The cavities were still too far off to lock simultaneously, but they should hopefully be better today.
Instead, we played with the Wenzel BluePhase 1000 phase noise test system.
Following the directions in the manual, we locked an IFR/Marconi 2023A to an IFR/Marconi 2023B, calibrated according to the manual, and got a curve very similar to what we had before . This means it's probably working correctly!
So then, just to be adventurous, we decided to connect the VCO in place of the 2023B to measure its phase noise. Apparently that was too adventurous for the BluePhase 1000 . We couldn't lock it (with the feedback loop going to the 2023A) with any input range below roughly 100 kHz and the output (after calibration as per the manual) was a flat line. After readjusting cables and reconnecting cables and finally reverting back to the 2023A vs 2023B to make sure the machine still worked, we decided the unity gain frequency was probably pretty high and the manual calibration did not take that into account. So we used a swept sine measurement to find the transfer function of the system with the VCO connected and locked. The UGF was around 5.3 kHz with the 100 kHz input range . This means that the calibration doesn't account for everything when the UGF is high . But it also means we may have found the problem with our data when the VCO is connected ! So I have to take the data we have, apply a zero at 5.3 kHz, and see if that gets it to line up correctly.
Meanwhile, we installed a program on the computer that, when connected to the BluePhase 1000, can control all the knobs and buttons and locking remotely. And we discovered you can do more on the computer than with the switches on the front! Like change the capacitor value.
So the summary of yesterday's activities is: don't ever completely trust the calibration the manufacturers tell you to use. They might not be taking something (UGF) into account.
And the calibration as per the manufacturers:
1) Adjust the offset until exactly 1 period is displayed on the oscilloscope
2) Divide the time divisions on the oscilloscope to 1/100th the original (this gives 0.02Pi radians)
3) Measure the voltage difference across the two ends of the line
4) Calculate your slope! (gives V/rad)
5) Noise [dBc/Hz] = [PSD]-[20log(slope)]-[amp gain]-[correction for SSB measurement]