[Enomoto, Sugimoto]

Today, we swapped the X side PLL servo with a spare one (S1605812→S1809505).

The newly installed one has the pole of LPF in slow path changed from 100kHz to 500kHz.

Attached figure show the measured OLTF of the new PLL loop with the previous results.

Although the phase margin was reduced by inserting the notch filter into the  loop (9140) , about 10 degrees was recovered by changing the low pass.

If the phase margin is allowed to about 30 deg, it is possible to increase the overall gain about 3 db.

It seems good.