According to the investigations done by Shoda-san and Yuhang last week, it seems that the excess of noise appearing sometimes in the BS oplev is due to the PSD itself.

Today I have replaced it with a simiar one, with the same gain. The spectra of pitch and yaw are exactly the same of the old one (when no noise was present) and for the moment no excess of noise has been observed with the new PSD. I could close the damping loop without change any gain.  I will keep monitoring the spectra in the next days.