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

[Stephan Sandenbergh]

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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