Hi, This makes sense.
Thanks, Stephan. On 8 March 2011 21:08, Bruce Griffiths <bruce.griffi...@xtra.co.nz> wrote: > 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? >> >> Cheers, >> >> Stephan. >> >> > Aside from the aliasing problem inherent when using a DMTD (The sampling > rate is equal to the beat frequency but the the bandwidth is necessarily > greater than the nyquist limit to permit the DMTD ZCD to work unless of > course one uses a front end bandpass filter - however the associated phase > shift tempco usually precludes this ) the output phase difference measures > also include contributions from the reference source and the offset source. > The contribution from the offset source depends on the phase difference > between the 2 sources being compared. Using a low noise reference and phase > locking the offset to the reference source will help somewhat as will > minimising the phase difference between the reference and the source under > test. > > Bruce > > > _______________________________________________ > 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.
