[time-nuts] Notes on tight-PLL performance versus TSC 5120A
Bruce Griffiths
bruce.griffiths at xtra.co.nz
Thu Jun 3 07:27:53 UTC 2010
Steve Rooke wrote:
> On 3 June 2010 15:46, Bruce Griffiths<bruce.griffiths at xtra.co.nz> wrote:
>
>> 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.
>>
> Bruce, I don't understand you, when presented with visual evidence
> that this method works you still deny it.
>
>
What visual evidence??
There is no proof that the technique works well in every case.
Only that for the range of Tau tested and for the particular source pair
used that it appears to.
>>> 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.
>>
> Well, what are are the "woefully inadequate" conclusions then? Please
> give us your full reasoning.
>
>
A simple example is that for a small number of samples a stability
metric like the ordinary (unfiltered) phase variance standard deviation
may appear to be stable, whereas with a sufficiently large number of
samples the instability of the metric itself becomes evident whenever
divergent noise processes like flicker phase noise, random walk
frequency noise are present.
/"Each Tau0 Frequency sample is ideally completely independent from all
others."
/
The above statement is incorrect as the finite bandwidth necessarily
imparts a correlation between samples they can only be strictly
independent if the bandwidth is infinite.
/"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."
/
The above statement imparts no useful information.
It would be much easier and less bandwidth wasted if the circuit
schematics and useful documentation on the algorithms employed were
available.
Extracting any useful information seems somewhat akin to pulling teeth.
>> The crucial integration/averaging to get good tau0 data, that Bruce can not
>> see for some unknown reason, is done
>>
>> Only in your imagination.
>>
> One would assume that this method only works when Warren does it as
> his "imagination" is required for it to work, but wait, John Miles has
> managed to get point for point identical data against a TSC, how can
> that be Bruce? Please give answers, not insults.
>
>
Read the following paper:
http://hal.archives-ouvertes.fr/docs/00/37/63/05/PDF/alaa_p1_v4a.pdf
which shows the relationship between AVAR etc, filters and the ordinary
phase variance.
The paper also outlines the techniques that should be used with the
sampled frequency difference data from a tight PLL.
>>> 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.
>>
> Again, it it does not work, how come the evidence shows that it does,
> how do you explain that Bruce?
>
The evidence doesn't show this at all.
It merely indicates that for the devices tested that the phase noise
spectral components in the region where the filter responses of the ADEV
and WDEV differ (its not ADEV so it shouldnt be labelled as such) dont
appear to be significant for the 2 sources compared and the tau range
over which the testing was done.
Extrapolation of such results to predict that the technique will produce
such agreement with other devices with differing phase noise
characteristics is unrelaible.
You are confusing producing the same numbers in specific cases with the
ability to do so in general.
There is no guarantee that such agreement will occur with a given pair
of sources.
Such agreement in general isnt possible as the equivalent phase noise
filters have different frequency responses.
Stability measures like AVAR can be shown to be the equal to the
ordinary variance of the phase difference at the output of a very
specific phase noise filter.
WDEV has a phase noise filter with a different frequency response so
that it doesnt actually measure ADEV.
>>> 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.
>>
> You sound like a school teacher marking a pupil's work. Perhaps not
> everyone is as eloquent as Shakespeare with their English, there is no
> need to resort to this form of denigration. I find your explanations
> on things very cryptic and hard to follow but I don'r resort to this
> sort of abuse.
>
>
It would be much easier if Warren limited his commentary to the actual
results and omitted the wild speculation (and the metaphysics).
>>> 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.
>>
> Has been explained by John who wrote the method and is available for
> you to review.
>
>
I've seen it and its somewhat shy of the optimum signal processing
technique.
>>> 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.
>>
> So the visual evidence before your very eyes which clearly shows that
> this works is not sufficient for you to understand that this
> measurement works.
>
Read the theory outlined in the paper and maybe you''ll begin to
understand my objections to statements that the technique measures ADEV.
>
>>> 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.
>>
> Bruce, you do know that this is the NIST tight-loop PLL method which
> produces frequency measurements and not the loose-loop PLL method
> which produces phase difference data I hope.
>
>
Of course I am aware of that.
Phase is the integral of frequency so phase differences sampled at
intervals of say T are equivalent to frequencies averaged over time T
and sampled at the end of the sample interval. Thus sampling the time
average frequency every T seconds is equivalent to sampling the phase
difference every T seconds.
>
>>> 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.
>>>
> Warrens implementation improves on the original NIST implementation by
> oversampling.
>
Actually it degrades the simplicity and accuracy of the NIST
implementation by replacing the integration inherent when using the
counter and VFC with an approximation to the required frequency
integral. Fortunately the accuracy can largely be recovered by using the
appropriate signal processing algorithms.
>
>>> 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.
>>>
> I'm intrigued Bruce, please explain to us in detail what you are
> actually getting at here?
>
>
Read the paper on stability variances and filters.
>>> 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.
>>>
> The needle is stuck again, Bruce, look at the results, as rose by any
> other name would smell as sweet.
>
>
Poetry is irrelevant, the fact that the equivalent filters for AVAR and
WVAR differ should be of concern.
>>> 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.
>>>
> You keep coming up with imaginary ways that you think this method
> would fail to produce the right answer but you've not produced a
> source that can be tested in the REAL World. You talked about Warrens
> imagination earlier, well I'm calling this on you now. Lets have some
> concrete example that shows this method is not usable or shut up.
> Warren has put his money where his mouth is, now it's your turn.
>
>
Its obvious if you understand the theory.
Otherwise an infinite set of tests with an infinite number of sources
with an infinite variety of phase noise spectra are required to show the
technique works in all cases.
> Bruce, I really do admire your knowledge and intelligence generally
> but sometimes you really need to take a step back and smell the
> coffee.
>
>
Its not that the method cant be easily fixed so that it produces
accurate ADEV measures for an extremely wide range of sources with
divergent phase noise spectra, its the extreme reluctance to do the
signal processing correctly (its not that this even incurs extra
hardware costs) that is perplexing.
> My apologies to the list for openly expressing my feelings on this.
>
Bruce
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