[Patrick, Gabriele]
As reported in 43475, one of the PZT used to steer then ALS green beam into the Y arm (using the transmon QPDs) was oscillating at ~400Hz.
Today we went to EY, pulled the PZT driver, and checked the J9 jumpers which controls the bandwidth of the closed-loop PZT controlled (see also E1300870). The goal was to change them from the "full bandwidth" to the "half bandwidth" configuration.
Unfortunately, the jumpers were already in the half bandwidth configuration. Just to be sure, we moved both of them (X and Y) in the "full bandwidth" configuration. The 400 Hz oscillation in the QPD signals got worse.
We put everything back in the previous configuration. No change in the oscillation.
If we really think this oscillation is a big issue, here are some possible options:
- Contact the manufacturer and see how they suggest we modify/tune the closed loop controller
- Replace the thick 2" mirror with a lighter version, so that the resonance frequency will move up and hopefully out of the control band
- Add some "rubber" to damp the PZT resonance, so that the peak moves farther away from the unity gain
