If you can solve the aliasing problem then its easy to average the noise
at frequencies fbeat-foffset and fbeat+foffset to synthesize the
measured phase noise values with a beat frequency of 0 Hz as would be
measured by a conventional phase measurement setup.
Bruce
Stephan Sandenbergh wrote:
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
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