[time-nuts] Re: Do time nuts only use and measure 10MHz clocks?

[Joseph Gwinn]

Re: time-nuts Digest, Vol 222, Issue 9

> 
> ----------------------------------------------------------------------
> 
> Message: 3
> Date: Sun, 9 Oct 2022 12:19:52 +0200
> From: Erik Kaashoek <erik at kaashoek.com>
> Subject: [time-nuts] Re: Do time nuts only use and measure 10MHz
> 	clocks?
> To: Discussion of precise time and frequency measurement
> 	<time-nuts at lists.febo.com>, Erik Kaashoek <erik at kaashoek.com>
> Message-ID:
> 	<CABj+HDNMxPYDeuJPwjf3ze5Ld_4W0K6=yMGqtgXCmfaUFxUwwA at mail.gmail.com>
> Content-Type: text/plain; charset="UTF-8"
> 
> Thanks for the input.
> After some more testing the coherence turned out to be lost even when the
> inputs use the same mixing frequency. The current assumption is that the
> two fractional dividers used to generate the two mixer input frequencies,
> although using same settings, already cause the loss of coherence. Probably
> because they are not started at the same moment so the fractional shifts
> happen at different moments. Next test is to us only integer dividers to
> generate the two mixing frequencies but this will lead to two IF
> frequencies and it is not sure if this can be implemented on the not so
> powerful MCU used.
> Erik
>

It occurs to me that there are two things able to be coherent here, 
bing the carrier (where coherence requires they be at the same 
frequency), and the noise signal being (weakly) modulated onto the 
carrier.  coherence of noise signal is independent of coherence of 
carrier.

This is easily seen if one performs the analysis in the time domain, 
not the PN spectral domain.


> On Sun, Oct 9, 2022, 11:30 zfe via time-nuts <time-nuts at lists.febo.com>
> wrote:
> 
>> I did, up to now, not experience a loss in measurement resolution  using
>> different frequencies for DUT and REF.

Because it is the modulating signal that's coherent.

Joe Gwinn