[time-nuts] ensemble oscillators for better stability
actast at hotmail.com
Sun Dec 30 04:59:07 UTC 2012
I think the key is to always obtain the best oscillator/s possible.
Combining oscillator for lower Phase Noise does work but with
diminishing returns. If I am not mistaken 2 perfectly matched phase
locked oscillators can theoretically lower Phase Noise 3dB, four can
lower Phase Noise another 3dB etc. Dual oscillators in Cross Correlated
measurements will also produce a 3dB theoretical reduction in a Phase
Noise measurement system.
> Date: Sat, 29 Dec 2012 23:33:52 +0100
> From: magnus at rubidium.dyndns.org
> To: time-nuts at febo.com
> Subject: Re: [time-nuts] ensemble oscillators for better stability
> 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
> 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.
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