Jennie W, Marc P, Fil C,

Summary: We still haven't found the issue with the segment 4 on QPD B and why it's noise is ~40 times higher than the other segments.

Marc and I checked (with a voltmeter) that the cable that takes the QPDB signal from chamber to the TIA variant 2 chassis is not connected to chamber ground and that it is connected to chassis ground at the other end that plugs into the TIA chassis.

The chassis input socket shield is also grounded to the rack ground through the front panel of the chassis.

To check that the high noise in segment 4 is not due to any problems with the shield, we plugged in a breakout board (which disconnects the cable shield from the socket shield) and checked the dark noise spectras again using the method and offsets detailed in alog #92010.

The high noise on segment 4 was unchanged by this.

This is making it look more likely that the problem is in chamber.

Tomorrow Marc and will use the breakout board to swap segment 3 and 4 at the TIA chassis input just to check this.

If the problem is only shown on the channel connected to segment 4 on the QPD then the problem is in chamber.

If this test shows that the problem persists on the same TIA channel then the next thing we might try is opening up the chassis to check if whitening is engaged for each quadrant (see D10001974 for wiring diagram of aLIGO ISC QPD Transimpedance Amplifier Primary Filter PCB). If the problem is only shown on the channel connected to segment 4 on the QPD then the problem is in chamber.

As a cross check I then turned off the measurement offsets in the QPD filter banks and re-opened the SPI shutter. Here is a graph of the spectra with light on the QPDs and you can see all four quadrants respond similarly for each QPD.