Nakano, Akutsu

To know the CARM noise contribution to Displacement, we measured a transfer function from CARM to DARM.

The details

The first issue was measuremnt of the transfer fucntion from CARM to displacement. Concretely speaking, from which to which channels do we need to measure? In the end Nakano-kun measured

K1:CAL-CS_PROC_DARM_DISPLACEMENT_DQ / K1:LSC-REFL_PDA1_RF45_I_NORM

with exciting CARM via K1:LSC-DGCARM_EXC.

See FIg. 1. It is about 0dB around 200-300Hz, and rolling up at the rate of 20dB per 500Hz around 1kHz. So simply speaking, if you plot the above two signals at once, if the two signals are at the same level, you can estimate the tendency. By the way, NORM was not a DQ channel. Well, to soleve the incovenient we found, he modify the digital model to include the normalized channel to the DQ list, and rebooted the system. Anyway when he did the measurement, the NORM channel was not a DQ channel...

Then I fit it; see Fig 2. As mentioned above, the transfer function can be fit more easily by eyeballs, since we don't even need the phase, but my curiousity did not stop do this kind of serisous fitting. I just used well-coherence frequency range referring to FIg.1's left, where coherence between displacement vs exc, and RF45I vs exc are found. 100 Hz -7 kHz are basucally used this time.

(Projection...)