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

Bob kb8tq kb8tq at n1k.org
Thu Aug 18 13:32:08 EDT 2016

```Hi

> On Aug 18, 2016, at 12:59 AM, Charles Steinmetz <csteinmetz at yandex.com> wrote:
>
> 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.)

ADEV is quite poor at frequency discrimination. That problem really nailed HP back in the 1970's. You will have a much easier time doing it with a FFT / frequency domain data set.

>
> 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.)

Thus no equal noise and the obvious question....

Bob

>
> Best regards,
>
> Charles
>
>
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