Summary: QOSEM QPD difference channels are sitting -x2-3 higher than shot noise even at high frequencies, but the measured dark noise is well beneath the shot noise level. Doesn't say much about source of noise, but has to be somewhere in the sat amp or on SUS QOSEMs.

This is one in a series of posts investigating the excess noise seen on the BBSS M1 QOSEMs. In short, at both LHO and LLO, the noisefloor of the QOSEMs appears to be around 8pm/rtHz at 100Hz, quickly increasing to 50pm/rtHz by 1Hz; where we expect the noisefloor to be flat at ~5pm/rtHz across this bandwidth. See more details in LHO alog 91466. 

Here we look at the voltage noise spectrums as measured out of the sat amp at LHO. In Figure 1, we plot the raw QPD sum, X difference and Y difference signals as produced by the sat amp (See the schematic for more details D2500190). The signals have been dewhitened. The difference channels, which give the displacement readout, are sitting a factor of ~2-3x above where the expected shot noise levels is. This is while the measured dark noise of this exact chassis is well below the shot noise level. Note that this measured chassis dark noise was taken prior to the whitening filter changes of ENG Log 1028, so there is a possibility is that these component swaps effected the dark noise, but this is unlikely.

Unsurprisingly, this indicates the problem is somewhere in the sat amp or on the sus QOSEMs, polluting noise into the difference channels. 

(Note: sum channel appears to sit below shot noise level because its an in loop signal due to the sum to led current feedback, see schematic)