[time-nuts] ensemble oscillators for better stability

[Tom Knox]

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.

Thomas Knox



> 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
> 
> 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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