Craig, Jenne, Evan

We have been running for a long time with the PUM analog filtering in a medium-noise state (ACQ off, LP off, aka state 1). Today we tried flipping a few times to the fully low-noise configuration (ACQ off, LP on, aka state 3). We repeatably observed a reduction in noise between 9 and 20 Hz. Nothing repeatable was seen at higher frequencies.

Attachment shows this result at the following times:

• Purple trace: 5 minutes starting 2022-07-29 20:23:57
• Green trace: 5 minutes starting 20:41:47
• Red trace: 5 minutes starting 21:29:12

Also during this state we were running with one stage of low-pass filtering on the ETM UIMs, whereas the nominal configuration so far is no low-pass filtering on the ETMs (and three stages on the ITMs). We were able to acquire lock like this just fine. The largest drives occur during CARM offset reduction, reaching around 120000 ct peak with the rms dominated by high-frequency (>100 Hz) content. There was no appreciable change in the DARM noise.

The new PUM filtering has not been put into the guardian, but would simply require changing the coil state code during the lownoise coil driver guardian step. UIM filtering does not seem to be controlled by the guardian, so I left it with one stage of low-passing on the UIMs as described above.

Similar tests were last done here in May of 2021 (LHO:58927).