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

Bruce Griffiths bruce.griffiths at xtra.co.nz
Thu Mar 10 13:36:23 EST 2011


Stephan Sandenbergh wrote:
> Hi,
>
> Thanks this is good advice.
>
> Pointing the spectrum analyzer to fc + delta seems to be similar
> than deducing phase noise from the ZCD output since this would be with
> reference to (fc + delta) in any how? Provided the aliasing issue can be
> sorted.
>
> Regarding the aliasing issue - in order to plot phase noise up to 100kHz I
> would use a 300kHz beat sampled at 100MHz (which is the sampling system I
> got available). Obviously, making sure I have sufficient bandwidth in all
> areas.
>    
The definition requires that both the noise at fc + delta and the noise 
at fc-delta be added to obtain the phase noise at an offset of delta.
This is easy to do either in an FPGA or in post processing.

If you are sampling at 100MHz then the post mixer filter only need limit 
the bandwidth sufficiently to eliminate the mixer sum product and keep 
the noise signal within the bandwidth of the following amplifier. 
Additional filtering if required can be implemented as digital filters.

Note with a 300kHz beat frequency phase noise components at offsets 
greater than fcarrier + 300KHz will be folded into the mixer output 
spectrum so using a carrier  bandpass filter with a bandwidth smaller 
than the beat frequency may be advisable.
> Cheers.
>
> Stephan.
>    
Bruce

> 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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