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<mag...@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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