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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Jun 2 23:31:20 UTC 2010

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


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