Stefan, Kiwamu, Matt

To test the theory that the ILS is gain limited, and that more gain might help with DARM noise, we measured the loop and used the TF input to add some gain at low frequency.

The open-loop TF was as previously reported (see 31121 and photos to follow).  We had computed that an SR560 with a gain of 10 and a pole at 100Hz accompanied by a 10kHz passive low-pass would make for a good test without destabilizing the loop (see attachment 1).  With this test-rig attached the TFB and TFIN ports on the ILS front panel (photos to follow), we relocked the IFO and found that the coherence with DARM had been reduced as expected (see attachment 2).  Noteabley the PMC HV signal was significantly reduced (black to brown in attachment 2), indicating that this gain was preventing noise in the PMC.  Unfortunately, this simple test was not able to apply much gain to the 1kHz, where the highest coherence with DARM is seen.

Kiwamu has offered to investigate the possibility of using a new CDS FE running at 65kHz to implement a tailored gain profile which will suppress the 1kHz peak and other HPO features while keeping the ILS loop stable.