Related: PUM P2P and Y2Y inversion filters:
https://alog.ligo-wa.caltech.edu/aLOG/index.php?callRep=10610
I injected into PUM drivealign P2P, through P2P inversion filter, and measured the TF from injection to the OL P (red thick) and Y (red thin dashed). Did the same thing to Y2Y to OL Y (blue thick) and P (blue thin dashed).
In a frequency band where I managed to get a high coherence (0.2Hz to 4Hz) P2P and Y2Y look pretty good. Also P2Y is good. However, Y2P (blue thin dashed) looks problematic between 1 and 2Hz (and maybe 0.4-0.6Hz).
I'll make a Y2P filter that eliminate this.