Following on from a previous attempt (see alog  #4173) where the PZT kept oscillating even when both the common and fast gains were set to zero.
It was found increasing the fast gain to 0 dB and then increasing the common gain stopped the PZT from oscillating.  OUT2.png
shows the error siginal for a few gain settings.  The highest I could push the gains was a common gain of 26 dB and a fast
gain of 3 dB (see C26F3.jpg) at which the UGF was 835 kHz - past where you'd want to be from a phase margin standpoint.  The
loop at this setting seemed sensitive and prone to oscillating, most likely because of something beyond 1 MHz (see IN2.jpg
for the mixer spectrum).