error signal = signal measured behind the low-pass filter
feedback signal = output of the gain servo, going to the PZT
First of all both signals in V/sqrt(Hz) just in case I mess up the next calibration step.
The 60 Hz line (and its multiple) are a new feature. They show up as soon as the feedback loop is closed. So far, I couldn't find their origin.
For the next calibration step: