I connected the second SR560 to the HPO injection lock HV monitor with the same setting as the PMC length PZT monitor. Blue is the injection lock, red is the PMC length PZT.

Known peaks in PMC PZT, i.e. 240Hz, 320, 480, 600 and 1kHz are all much more prominent in the injection lock feedback.

Unsurprisingly, opening/closing DBB shutters, plugging off the shutter cables, turning off HV of DBB, and completely powering off the DBB crate didn't affect the peak height of the PMC PZT as well as injection lock. (green and brown are when the DBB crate was fully powered and the shutters were closed, red and blue are with the DBB crate totally powered off and the shutter cables disconnected.)

I left DBB crate unpowered and shutter cables disconnected.

I left the injection lock monitor connected to the SR560 and the second LSC-EXTRA_AI channel.