Stephan Sandenbergh wrote:
Hi,
Thanks for the replies. I agree. One is interested in the timing of beat
notes.
But, I'm slightly confused now. It might be a language problem on my side.
Quoting Howe, Allan and Barnes, 1981, "...adjust the phase so that the two
beat frequencies are nominally in phase; this adjustment sets up a nice
condition that the noise of the common oscillator tends to cancel..."
I can see a few issues here:
1) if the beat frequencies are in phase, there will be a very small time
interval between their zero crossings. This might be difficult to measure
with accuracy.
No, it is not a particular issue, unless you have problem with
isolation. The method to workaround isolation problems that I described
is just one possible approach out of many, including designing it out of
being an issue.
What they are after is that as coincidence in time makes signals from a
DTMD perspective correlate better in the regard as supressing the
transfer oscillator phase stability. If you keep the coincidence time to
say within 100 us much of the low-frequency noise of the transfer
osicllator beyond 100 us correlate quite well, so cross-correlation will
perform well to supress it where as instabilities with shorter times
will not correlate and thus polute the measurement. As time-separation
increases, more integrate time-noise of the transfer oscillator will be
exposed and induced into the measurement results, for which only
time-averaging will help.
2) I agree that reference oscillator noise will cancel to some extend
because the measurements are done closer to the same time, which makes the
reference oscillator noise better correlated between the start and stop
edges.
Correct.
3) Also, one would like to compare both clkA and clkB at the same time. Not
one at t=0 and the other at t=1day to exaggerate a little. I'm not planning
to measure atomic standards, or the best OCXO available, so I doubt this
will bother me.
The closer together they are, the better cross-correlation gain can be
achieved. On the other hand, tbe closer the edges is and thus
cross-correlation losses may also be found.
Other than the above, I agree: it is better to have a greater phase offset
between the beat notes.
Does my thinking my sense at all?
Sure thing.
Cheers,
M'agnus
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