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:

• width of a typical error signal, i.e. the frequency band width of the carrier slope, ~1.4 kHz
• height of a typical error signal 182 mV