On the weekend, I measured the voltage noise on the ITMX and ITMY ring heaters. I found no evidence of a ~1 Hz peak (the Hartmann scan frequency at that time).

To do the measurement, I put a 25-pin breakout board in between the RH driver box and the cable. There are 3 conductors each for the positive and negative current drives. I used the SR785 with floating input (very important) and hooked up channel 1 through a single BNC to mini-grabber to the +/- pins of the breakout. Since the 785 is floating, the ring heater driver does not short to ground.

We noticed earlier that the coupling of the RH to DARM doesn't depend on the heating state of the ring segment. The differential voltage noise across the RH is ~1-2 uV/rHz in the 10-100 Hz band. This measurement didn't find any lines from the Hartmann camera, so either its not coupling through the RH, or the Hartmann power supply infection causes a common mode variation. This could be checked by measuring the voltage of one leg of the RH relative to rack ground with the floating SR785.

The next step to do, in ring heater noise budgeting, is to directly inject some noise onto the RH. This can be done with the same breakout connection; just disconnect from the driver box and drive directly into the RH using the SR785, or more flexibly, take a DAC output, buffer through a battery powered SR560 and use its 50 Ohm output to drive the RH from the control room.