As discussed at the Wednesday meeting last week, I tried moving the demodulation of the PMC error signal off the PMC servo board, by using some minicircuits components. This is just a quick summary elog, more details to follow tomorrow.
- I used the Mini Circuits ZAD-6+. This is a level 7 mixer, and the LO board puts out ~16dBm, so I replaced the existing 3dB attenuator between the LO board and the input to the PMC servo board with a 9dB attenuator.
- On the RF side, I retained the 35.5 MHz bandpass filter on the PD input.
- On the IF output, I used an in-line 50ohm terminator in series with a minicircuits BLP1.9+ low pass filter
- The mixer output was routed to the FP1 test input of the servo board
- After some twiddling with the demod phase MEDM screen, I was able to lock the PMC. I've not done a thorough characterization of the loop with the current configuration, this will be done tomorrow. But the PMC and IMC have been stably locked for the last couple of hours...
During the course of this work, I noticed that there was a 35.5MHz line (at ~-55dBm) in the 4-pin LEMO DAQ outputs even when all other inputs to the servo board were terminated. So it seems like this pickup is not coming from the RFPD or demod path. The LO board has a shield enclosure similar to what we have on the LSC demod boards, but perhaps this shield does not enclose the full RF path, and there is some residual pickup between the two cards in close proximity in the Eurocrate?
On the bright side, with this demod setup, the higher harmonic peaks seem to be significantly suppressed.
In particular, the 3x35.5 MHz peak which was very prominent when I looked at these spectra with the nominal demod setup, seems to be much suppressed.
I'm leaving the PMC servo in this configuration (off servo board demodulation using minicircuits parts) overnight.