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

Charles P. Steinmetz charles_steinmetz at lavabit.com
Thu Jun 3 20:32:22 UTC 2010

If I may be allowed to summarize, it appears that Warren and Bruce 
agree that integration is necessary to produce true ADEV 
results.  Warren asserts that the low-pass filtering his method uses 
is "close enough" to integration to provide a useful approximation to 
ADEV, while Bruce disagrees.  So, the remaining points of contention 
seem to be:

1.  How close can a LPF implementation come to integration in ADEV 
calculations, and

2.  How close to true ADEV is "good enough"?

I humbly submit that trading insults has become too dreary for words, 
and that neither Warren nor Bruce will ever convince the other on the 
latter point.

I thus humbly suggest (nay, plead) that the discussion be re-focused 
on the two points above in a "just the facts, ma'am" manner.  One can 
certainly characterize mathematically the differences between 
integration and LP filtering, and predict the differential effect of 
various LPF implementations given various statistical noise 
distributions.  If one is willing to agree that certain models of 
noise distributions characterize reasonably accurately the 
performance of the oscillators that interest us, one can calculate 
the expected magnitudes of the departures from true ADEV exhibited by 
the LPF method.  Each person can then conclude for him- or herself 
whether this is "good enough" for his or her purposes.  Indeed, 
careful analysis of this sort should assist in minimizing the 
departures by suggesting optimal LPF implementations.

Best regards,


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