During the O3GK, we often saw 1 Hz oscillation, which is a resonance of pendulum mode below BF stage, which cannot be seen well at BF LVDT.
So, I implemented a narrow band damping filter at BFL by using TM_OLDAMP_L signals.

In the case of ETMX, the resonant frequency is 0.977 Hz, so I designed band-pass filters and test it.
Figure 1 shows the OLTF of the designed loop.
Figure 2 shows the spectra of TML OpLev with (red) and without (pink) the new damping filter.

1 Hz peak seems to be well damped without large excess noises, so the damping filter looks fine.

Note:

The followings are the differences from the other NB damp filter implemented in the payload.
1. I used VIS-ETMX_BF_OLDAMP_L filter bank insted of NBDAMP_L filter because the NB model doesn't have a path to BF actuators.
2. I broaden BP filter from 10% to 30% to reduce the gain peaking around the UGF. During the work, I slightly modify the NBfilter_design.py to add a function to broaden the pass-band width. So, if you press the command "python3 NBfilter_design.py 0.977 0.3", you can obtained the low and high cut-off of the necessary BP filter.