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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 10 08:50:25 UTC 2011


For conventional phase noise measurements at offsets in the (10Hz, 
20kHz) range one can use a sound card with a low noise preaamp.
Suitable sound card preamps with lower noise floors than Enrico's or 
Wenzel's designs can be built using readily available components.
Wider bandwidths ( up to 1MHz or so) are not difficult to achieve.

Bruce

Stephan Sandenbergh wrote:
> Hi,
>
> Cross-correlation a very clever idea! Thanks for the reference - Rubiola got
> some good sources of reference on his home page.
>
> One thing though - for a phase-noise kit one will probably need to replace
> the ZCD with a low-noise amplification stage of around 80dB to be to allow
> sampling at ADC voltage levels?
>
> Cheers,
>
> Stephan.
>
> On 8 March 2011 22:28, Magnus Danielson<magnus at rubidium.dyndns.org>  wrote:
>
>    
>> On 03/08/2011 07:46 PM, 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?
>>>
>>>        
>> An architecture not completely different to the DMTD architecture is used
>> in phase-noise kits. Instead of having two sources and one intermediary
>> oscillator is instead there one source and two intermediary oscillators. The
>> oscillators is locked to the carrier frequency rather than an offset. The
>> mixed down signal is then cross-correlated to get the spectrum. Increasing
>> the averaging factor and the spectrum can be suppressed below that of the
>> intermediary oscillators. Since the two intermediary oscillators have
>> uncorrelated noise, the external noise is what correlates over time. This
>> technique is simply called cross-correlation. Such a cross-correlation setup
>> can run very close to the carrier in terms of offsets.
>>
>> In contrast will a DMTD with it's offset frequency be problematic at low
>> offsets since the positive and negative offsets noise will not occur at the
>> same frequency in a DMTD setup. Consider a a DMTD with a 10 Hz offset,
>> pointing a spectrum analyzer on 100 Hz will measure the down-converted
>> average of carrier+(100-10) Hz and carrier-(100+10) Hz, thus carrier+90 Hz
>> and carrier-110 Hz.
>>
>> Creating a mixed-mode setup for phase-noise/DMTD will however be possible.
>>
>> So, DMTD as such is relatively limited, but add an RF switch and another
>> oscillator and you get a cross-correlation phase-noise kit.
>>
>> To turbo-charge the phase-noise kit use a quadrature combiner and amplitude
>> adjustment to create a interferometric mixdown, working around part of the
>> mixer limitations. Enrico Rubiola has writen about this approach.
>>
>> Cheers,
>> Magnus
>>
>>
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