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

WarrenS warrensjmail-one at yahoo.com
Thu Jun 3 00:49:44 UTC 2010

As Bruce says "It remains a mystery"  to him why this works.

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.

The crucial integration/averaging to get good tau0 data, that Bruce can not see for some unknown reason, is done 
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.
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.

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.
That alone should be enough information for any knowledgeable designer to understand. 


Do note, I'm working with Frequency here and not phase, that may be what is confusing some.

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.

John Miles wrote:
> For those following this strange and wonderful saga:
> http://www.ke5fx.com/tpll.htm
> -- john, KE5FX

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