[time-nuts] GPSDO - probably a stupid question.

Charles Steinmetz csteinmetz at yandex.com
Thu Aug 18 00:59:50 EDT 2016

Bob wrote:

> The point is still looking at the noise characteristics of the oscillator and the reference.
> It is best done in the frequency domain as phase noise. We substitute ADEV, but that
> is not an ideal proxy.

Phase noise and xDEV measure the same thing -- the stability of an 
oscillator at different time scales.  They just express the result 
differently.  Phase noise expresses it as PM in the frequency domain, 
and xDEV expresses it as "parts per" in the time domain.  (Yes, this is 
a somewhat simplified view of it, but it captures the essential point 
without undue complexity.)

Conventionally, we switch from using PN to using xDEV at a time scale 
(reciprocal frequency scale) of around 1 second, but there is no 
mathematical reason why they both cannot be extended indefinitely in 
either direction.  The convention arose largely because the equipment 
and techniques we use[ed] to quantify them have traditionally been 
different at time scales (reciprocal frequency scales) greater than and 
less than about one second.  Now that we are in the era of "digitize 
everything, and let Laplace sort it out," we needn't view it as the 
rigid convention it once was.

> Either way you want the loop to cross over from one to the other
> somewhere in the vicinity of the “equal noise” point if it exists. If there is no equal noise
> point, that makes you wonder a bit about why you are locking one to the other

Not really, if one has lower noise at all time (frequency) scales, just 
lock to that one at all scales.  (It may call into question why you're 
fiddling with two oscillators, rather than just using the output of the 
quiet one, if they are both at the same frequency -- but there are a 
number of reasons one might want to do that.)

Best regards,


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