Kiwamu, Evan

There was some discussion about whether we could improve on the current high-frequency DARM rolloff (a 950 Hz fourth-order elliptic).

We came up with a gentler filter (see attached foton plot) that has no extra phase loss below 100 Hz, but still manages to reduce the rms drive to the EY ESD by almost a factor of 2 (see attached DAC spectra) through a combination of more thoughtful shaping and judicious notching. In particular, we installed elliptic bandstops around 1237 Hz (not sure what this is), 2162 Hz (the OMC angular dither lines), and 5050 Hz (presumably a forest of upconverted violin modes). (Fotonistas will note the deliberate tuning of the elliptic notches for the OMC dither lines, although in this case it may be overkill.)

However, we have not implemented this new filter, since we do not want to disturb the current calibration efforts.