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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