I'm hoping to reproduce this cavity stabilized oscillator design from NIST but at 12 GHz as an ultra low phase noise reference clock for the vector signal generator. This nonsense is optional, the clock could be as simple as an LMX2594.

These pipe couplings are very close to what I need for a TE015 resonator at 12 GHz. They need some minor machining, polishing, endcaps, coupling loops.

https://tf.nist.gov/general/pdf/1887.pdf
https://tf.nist.gov/general/pdf/2729.pdf

NIST got some impressive phase noise out of this thing, comparable to room temperature sapphire whispering gallery oscillators. Fabricating copper is much cheaper and easier than fabricating sapphire. Won't be as pretty as those big chunks of sapphire. Still not trivial.

It's going to require insulation, temperature control, will be fairly large once complete. There will be a few watts of heating from losses in the walls so will have to remove some heat.

Any ideas for a power amp at 12 GHz, 2 W or so? I see a few options from Hittite/Analog and Qorvo.

One advantage of a chunky copper resonator is you can drive it hard. That's one of the reasons for the performance.

I ordered the copper for the endcaps. That was expensive. Hopefully my bandsaw will be up to roughing them out. Probably have to use a bigger lathe I have access to as my Sherline is likely to have trouble with a 66 mm OD in copper.

One spot the NIST papers are light on is the coupling probes. This may require some simulation and experimentation so I got some extra copper. Hopefully a bunch of clamps can hold the probe end on temporarily before I commit to soldering.

@dlharmon can you machine brass then electroplate copper or silver on it?

@whitequark I could but proper chemical disposal is difficult around here. I think the copper will be OK, just not fun to saw. I've cut worse before. NIST actually proposed the next iteration to be silver plated ceramic for temperature stability. They did a 40 GHz version in silver plated aluminum.

@dlharmon they regulate disposal of copper sulphate or silver nitrate? that's rough

@whitequark It's sort of vague. Being sort of associated with my business, I'd be supposed to pay a disposal company, large minimum fees. As a private individual, I could drop it off at household hazardous waste for free.

I err on the side of caution given my business license at a residential address restricts me quite a bit, I don't want to get on the wrong side of the local government.

@dlharmon I see, that's very understandable

Rather than swap my bandsaw to a metal blade, I tried the jigsaw. Worked great.

Also, after getting the copper, I realized that for probably not much more money I could have had the parts laser cut.

Next up, I need to make some copper collets to hold the feed probes allowing adjustment of insertion depth. Then I should be able to test it with clamps instead of solder. The coupler will need to be cut down a bit but we should get the TE015 resonance at a lower frequency with it unmodified.

@dlharmon thanks for the pointer to those papers. I wonder what else NIST has there…

@dlharmon is this the sort of project where you end of with a little screw that "slightly" distorts the shape of the cavity to tune it?

@RueNahcMohr That's something I plan to explore. It would be nice for it to be exactly 12 GHz but I may accept it being very close, deal with the difference in the FPGA, continuous measurement of the free run frequency against the external reference. There is temperature control as another option, a tiny amount of electrical tuning.