Filiberto, Dean, Daniel, Rich

More to follow, but today we reversed the installation of the recently modified OMC Piezo Driver.  Removing the driver fixed the broad 180Hz peak in DARM.  There was a suspicion that the installation of the modified chassis was causing an increase in a broad peak in DARM (about 50Hz wide at 180Hz center).  Initially, we terminated the inputs of the newly installed chassis, and removed the monitor readback cables used to monitor the newly enabled bias function (the possibility existed that there was a ground loop associated with the monitor cable).  None of these attempts seemed to remedy the broad noise around 180Hz.

We even speculated that the increased dither associated with the change to the new system may have manifested in an increase in the 180Hz component, but after reverting  to the old chassis and observing a "normal" 180Hz component with the OMC locked, we increased the dither to see if the broad 180Hz noise came back, but it didn't.  At this point, we must conclude that there is some aspect of the newly modified approach that causes an unfortunate addition of noise at 180Hz.

There is a possibility that the observed noise may be related to the change in the source impedance associated with driving the second piezo with the new HV driver board.  With this in mind, we have prepared an inline circuit that will emulate the output impedance of the new HV driver. We plan to insert this circuit tomorrow as a test.  This device may yield insight into whether the addition of the second HV driver causes some form of environmental pickup in the wiring leading to the OMC that manifests in a broad 180Hz peak.