[time-nuts] Plot phase noise spectrum from DMTD measurement?

[Bruce Griffiths]

Stephan Sandenbergh wrote:
> Hi,
>
> I recently noticed something interesting: The DMTD measurement gives a set
> of phase values x(t). From which fractional frequency y(t) is calculable. So
> now it seems viable to plot the spectrum, Sy(f) and if you scale it properly
> you arrive at Sphi(f). If I'm  not making a gross error somewhere the math
> seems to check out. But, I'm wondering is there a physical reason why this
> isn't valid?
>
> I have not seen this being done anywhere - so I assume there is. However, it
> seems possible to plot Sphi(f) for 1Hz<  f<100kHz when having a vbeat =
> 100kHz sampled for 1 second.
>
> I'm familiar with the loose and tight phase-locked methods of measuring
> phase noise, but am quite curious to know if phase noise from a DMTD
> measurement is a valid assumption.
>
> I would guess that if the frequency domain phase noise measurement requires
> phase-lock then the time-domain measurement requires as well. However, here
> in lies my real interest - two GPSDOs are phase-locked (not to 1Hz,
> something far less I know) so can it be possible to measure GPSDO Adev and
> phase-noise using a single DMTD run? Am I making a wrong assumption
> somewhere?
>
> Cheers,
>
> Stephan.
>    
Aside from the aliasing problem inherent when using a DMTD (The sampling 
rate is equal to the beat frequency but the the bandwidth is necessarily 
greater than the nyquist limit to permit the DMTD ZCD to work unless of 
course one uses a front end bandpass filter - however the associated 
phase shift tempco usually precludes this ) the output phase difference 
measures also include contributions from the reference source and the 
offset source. The contribution from the offset source depends on the 
phase difference between the 2 sources being compared. Using a low noise 
reference and phase locking the offset to the reference source will help 
somewhat as will minimising the phase difference between the reference 
and the source under test.

Bruce