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 > > > _______________________________________________ > time-nuts mailing list -- time-nuts@febo.com > To unsubscribe, go to > https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts > and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.