Hi, Ok some cool advice - this thread is an interesting thought exercise. I'm going to think about it a some more, but it seems, in comparison at least, the loose phase-lock technique remains the simplest. Provided you have a low-frequency spectrum analyser handy.
The sound card idea is clever as well - however, I'd assume one needs to measure the ADCs clocking oscillator offset, since that will be apparent when plotting the beat frequency phase (what I mean is that sampling will then look like another mixing process). What I usually due is to clock the sampling system off a clock that's correlated to the clock under test. This resolves that issue. However, I'd like to experiment with the cross-correlation idea, since I've got a setup that will lend itself perfect to that. Maybe I could save myself some time, with clever post-processing. Can anyone recommend a fundamental text on the cross-correlation technique? Regards, Stephan. On 10 March 2011 20:53, Bruce Griffiths <bruce.griffi...@xtra.co.nz> wrote: > Without nulling the carrier the dynamic range of your ADC will limit the > measurement system phase noise fllor. > > With a 300KHz beat frequency the post mixer preamp response need not extend > to dc. > > Bruce > > > Stephan Sandenbergh wrote: > >> Thanks, >> >> I'm familiar with the designs you posted to measure voltage noise ect. on >> you home page. These, with some modification, mainly removing the blocking >> caps, seems that it would do the trick. >> >> Cheers, >> >> Stephan. >> >> On 10 March 2011 10:50, Bruce Griffiths<bruce.griffi...@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<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. >>>> >>>> >>>> >>>> >>>> >>> >>> >>> _______________________________________________ >>> 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. >> >> >> > > > > _______________________________________________ > 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.