[Enomoto, Yuhang, Matteo, Eleonora]

In the past days we observed an oscillation in the SHG lock at about 20 kHz.  This is likely to be due to the change of the loop shape as we removed both a low pass fiter and a high pass filter in the control servo.

Before finding out that the LP filter in the High Voltage Piezo Driver could be disabled (see entry #1016), Matteo had designed a HP filter with the goal of componsating its effect and allow for a larger loop bandwidth.

The TFs of this two filters and their combination are shown in the attached picture (taken from entry #585).

As suggested also by Raffaele, the LP and the HP where non exactly compensating above 10 kHz and this could avoid the oscillation at 20 kHz (likely due to a Piezo resonance) that we see when we remove both of them. 

As a temporary solution we decided to keep both the filters on. We will study a better filter to be implement in a new version of the control board.