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 -- [email protected]
To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
and follow the instructions there.
