[time-nuts] Low Phase-Noise measurements and design (was Re: HP 5071A Electron Multiplier of Cesium Beam Tube)
magnus at rubidium.dyndns.org
Sat Sep 12 10:17:40 UTC 2009
Shouldn't this thread have a more appropriate subject?
> > A better approach IMHO is to work on pushing the limits of what
> > can be done with homebrew crystal oscillators. The excellent
> > broadband floor of Wenzel and similar oscillators is not due to
> > their use of exotic crystals, but to their use of good oscillator
> > circuit topologies (and no buffering to speak of).
> This is very interesting news. I thought it took excellent high
> quality quartz and very good low noise circuitry.
> Can you tell more about how it is done? Do you happen to know of any
> schematics? What kind of crystal would be suitable? I would be very
> interested in any additional info.
What a good crystal gives you is a high Q value and high purity and
mounting which gives you low drift. Ovenizing aids to lower the
frequency deviations due to temperature sensitivity, as usual.
The low noise oscillator and buffering amps makes the next important
step. The 1/f noise and white noise is of importance. I recommend
Rubiolas book and do read it through carefully. He drives the points
down very well. In the end, the concepts are simple and the way
interaction occurs isn't too complex. For phase-noise, the 1/f noise and
white noise of the oscillating amplifier becomes crutial, as they
convert into 1/f³ and 1/f² noise within the bandwidth defined by the
crystal Q-value. The output buffer amps noise can then hide part of that
spectrum, but as Rubiola points out, spending too much money on the
buffer amps and not on the oscillating amps will be wasted as the 1/f²
and 1/f³ noise rises quicker than the 1/f noise of the buffer amp.
> > The crystal's job is stability, not noise, and unlike low noise,
> > good stability is relatively cheap and trivial nowadays thanks to
> > cheap GPS clocks, rubidiums, and good-quality OCXOs.
> Yes, I very much agree. GPS solves a lot of problems.
> >> So the question is what kind of tweaking is needed to get the
> >> best performance in a regenerative divider, and what kind of
> >> equipment is needed to do it? Then, is perfection really needed
> >> in order to beat the Wenzel ULN? Maybe put up with lower
> >> performance in the beginning, then upgrade later.
> > In practice many applications for ULN-class oscillators put the
> > broadband floor at risk in other ways. Very few buffer amplifiers
> > have a noise floor below -170 dBc/Hz, for instance. Fortunately,
> > apart from timing metrology, ULNs often end up driving high-end
> > ADCs, where the application is likely to be a good test bed in
> > itself.
> I thought the noise in a 50 ohm resistor set the lower limit to
> -174dBc. Modern amplifiers are better than that. For example, a 50
> ohm resistor has 0.894nV/sqrt(Hz) noise, but you can get wideband
> amplifiers with 0.7nV/sqrt(Hz) noise, which is equal to the noise in
> a 30.6 ohm resistor. (Of course, flicker noise is not included)
-174 dBm, but not -174 dBc... the intrinsic noise of the resistor does
not change with the amplitude of the carrier, it has a fixed amplitude
with relation to the carrier... unless you consider the heating effect,
but it needs to be very high to make any larger contribution as you most
probably start with room temperature for most cases.
> High speed adcs have very low jitter requirements to maintain ENOB,
> so anything that can improve the noise is helpful.
> >> One trick I have found that really helps isolate circuit blocks
> >> is to put them on their own small island pcb, which is then
> >> soldered to the main ground plane to hold it in place. Then find
> >> the location of ground connections that give the lowest
> >> crosstalk. A brief description is here.
> > Yep, totally, and the islands become reusable components in their
> > own right.
> > That's a valid thing to do, although I find that when I'm that
> > concerned with isolation, I probably want a full shield anyway
> > (hence the use of lots of discrete Hammond boxes). Sometimes even
> > this approach is self-defeating, as when I find that my
> > tightly-sealed Hammond enclosures make good cavity oscillators.
> I'm probably preaching to the choir, but do you find the waveguide
> cutoff frequency for the box? It's pretty easy - you can do it in
> your head. For example, the cutoff frequency is
> fc = c / 2w, where
> fc = cutoff in GHz
> c = speed of light, 30 cm/ns
> w = width in cm
> So a box 4 inches wide would be
> fc = 30 / (2 * 10)
> = 30 / 20
> = 1.5 GHz
> Here's a calculator that gives the attenuation at any desired
> frequency below cutoff:
> Another problem is the pcb will resonate at some frequency, just
> like a patch antenna.
> For example, a 100mm x 50 mm (4 inch x 2 inch) pcb will resonate at
> 700MHz. But drop the size to a 1 inch square, and the resonance
> moves up to 2.768 GHz. This is a bit more difficult to do in your
> head, so here's a calculator to help:
There is of course ways to mitigate that resonance by design.
However, for homebrew it may just be simpler to obey the simple rules.
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