Peter, Georgia

After looking into the OMC ASC error signals on dither vs QPD last week Peter asked me to make some spectra of the signal sent to the DAC for OM3, to see if our OMC ASC is limited by DAC noise. The actuators for the OMC ASC are OM3 and the OMC suspension. Long story short - the drive to OM3 is dominated by suspension damping, not the OMC ASC control signal.

The spectra of the MASTEROUTs (first attachment) looked very different to the error signals - the calibration lines around 16 Hz and bandstops around 9.5 and 14 Hz (these are put in in the OMC_ASC_DOF filter banks) were absent. The excitation (in this case in POS_X with the OMC on QPD alignment) is ~3 orders of magnitude above the noise.

Peter suggested there must be some extra noise coming from the damping path (the OMC ASC signal comes from the LOCK/DRIVEALIGN path). It's hard to make a direct comparison - the DAMP OUT's are not DQ'd, so instead I plotted the inputs to the damping filters, compared to the DRIVEALIGN out (second attachment). This looks very suspicious - the features at 60 and 70 Hz in the damp inputs match what we see in the MASTEROUTs. To make a proper comparison of the paths which will actually go to the OSEMS we need to apply the DAMP filters - the transfer function is shown in the third attachment.

Jeff K has said he will look into making the damping better -- Thanks Jeff!