Low Phase-Noise measurements and design (was Re: HP 5071A Electron Multiplier of Cesium Beam Tube)
Hi!
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:
http://www.k5rmg.org/calc/waveguide.html
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.
Cheers,
Magnus