[Matt, Carl, Jenny, Sheila, Evan, Jeff]

The SR785 monitorring the ETMY QPD transmission has not uncovered any large hidden test mass accoustic modes in the 65-75kHz band.  It has however highlighted the fact that there is a strange 3kHz comb on these QPDs.  See this first image showing the comb (3 peaks) and the second image showing the 65-75kHz band at 50W.

This comb is present on both the fast 10-80kHz PI channels and the normal QPD A channel.  The comb appears in the X and Y arm but they are not identical.  See third image.  Also in this image the spectra before and after the whitening gain is changed (in guradian state DRMI_ON_POP) indicates that the source of these peaks lies before the whitening gain stage.

There is little indication of anything in OMC DCPDs other than these peaks being in the vicinity of violin mode harmonics - see last image.