[Rana / Koji]
The MC servo loop has been investigated as the MC servo was not an ideal state.
With the improved situation by us, the attached setting is used for the MC and the FSS.
The current UGF is 24kHz with phase margin is ~15deg, which is unbearably small.
We need to change the phase compensation in the FSS box some time in the next week.
- We found the PD has plenty of 29.5MHz signal in in-lock state. This was fixed by reducing the LO power and the modulation depth.
- The LO power for the MC demodulator was ~6dBm. As this was too high for the demodulator, we have reduced it down to 2dBm
by changing attenuator to 12dB (at 6 oclock of the dial) on the AM stabilization box.
- The RF power on the MC PD was still too high. The PD mush have been saturated. So the modulation slider for 29.5MHz was moved
from 0.0 to 5.0. This reduced the 29.5MHz component. (But eventually Koji restored the modulation depth after the servo shape has been modified.)
- The openloop gain of the loop has been measured using EXC A/TEST1/TEST2. The UGF was ~5kHz with the phase mergin of ~10deg.
- This quite low phase margin is caused by the fact that the loop has f^-2 shape at around 4k-100kHz. The reference cavity has
the cavity pole of 40kHz or so while the IMC has the pole of ~4kHz. Basically we need phase lead at around 10-100kHz.
- We decided to turn off (disable) 40:4000 boost of the MC servo to earn some phase. Then MC did not lock. This is because the LF gain was not enough.
So put Kevin's pomona box in the FAST PZT path (1.6:40). By this operation we obtain ~75deg (max) at 560Hz, ~35deg at 5kHz, ~20deg at 10kHz.
- In this setup the UGF is 24kHz. Still the phase margin is ~15kHz. This phase lag might be cause by 1) the MC servo circut 2) PMC cavity pole
- Put/modify phase lead in the FSS box.
- Measure the PMC cavity pole
- Measure and put notch in the PZT path
- Increase the UGF / measure the openloop TF