Summary: These is no correlation between the DC level of the QPD difference channels and the output voltage noise, suggesting excess resistor noise is not to blame.

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. 

Thus far we have rulled out real motion being the casue of this excess noise, as motion measured by the BSC2 GS13's, propagated down to M1 isn't enough to explain it. Leading suspicions are excess noise arrising from the new resistors used in the whitening changes (which in the past have been observed to cause issues, see ENG Log 1028, for more details) or LED noise. A trait of excess resistor noise is a roughly linear increase with increrasing DC voltage across the resistor. As such I would expect a correlation between the DC difference voltage and the noise on the difference signals if this is the culprit.

In Figure 1, we plot the DC level of each of the 12 difference signals from the LHO BBSS QOSEMs, against the voltage noise level of that signal at 15Hz (frequency away from any suspension resonances). Here we see no clear correlation between the DC level and the noise, indicating that its likely not excess resistor noise driving this.