Problem
We started checking the health of QPDs and were puzzled that all four quadrants of the OMCA QPDA are much, much noisier than OMCA QPDB.
Note, there's a naming inconsistency about the QPD numbering between D2200276 wiring diagram and D0981811 (see "cartoon version").
| D0981811 | OMCA QPD-1 | OMCA QPD-2 | OMCB QPD-1 | OMCB QPD-2 |
| D2200276 | QPD-2 | QPD-1 | QPD-8 | QPD-7 |
Cause of the problem
Anyway, the problem was tracked down to short-circuit of common mode noise sensing line for OMCA-QPD1 (pin 11 of DB25 inside the vacuum, pin 3 outside of the vacuum, see attached) to the metal part of the suspension structure (i.e. ISI surface and the chamber) which in turn is grounded to the lab ground.
OMCA QPD-2 as well as both of the QPDs for OMCB are fine.
All QPD segments (incl. OMCA QPD1) responded to the flashlight.
Dark noise test: OMCB QPDs looks OK, not sure about OMCA QPDs
We connected a dual-QPD trans impedance chassis (S1102832, which is the original version of D1002481) to the OMCB QPDs like this:
QPDs-DB25 assy - in-vac cable (DB25F-DB25F) - feedthrough simulator - DB25F-to-DB25M-cable - S1102832 front panel.
We looked at the output of the TIA with SR785 when there's only ambient light on QPDs (not much), segment by segment. No segment was extra noisy or anything.
OMCA QPDs are a different story because of the 60Hz problem.
We'll see if the problem could be solved by somehow reassembling the QPD or cutting the noise line.
1st attachment shows the connection from the QPD to TIA. ("S1P" "S2N" etc. mean segment 1 positive pin, segment 2 negative pin etc.)
2nd attachment shows how the output of the TIA (all differential) was connected to SR785 or oscilloscope, using QPD1 seg1 as an example.
3rd attachment shows that CH1 (yellow), which is the segment 1 positive pin of the QPD1 for OMCA, was terrible while the segment 1 positive of QPD2 for OMCA (blue) was not. Though there's no picture, the terrible 60Hz noise appeared in the negative pin with the opposite sign (for QPD1) so it's not cancelled.
For OMCB QPDs, DC offset of all segments were smaller than 1mV when there was no light (used a fluke DVM to measure across positive and negative pins). For OMCA QPD2 that was also the case.
OMCB_QPD[12]_SEG[1234].jpg are the noise measurements for OMCB QPDs using SR785. The TIA output (which receives 0.4:40 whitening in the chassis) just shows the TIA noise from 200Hz and higher without any light on QPDs, which looks fine to me. There's no reason to suspect that one or more of these segments/QPDs are broken.
The last attachment (QPD_removed.jpg) shows a measurement of OMCB_QPD2_SEG4 after disconnecting the QPD from the front panel, showing the TIA noise floor. SR785 noise floor is lower than this.
