[time-nuts] Notes on tight-PLL performance versus TSC 5120A

Steve Rooke sar10538 at gmail.com
Thu Jun 3 01:26:09 EDT 2010


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.

>> 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.

> 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.

>> 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?

>> 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.

>> 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.

>> 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.

>> 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.


>> 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.

>> 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?

>> 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.

>> 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.

Bruce, I really do admire your knowledge and intelligence generally
but sometimes you really need to take a step back and smell the
coffee.

My apologies to the list for openly expressing my feelings on this.
-- 
Steve Rooke - ZL3TUV & G8KVD
The only reason for time is so that everything doesn't happen at once.
- Einstein




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