Today, I installed a notch filter for the phase loop lock system for the X arm.
This filter is inserted between PZT and PLL servo.
The attached figures are the notch filter configuration and the measured OLTF of PLLX.
Because the inductor is variable, the value of the inductance in the figure may not be accurate.
The inductor has a 6 ohm resistance.
From the measurement results, it can be seen that the PZT mechanical resonant peaks above 200 kHz are successfully eliminated.
The phase margin decreased from about 50 degrees to about 25 degrees.
At present, the PLL servo includes a 100 kHz LPF for attenuating the PZT mechanical resonant peaks, it is expected that the phase margin can be recovered by removing this filter.
