Rick,
"continuous count" as in counting/time-stamping each individual cycle
forms a sample-rate limit. However, this is not what is meant with
continuous conting today, as that is that you have a continuous
time-stamping for some time-base. In that some number of counted cycle
(+/- 1) occurs between each time-stamp. Unless one attempts to use
time-base very near the maximum sample rate per second, it cease to be a
practical concern as one does not want to miss samples.
I have a counter that can time-stamp at 10 MSa/s and 13.333 MSa/s
depending on mode. I extremely rarely use that even close to the
extreme, as continuous counting I normally need is maybe up to 100 Sa/s.
Cheers,
Magnus
On 2021-11-10 01:53, Richard (Rick) Karlquist wrote:
Let me just mention that when I worked at the HP Santa Clara
Division counters section, they came out with a "feature"
that they called "continuous count". However, it was limited
to something like 3 MHz. So a 100 MHz counter would only
continuously count signals below 3 MHz.
So you need to verify for what bandwidth your specific counter
model is truly doing continuous count.
Rick N6RK
On 11/9/2021 2:29 PM, Magnus Danielson via time-nuts wrote:
Hi Erik,
On 2021-11-09 18:26, Erik Kaashoek wrote:
As far as I understood the ADEV at a Tau of 1 second is a statement
about the amount of variation to be expected over a one second
interval.
Rather, the variation of readings of a frequency estimation done over
a span over 1 second.
It would be nice if we would be able to measure a frequency in an
infinite short interval but any frequency measurement takes time.
Turn out that basic white noise and systematic noise will limit our
frequency resolution to form a 1/tau limit slope, so infinite short
interval will bury it well into that noise whatever we do.
What if the frequency counter does a complete measurement of a
frequency source every second and all the variation within that
second is hidden because of the "integration" that happens over the
second?
That is what happens, but that is not what the ADEV is about, it's
about the variations of these measures as we look for a bunch of
them. So if we now have say 1000 of these frequency estimates, how
much variations in these can be contributed to the random noise of
the source, and to analyse that, we need at least a tool like ADEV
since standard deviation will not even converge for white and flicker
phase noise modulation.
What ADEV actually aims to do is to provide a low-frequency
spectroscopy method at a time when time-interval counters was about
the only tool at hand, and even those where very rare. We now have a
much wider palette of tools, but ADEV is relevant for how we measure
frequency stability and a few other applications.
This is specially the case with continuous time-stamping counters.
They can provide a precise number by applying statistical methods on
many measurements done during one second but they can not provide
information exactly at the end of a second.
Is this kind of statistical measurement over a period of a second
still valid for determining the ADEV at the Tau of one second of a
frequency source?
Not for ADEV, but if you use averaging counter you get the result of
MDEV and for linear regression / least square counter you get the
response of PDEV. That is the result of various statistical measures
and then applying the ADEV processing on these frequency estimates.
The upcoming IEEE Std 1139 revision, which is in approval process now
include language to reflect that.
Or should there be a correction factor depending on the method used
in the frequency counter?
Yes, you then need to use the appropriate bias function for ADEV/MDEV
and ADEV/PDEV to convert between these scales. Knowing the response
of ADEV, MDEV and PDEV for a particular noise-type which is dominant
at the tau of interest, you can readily convert between them by
forming the bias functions.
You may find NIST SP-1065 a useful and handy tool, even if it does
not cover the more recent work such as PDEV.
https://www.nist.gov/publications/handbook-frequency-stability-analysis
I tried to read some scientific studies on this subject but I am not
smart enough to understand.
Hope one of you can provide some information.
It is scattered over a large number of articles, and quite a lot of
folks get confused. Hopefully the updated IEEE Std 1139 will be of
aid to you. It also has lots of useful references.
Cheers,
Magnus
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