
>
> Hi all,
>
>
>
> I hope you can clear up some confusion that I have regarding ADEV
>
> measurement. I was under the impression that you need a
> counter that is able
>
> to timestamp each rising edge of the clock under test, or equivalently
>
> measure period time continuously with no dead time. Now,
> while the CNT-90
>
> can do this, the CNT-81 ( = PM6681 ) can't. Still, a message
> last December
>
> to the list here
>
> (http://www.mail-archive.com/time-nuts@febo.com/msg10963.html)
> seems to
>
> indicate that using TimeView you can have ADEV plotted with
> the "lesser"
>
> model, too. I have a PM6681 and TimeView, but I couldn't find
> out how to do
>
> it. Maybe it can't be done.
>
>
>
> So what's the deal on this? Can it be done, with or without
> TimeView, and if
>
> yes, how? If this has been answered earlier, a link would be
> just fine.
>
>
>
> Thanks and cheers
>
> Stefan
>
>
>
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