Terra, Carl B., Rich

Terra and Carl had noticed a ~20mVp-p 1.3MHz oscillation on the ETMX ESD Driver at viewed with an oscilloscope at the output (SHV connectors on the front panel) leading to the ETM.  After confirming that the oscillation was really generated by the ESD Driver, Carl and I opened the spare to see if we could find a set of conditions that might produce such an oscillation.  By examining the spare, we were able to trace the potential instability to the first stage of the two stage pole-zero dewhitening filter.  Poking at this stage with my finger will occasionally get it to latch into the 1.3MHz oscillation state.  We drove the output of the opamp in question with a 1vp-p, 1kHz square wave through 400 ohms to get a better feel for the dampling and phase margin under normal operating conditions.  This stage is completely stable with the pole-zero stage engaged, but becomes marginally stable with the pole-zero stage bypassed.  The problem can be cured by the addition of a small (~30pf) capacitor across the feedback 21kohm resistor implying it is worsened by input capacitance.

We opted not to pull all the drivers and fix this problem.  The reasoning is that this stage is in its stable state (pole-zero engaged) while the driver is used as a low noise driver for length control of the ETM.  There is really no driving need to fix this at this point, but we wanted to document its existence for optional repair at a later date.  A fault report will be written to further document the issue.