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

Magnus Danielson magnus at rubidium.dyndns.org
Tue Mar 8 20:28:44 UTC 2011

```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