I took beatnote measurements and spanned gain values in the PMC to see if stability issues in PMC loops can be affecting the FSS downstream.

Method

• I used BNspec.py with default settings (10 kHz bandwidth at 15.625 kSa/s for 60s) while spanning the parameter space of PMC gains.
• The beatnote frequency was robustly within 2 kHz of 27.34 MHz during this whole measurement.
• Overall experiment was run by spanParamSpace.py script.

Inference

• I calculated the integrated total beatnote frequency noise in the frequency range 200 Hz to 1 kHz where the total noise is expected to be dominated by coating Brownian noise.
• Unstable regions in the parameter space are clearly visible.
• Ideally, the noise in beatnote frequency should have very minimal coupling with the gain values in PMC loops.
• This has been found true indeed for the south side but not so much on the North side.
• Very clearly there is a corridor from -3 dB to 8 dB in NPMC gain where noise remains low. We generally use 7 dB value.
• But this might just be too bad PMC loop causing light amplitude noise in the output.
• There is a bulge at 400 Hz in the beatnote freqeuncy noise. I can also see a broadened oscillation in PMC error signals around that frequency on both sides.
• I'm afraid that maybe the North PMC is causing big mode mismatch at 400 Hz causing the noise in the beatnote frequency. Or is that not possible?

Data