[time-nuts] Measuring ADEV with a PM6681 or CNT-81
Tom Duckworth
tomduck at comcast.net
Sat Oct 25 01:10:57 UTC 2008
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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