In 46983 and 46083 Robert showed that the fan frequencies of the IO chassis at EX were showing up in DARM. After the work today that electrically isolated the EX ESD electronics from the rack it is installed in, it appears that this coupling has gone away.

• First figure: coherence between DARM and the differential ground monitor that Robert set up (between the beam tube and the ESD LNLV chassis). There are several peaks in the 260-290 Hz band that previously had high coherence, and now show no coherence. Also there was a peak at 73 Hz that had quite high coherence (0.9) which is also now not coherent.
• Second figure: DARM spectrum, showing that the 73 Hz peak has gone away, as well as a couple of peaks around 265 Hz. There is a new peak in the post-isolation data at 274 Hz; don't know what this is. Also the post-isolation spectrum is worse at low frequencies, but this seems to be some unrelated scattering shelf that we are currently trying to debug. The height of the 60 Hz peak is the same before and after the isolation. This particular spectrum shows higher sidebands around the 60 Hz, but that's because of the higher low frequency noise we have now; there was an earlier stretch of data where the sidebands were also the same as pre-isolation.

The EX ESD isolation seems to be a good move, and should be considered for the Y end as well.