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

Stephan Sandenbergh ssandenbergh at gmail.com
Thu Mar 10 08:32:23 UTC 2011

```Hi,

This makes sense.

Thanks,

Stephan.

On 8 March 2011 21:08, Bruce Griffiths <bruce.griffiths at xtra.co.nz> wrote:

> 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
>
>
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```