[Yokozawa, Komori]

Abstract:

We began implementing a new filter for PR3 pitch control to reduce control noise coupling into DARM around 20 Hz.

Details:

As discussed in klog:33437, one of the next commissioning goals is to mitigate PR3 pitch coupling noise, which currently dominates the DARM sensitivity in the 20 Hz region.
To address this, I designed a new local control filter, shown in the attached figure (blue: current filter, red: proposed new filter).

Concepts of this modification are lowering the cutoff frequency of the elliptic filter to reduce the sensing noise feedback by more than one order of magnitude above 10 Hz.
The details are as follows:

• Lowered the cutoff frequency in FM4 from 15 Hz to 8 Hz

• Shifted the low-frequency pole in FM4 from 0.1 Hz to 1 Hz to keep the phase mergin around 2 Hz

• Reduced gain in FM4 from 38 dB to 15 dB to keep the overall gain

• Increased the resonant gain in FM6 from 30 to 40 to maintain gain at the pendulum frequency ~0.4 Hz

• Adjusted FM5 gain for PRC2 control from 0.5 to 0.35 to maintain the relative contribution between WFS and local control

In the initial trial, we attempted to switch the local control to the new filter after reducing the WFS gain.
However, the IFO lost lock during the transition.
Shortly after the lockloss, the gate valve was closed, so we suspended further work.

All modified settings were reverted, and we plan to retry the implementation soon.