Currently the OMC alignment dither lines are between 1675 and 1750Hz.  We'd like to move these lines above 2kHz to avoid polluting the analysis band of the CW searches.  Previously, when we moved the dither line frequencies from ~600Hz to ~1700Hz, we noticed there was a sign flip in the dither sensing matrix.  We expect the tip-tilt response to be flat at these high frequencies, so before moving the lines again I thought it was prudent to check what kind of resonance structure we were dealing with.

The attached plot shows transfer functions from OM1 pitch and yaw to OMC-DCPD_NORM while in full lock.  Overall the reponse of the tip-tilts is what we expect, but there are resonance features around 1900Hz and above 2.2kHz.  Also the phase changes smoothly by 180degrees between 500Hz and 1600Hz, so that explains the sign flip.  Note the dropout in coherence around the violin harmonics.  The range of the current dither frequencies is marked by the black dashed lines.

It looks like we can move the dither lines to just above 2kHz.  The phase will be a few tens of degrees different than the current frequencies, so the demod rotation phases will need to be changed.  The magnitude of the tip-tilt response is roughly the same, and anyways the current dither amplitudes are only a hundred counts, so we're well below saturation.  Should be a simple fix.