[time-nuts] PLL Tester performance

WarrenS warrensjmail-one at yahoo.com
Thu May 27 20:54:02 UTC 2010

I have been unable to explain to some of the upside down experts out there, in a way that they can understand, why the PLL Tester method works good enough for many.
Maybe now that they can see that the simple PLL tester gives nearly the exact same answers as the TSC 5120A over the whole range of taus, 
They will be able to set aside their prejudices long enough to see why it works as good as it does when using over sampling,
and why I say "It can work as good or better than Anything else out there".  (with the performance limited by it's reference oscillator)

When everything is set up correctly,
IF you feed an ADEV program like PLOTTER an over sampled raw ADC frequency data set, 
The program will calculate the correct ADEV values from about 1/10 the raw data rate out to about 1/3 of the length of the data run.

setup correctly means:
PLL_Bandwidth > ADC_sample_rate > ADC_LP_Filter > Tau0.  
Using a factor of 3 to 5 for each ">" works OK, (more is generally better).

an extreme example: using a factor of 10 for every  ">"
PLL_Bandwidth = 10 KHz
ADC_sample_Rate = 1 KHz
ADC_LPF = 100 Hz
Valid Tau data = 10 Hz and below (with 100 Hz BW noise)

If one wants to reduce the long 1KHz rate frequency data log that the tester produces in this example to say a more normal 0.1 sec tau0 data set, then down converting the longer freq data set using a simple averaging program that does (sum of n_samples / n)  does not change any of the remaining ADEV answers and reduces the length of the data set by n.

PS) If one wants to reference some obscure write up that they think says why this should not work, if they will also include an example of a data set that it does not work on, I'll try to show what they did wrong.


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