Tom,
On 29/12/12 18:11, Tom Van Baak wrote:
Corby,
So that's an interesting experiment. I think the key is keeping them
in tight phase so that what you gain in combined performance is still
better than what you lose with the additional mixing electronics.
If you just mixup, then you do not need to lock them up. You only need
that if you add them up in a power-combiner.
A couple of comments that come to mind.
1) This was a topic some years back -- for internal use, hp tightly
combined multiple 10811 oscillators so that the net phase noise or
short-term performance was significantly better than any one of the
constituent oscillators.
Care to share a reference on that? It would be interesting to see how
they did it and how well they where doing it.
2) It would be nice to be able to extend this to more than 2
oscillators, in such a way that you gain by sqrt(N) without
corresponding losses due to increased noise.
Using the mix-up strategy would be possible. Also, for three sources you
would get back to your starting frequency easily on the second mixer. A
mix-up strategy would allow to mix 5 and 10 MHz sources, but
unfortunately that would give the 10 MHz sources twice the weight of 5
MHz sources. The free-running measure and locked additive strategies
does not have that drawback.
3) You already realize that being able to keep coherence between the
standards as long as possible is highly desirable.
It depends on what strategy you try to achieve.
4) Consider that none of the UTC(k) timing labs use your technique.
The reason is that it's far easier to compare N frequency
standards in near-realtime (like every second or every 100 s,
etc.) combining the measurement *numbers* than it is to combine
the actual *electrons* coming out of the frequency standards in
realtime.
Also, they do not need the high-frequency phase noise benefit. If they
need low phase-noise, an active H-maser is used.
Another benefit of not locking the standards is that you can observe
them undisturbed by a control-loop, which make things easier for what
they try to achieve.
So this is one reason why I keep encouraging those of you building
amateur, inexpensive, high-resolution, multi-port phase comparators.
It is indeed an interesting thing do to. To benefit it needs to have
many channels, say 8 or so. Preferably expandable further as you have
more sources to look at and form an ensemble of.
If you had a couple of these comparators you'd simultaneously
measure each of your 5065A and perhaps several other standards all
using a common reference. It wouldn't really matter which standard
was the reference, since the data is all pair-wise relative.
As you compare many sources, doing M-cornered hat stuff becomes
possible, and you can get some confidence in the absolute phase-noise of
all involved sources.
It's trivial to create an ensemble in software, based on multiple
phase measurements that arrive by spi or gpib or rs232. With that
calculated mean phase you can then ex post facto apply a correction
to each of the oscillators in the ensemble. It's like sawtooth
correction; you take the pulse as you see it, but you apply a
freshly calculated correction factor.
A note on ensembles is that NTP actually features ensemble calculations,
as it is able to estimate the noise, do weighting of various sources
etc. Inspired by the work done at NIST. I'm not completely sure that NTP
will work well with unlocked frequency sources, but I mention it so
people can look in their NTP books and read up a bit.
The main point is that the past noise of a source is used to calculate
the weight it can have in order to form the optimum stability. This is
how the national labs create their time-scales, and then how EAL is
built for maximum frequency stability, then being corrected into the TAI
for phase stability and then synthesized into UTC to form a stable GMT
replacement.
Once you have started to walk on the ensemble path, you are not that far
off from looking at doing a full-blown time-scale.
Cheers,
Magnus
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