I looked at the coherence between DARM and the ESD power supply voltage monitors (the power supply voltages are run into temporary PEM channels through high-pass filters - soon to be permanent). There was quite a bit of coherence around 60 Hz, explaining some, but not all of the features around the 60 Hz peak in DARM (see Figure 1). Some of the other 60 Hz sidebands in DARM are associated with suspension resonances at about 0.4 Hz (Figure 2). There is also a roughly 1 Hz comb of side bands that is visible in the temporary channel that monitors the voltage between rack ground and the chamber (Figure 1) as well as in the mains monitors at all stations (Figure 3). Finally, at certain other frequencies, such as around 280 Hz, there is also strong coherence between DARM and power supply or rack-to-chamber voltage monitors (Figure 4).

We hope that at least some of the noise from the power supply will be reduced once the ESDs have dedicated power supplies (I think the plan is to replace them this week), but the 1 Hz comb on the mains, at least, will still likely couple and needs to be tracked down. I have doubts that the broad features around 280 will go away either, since they are in the rack-to-chamber voltage monitors as well as the power supply monitor.