Dean, Filiberto, Daniel, Jenne, Rich

Following up on work done by Koji wherein he discovered that multiples of the IFO sidebands can resonate in the OMC cavity, a revision has been made to the OMC High Voltage Driver Chassis (D1300485).  The OMC higher order mode (HOM) content changes due to deformation of the OMC cavity mirrors as voltage is applied to the OMC piezos.  The OMC piezos are donut shaped and do not simply move a mirror in piston motion.  The mirror, which is glued rigidly to the piezo, is stressed as well as translated in piston motion by the piezo, which changes the radius of curvature of the OMC cavity mirrors.  It is this effect that changes the HOM content of the OMC cavity.

Attached are some diagrams that will simplify the explanation of the changes made to the OMC High Voltage Driver Chassis.  In a nutshell, an additional high voltage driver was added to drive one side of the piezo used to perform the shutter function and inject a dither.  The other side of this same piezo is still attached to the Low Voltage Driver board within the OMC High Voltage Driver Chassis.

A result of this topology change is that the sign of the dither flips, and the amplitdue of the dither is reduced by a factor of 1.5.  This reduction in amplitude may be due to the source impedance of the high voltage driver circuit now terminating the other side of the piezo used to inject the 4.2kHz dither signal.  This source impedance looks like 50kohms in parallel with 0.47uF.  Effectively the piezo and the terminating capacitor form a capacitive voltage divider.  The factor of 1.5 was derived by Jenne looking at the change in the OMC length locking loop gain before (by archived transfer function thanks to Shiela) and after the modification.  This is a bit odd though as it would imply that the piezo capacitance is of order 1uF, which seems high.  Another possibility is that the archived transfer function differs from the transfer function taken today due to some other factor unrelated to dither amplitude.  We don't know yet, but once the value of the OMC piezo capacitance is known, the calculation can be made.

1.  The serial number of the chassis removed from ISC-R5, slot 13 is S1301298
2.  The serial number of the modified chassis returned to ISC-R5, slot 13 is S1301297
3.  The serial number of the chassis taken from 3rd IFO to donate the new HV driver board is S1301299.  D060283 circuit board S1301295 was removed from this chassis and is now installed in the modified chassis as the OMC length loop HV driver as shown in the attached PDF containing a high resolution photo of the modified chassis.
4.  A Y-cable was added to use two unused ADC channels previously associated with the HAM6 RFPD Interface chassis channels 3 and 4.  These channels are now used to read the DC (channel 3) and AC (channel 4) of the newly installed additional HV driver board within the modified chassis. Details of this cable are shown in the attached PDF.