[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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