Evan, Den
Tonight we asked ourselves a question whether 1/f (or 1/f^2) noise, seen in DARM around 100Hz, comes from the OMC or not. We put a bandstop filter to the DARM control loop at 92-127Hz and made a correlation measurement between AS 45 WFS SUM and OMC PDs at this frequency band. The idea is that if there is any OMC noise coherent between 2 OMC PDs, it should not be present at the RF detector.
Attached plot shows the results of the measurement. Cross-spectrum between WFS and OMC PDs around 100Hz is almost the same (15% lower) as cross-spectrum between two OMC PDs. We integrated for 3.5 hours and AS 45 WFS SUM channels are not DQ. We can significantly improve the measurement if we record these channels and integrate for ~20 hours.
At the current precision of the correlation we can not say that noise around 100Hz comes from the OMC. Right now it looks vice versa.
We have also measured coherence between DARM and voltage noise of +15V signal going into the vacuum. Noise level is 2uV/sqHz at 100Hz and coherence is <10-3. This noise is insignificant for the current sensitivity.
Den, which +15V signal is this that you measured?
Rich, we have measured the noise on pin 6 (+15V head 1, D1300502). This noise is highly coherent with noise on pin 2 (+15V on head 2) and partially coherent (0.4) with noise on pin 7 and 3 (-15V, head 1 and 2). For this reason, we did not measure DARM coherence with other pins.