On Tuesday when we measured dark noise of the QPDs we noticed what we think is some excess noise on QPD A.
First I checked the voltage on QPD A and B (we think thise dark noise spectra looks ok but I wanted to compare) with the light unshuttered and measured around 7 V Dc for each quadrant on QPD B and 6V per quandrant for QPDA.
With the shutter closed I looked at quadrant 1 on QPD B and then each quadrant on QPDA ( I used channel 1 and 2 for the opposite sign legs of the quadrants between qpd A and qpd B).
QPDB Q1 shows random noise which is a similar level to noise on QPDA Q1,2,3,4. I don't think any of these show an oscillation large enough to be the op-amp in the TIA oscillating. I also unplugged the input signal to the chassis and the noise on Q2 did not really change.
Summary: It's probably not the op-amps oscillating in the TIA chassis. Investigations ongoing.
Marc, Jennie
Marc measured the grounding of the signal input to the TIA and it was connected to the chamber ground. This would allow current to flow through the TIA v1 from chamber to SUS-R2 rack.
We removed the ground connection at input to TIA using a breakout board with pin 13 jumper removed.
Restesting shows good dark noise spectra limited by ADC noise for all four quadrants.
We removed this temporary solution and put in a work permit to fix this at the flange.
