Yesterday I measured the noise of the refcav PDH loops. Because of RFAM effects, possible nonlinearity in the RFPD response, etc., the correct way to measure the loop noise is to take the PSD of the error signal (Common OUT1 on the TTFSS) while the cavity is unlocked but light is still incident on the cavity and on the RFPD.

For these measurements, the south TTFSS gain was 642 common and 702 fast, and the north TTFSS gain was 802 common and 835 fast; these are the highest gain settings I could achieve before the loops started to oscillate when locked. There was 1 mW of light incident on each cavity.

Plots and data are attached. I've converted from voltage to frequency using the slopes I found in PSL:1339. Current thoughts:

• These plots seem to say that at high TTFSS gain, we're actuating on sensing noise rather than cavity frequency noise. I'm not sure I believe this yet.
• We're dominated by (what is probably) seismic noise below a few hundred hertz.
• There's something goofy with the north TTFSS; I would say it's a grounding issue, but the fundamental peak looks more like 75 Hz than 60 Hz.