The signal observed by the coarse frequency discriminator was actually dominated by the ADC noise above 30 Hz.
It means that once increasing the UGF more than 30 Hz the servo will feed the ADC noise to the test mass and shake it unnecessarily.
I guess this could be one of the reasons of the unstable behavior in the Y end PDH lock (#6071).
(But still it doesn't fully explain the instability).
To improve the situation I am going to do the following actions:
(1) Installation of a whitening filter (probably use of SR560s)
(2) Redesign of the servo filter
Here is a brief noise budget of the coarse sensor.
Gray curve: free running noise when no servo is applied
Green curve : in-loop noise when the ALS loop is closed with the coarse frequency-discriminator. The UGF was at 30 Hz.
Red curve : ADC noise of the coarse discriminator
So far I still kept failing to increase the UGF of the ALS servo for some reason (see #6024).