[time-nuts] ensemble oscillators for better stability

Magnus Danielson magnus at rubidium.dyndns.org
Sat Dec 29 22:33:52 UTC 2012


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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