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.
Bruce
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