WarrenS wrote:
As Bruce says "It remains a mystery" to him why this works.
It doesnt, it only appears to in a very restricted set of circumstances.
Not one of my best skills, but I'll try to explain it once again.
Now that they see it works, maybe someone else will be able to put this into
words that Bruce will be able to finally understand.
The only requirement needed for the Frequency data log to be give correct ADEV
readings, is to get good, Averaged, integrated, Frequency data, with no dead
time, and no aliasing, over the tau0 time period.
Each Tau0 Frequency sample is ideally completely independent from all others.
If it can do one right then it can get them ALL right.
In a single tau0 sample there is NO SUCH THING as a certain type of long term
noise, Just the average freq over that single time period.
Misleading as usual, your knowledge of statistics is woefully inadequate
leading to incorrect conclusions.
The crucial integration/averaging to get good tau0 data, that Bruce can
not see for some unknown reason, is done
Only in your imagination.
with an analog filter set to about the Tau0 Freq and by oversampling at about
about a 10 to one ratio, and averaging the oversampled frequency readings down
to tau0.
That doesn't work as it has the wrong transfer function.
It is not perfect, but plenty close enough for the plot to match the output of
the TSC 5120A over the whole tau range.
There are a few other subtle details on how to insure that aliasing and over
filtering do not become a problem, but first things first,
one needs to understand how the integration is being done.
Sloppy and misleading "explanation" as usual.
The integration secret (which is no secret to anyone but Bruce) is to analog
filter, Oversample, then average the Frequency data at a rate much faster than
the tau0 data rate.
Which again is misleading as you specify neither the averaging method
nor the analog filter.
That alone should be enough information for any knowledgeable designer to
understand.
Its not and you should know that it isnt.
You draw conclusions that are neither supported by measurement nor theory.
ws
ps)
Do note, I'm working with Frequency here and not phase, that may be what is
confusing some.
When will you understand that phase differences and differences of
average frequency (unit weight to frequency measures over the sampling
interval zero weight outside) are equivalent.
*******************
************************
The problem with that page is that you show the original NIST
implementation which actually produces valid ADEV measures whereas
Warren's implementation omits the crucial integration/averaging (his
figurative handwaving antics don't change this) and hence actually has a
different phase noise frequency response than that of the filter implied
by the definition of AVAR.
Why Warren omits this crucial step when all it requires is a little
digital signal processing as all the required information is available
from the sampled EFC voltage remains a mystery.
The method as implemented by Warren produces a frequency stability
metric which may be useful for comparing the stability of some sources,
however it does not measure ADEV.
Under a restricted set of circumstances such as when white phase noise
or drift dominate the measures so calculated my be close to the measured
ADEV obtained by a method wth the correct response to the various phase
noise frequency components, however this doesnt mean that the measures
are actually ADEV measures it merely means that the phase noise
frequency components in the region where the frequency response of the 2
methods differ significantly, are not significant.
Bruce
*************************
John Miles wrote:
For those following this strange and wonderful saga:
http://www.ke5fx.com/tpll.htm
-- john, KE5FX
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