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

WarrenS warrensjmail-one at yahoo.com
Thu Jun 3 05:10:42 UTC 2010


Bruce posted:
>"That doesn't work as it has the wrong transfer function".
If you'll send me a Freq data log of any reasonable type,  I'll show you it working just fine.

I have the Hardware that worked 'good enough's in all cases tested. (no restrictions, or exceptions)
Bruce has an idea that it shouldn't work some times, but has not give any example of when that may be.
I'm going with John's the H/W tests.


>"You draw conclusions that are neither supported by measurement nor theory".
You need to rework that argument now, because did you forget, it worked just fine EVERY time it was measured and tested.

BTW, now that you understand the basic method can work, go ahead and make yours with the VtoF, It will also work,  just not be as cheap or as good as the ADC.

Still sounds like you may be confusing how to process Phase data with how to process Freq data. 
They are not done the same way (as you say different transfer function needed). 
They both contain the same information, but in different forms, so can not do the same thing to them both and expect the same results.
If that basic thing is what you do not know, I can give you a simple example to show the difference.

If it is not phase and frequency that is your misunderstanding, then the only other thing I can see is we just don't agree on what is close enough.
If the two plots overlay on each other as these do, that is close enough for me.

ws

*****************

WarrenS wrote:
> As Bruce says "It remains a mystery"  to him why this works.
>
>    
It doesn't, 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 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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