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

[Stephan Sandenbergh]

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.

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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