Summary: With BBSS M1 Damping off, there is no coherence between the QOSEM readouts and coil drive monitors, ruling out DAC or CD noise as the excess noise culprit, which agrees with the noise budgeting. 

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. 

We recently switched the BBSS M1 damping loops off such that coherence measurements could be taken between the coil drive monitors and the QOSEMs readouts. This damping off time is summarized here LHO:92011.

Figure 1 plots the spectra of the QOSEM readouts (in the OSEM basis), alongside that of the coil monitors, projected into M1 motion in OSEM basis. We also plot the coherence between the QOSEM readouts and the coil monitors. During this time period the QOSEM sum feedback was on. Note that the coil monitors are AC coupled and not useful below ~10Hz.

We see that there is no virtually coherence between the coils and QOSEM readouts, which is unsurprising as the coil monitors appear to be sitting at the ADC noisefloor. However they appear to still be sensitive enough to rule out DAC or CD noise as the cause of excess noise, as predicted by the noise budgets done in LHO:91871.