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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 10 13:51:05 EST 2011


Stephan Sandenbergh wrote:
> Thanks,
>
> I'm familiar with the designs you posted to measure voltage noise ect. on
> you home page. These, with some modification, mainly removing the blocking
> caps, seems that it would do the trick.
>
> Cheers,
>
> Stephan.
>
> On 10 March 2011 10:50, Bruce Griffiths<bruce.griffiths at xtra.co.nz>  wrote:
>
>    
>> 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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>>>>
>>>>
>>>>          
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>>
>>
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