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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 10 08:44:32 UTC 2011


If you can solve the aliasing problem then its easy to average the noise 
at frequencies fbeat-foffset and fbeat+foffset to synthesize the 
measured phase noise values with a beat frequency of 0 Hz as would be 
measured by a conventional phase measurement setup.

Bruce

Stephan Sandenbergh wrote:
> 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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