Jennie W, Khanh V,
Since Keita and I realised that the WFS A and B quadrants 2 and 3 are swapped between the RF channels and the DC channels, I have been trying to trace down where the error is. Last week we fixed the problem by swapping the cables for WFSA segment 2 and 3 and WFS B segment 2 and 3 cables at the IOT1 feedthrough panel.
I don't think it is in the RF signal chain as when we unplugged WFS HF segment 2 from the outside of the feedthrough, the channel that corresponds to WFS segment 2 went dark in EPICS.
Since we are still laser SAFE in the corner, Khanh and I took a laser pointer onto the JAC table and tried to see if we could only shine it on one quadrant of the QPD. Since the QPD is as large as the beam this was not really practical. We instead moved a beam card in front of the PD and slowly drew it downwards.
Since quadrants 2 and 3 are the two we suspect are swapped, we looked at which of these showed some light first.
We used the response to the table lights being blocked with the card to see if there was a difference in the readout from each QPD segment. Using the laser point for this did not work as the beam reflects off the card and the QPD casing.
Segment 3 showed light before segment 2 when moving the card down from the top of the diode.This implies the readout channels for 2 and 3 are swapped. The step in power was not as obvious as with a laser beam so I would like to recheck this once we go laser hazard.
Khanh and I also took the side panel off the table and traced the individual WFS cables from the QPD boxes to the feedthrough. These cable are all plugged in correctly on the inside of the feedthrough.
Next step is to get Fil's help to either check the DC pin outs on the WFS boxes or check the channels on the RF PD chassis at ISC-R1.
Summary: We think the JAC WFS QPDs have two segments wired incorrectly but only on the DC readouts, not the RF readouts.
There's no reason to suspect that DC connection is somehow wrong, the issue was RF cross-wiring (which we "fixed" by making another cross-wiring) and we already knew that.
FYI, this is what happened on Monday:
We were able to move the JAC refl beam spot on the WFS using JM1 as well as picos and nothing weird was observed. PIT was PIT, YAW was YAW, you can move from e.g. segment 1 to segment 2 by YAW motion, then from 2 to 3 by big PIT etc. If segment 2 and 3 were swapped in DC, the beam would have hopped from segment 1 to segment 3, not to 2, after YAW motion, but that was never the case. So QPD connections seemed good.
We confirmed (by pico-ing the beam on WFS while using DC signals to guide us) that DC segment 2 corresponded to RF segment 3, and DC segment 3 to RF segment 2, both for WFS A and B. Clearly the RF chain was somehow cross-wired. When we disconnected the segment 2 RF cable connecting IOT1 and the field rack on IOT1 feedthrough, segment 3 signals in digital world (e.g. H1:JAC-WFS_A_I3_OUT etc.) showed big jumps in the dark offset, and vice versa, if I remember correctly. As a quick "fix" we made another cross-wiring to undo whatever cross-wiring that existed by swapping the RF connection of segment 2 and segment 3 on the feedthrough on IOT1 (alog 91272). This caused an inconsistency between the cable labels and the feedthrough marking, i.e. somethingsomething_A2 cable is now connected to WFSA segment 3 TNC on the feedthrough, A3 cable to segment 2, and the same thing for WFSB.
Considering the above, it's unlikely that the RF cross-wiring is in IOT1. It should be downstream somewhere.
JAC WFS analog whitening was in a weird state where it didn't match digital anti-whitening. Fixed it by pressing "ON" for the first and the second whitening filter.