Hi Corby -- I've often thought of the idea of mixing two 5 MHz sources in a DBM and using the resulting 10 MHz, so I'm really curious to see how this works out.  Seems like injection locking  is the most likely problem you'd have to deal with.

Let us know how this goes!

John

On Dec 28, 2012, at 2:12 PM, cdelect@juno.com wrote:

Seems like I saw an article in QEX a year or two back where a guy did just
that, except he used a single 5 MHz oscillator and used the mixer to make a
doubler, but that isn't very much different from this.

Tom Holmes, N8ZM
Tipp City, OH
EM79

a DBM

works out.

deal with.

Which reference did you use to time your measurements?

On Fri, Dec 28, 2012 at 9:32 PM, Tom Holmes tholmes@woh.rr.com wrote:

On Fri, 28 Dec 2012 15:19:43 -0500
John Ackermann  N8UR jra@febo.com wrote:

Stupid question, but wouldnt the use of a DBM make the noise being
added up, instead of being awaraged out?

			Attila Kinali

--
There is no secret ingredient
-- Po, Kung Fu Panda

Hi Corby,

On 28/12/12 20:12, cdelect@juno.com wrote:

Interesting that you take up the concept of ensembles, as I have been
pondering a little about this too.

My first idea would be to mutually lock the oscillators. You can even do
weighted locking without much difficulty. You toss the oscillators into
a mixer, use it as a phase detector, do the usual PI control and then
steer both oscillators the same EFC but with opposite sign. One can add
a common EFC steering to both:

Vd is the detected phase from the mixer (low-pass filtered)
VI = VI + IVd
EFC_loop = VI + PVd
EFC_A = EFC_common + EFC_loop/2
EFC_B = EFC_common - EFC_loop/2

With the right sign, the loop will steer the oscillators towards each
other and the common frequency will be the average. Weighting can be
applied in how much the EFC_loop steers the oscillator, and should be
assigned to how great part they have in the noise compared to the
ensemble noise, noisier gets more steering.

The benefit of mutual locking (which really just is two oscillators
locked together with the other as reference) as well as mixing is that
high-frequency noise gets averaged.

Another way would be to measure them and then steer one to represent the
ensemble. This is what people like NIST do, but they use a microstepper
to re-synthesize but it is essentially the same thing. This method
however does not get the high-frequency noise benefit, as the output of
the individual clocks isn't combined.

Mixing would work, but you will not be able to weight the oscillators.
The simple combination provides rather fixed weighting (actually not,
but for rubidiums the difference in frequency is small enough not to
have large effect on the weighting between the oscillators), but can be
useful in that context.

It would. The downside is that you won't be able to get best possible
performance out of the ensemble, but you can get pretty good results for
the cost of combining them.

Happy New Year!

Cheers,
Magnus

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.

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.

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.

3. You already realize that being able to keep coherence between the standards as long as possible is highly desirable.

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.

So this is one reason why I keep encouraging those of you building amateur, inexpensive, high-resolution, multi-port phase comparators.

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.

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.

/tvb

Tom,

On 29/12/12 18:11, Tom Van Baak wrote:

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.

Care to share a reference on that? It would be interesting to see how
they did it and how well they where doing it.

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.

It depends on what strategy you try to achieve.

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.

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.

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.

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

Magnus,

Yes, it came up in a long thread called "GPS noise reduction" back in April 2008. I'll mention it again  -- when I visited the HP facility in Santa Clara years ago I saw or was told about a special low noise 10 MHz reference that was used to measure the phase noise of shipping 10811 oscillators. The reference itself was a set of several hand-picked 10811's that were combined in some clever way so that they all steered themselves creating a coherent single output with significantly lower noise than any one individual 10811. I don't have further details but assume the RF and PLL experts here on the list can figure out what is meant by this and propose a paper design. It was probably more than 2 oscillators, but I don't remember now if it was 3 or 6 or more. Rick Karlquist may also know about this system; either when/how it was built or what level of performance it delivered.

/tvb

I know that this is not germane but I keep thinking of this video:

http://www.youtube.com/watch?v=kqFc4wriBvE

32 mechanical metronomes on a moveable floor. They all sync up in a bit more
than two minutes.

Dave