Following up on the discovery that the 1.34Hz peak in DARM was injected by the SRCLFF, I implemented a more aggressive high-pass with a 1.34 Hz notch in the SRCLFF path.

The new high-pass and notch completely remove the 1.34 Hz peak from DARM, and improves the DARM noise in the 1-2 Hz region. The reduction in RMS is visible also in the time trace of the DARM error signal.

As expected, the new high-pass introduces a bit of phase rotation at 10-20 Hz, and this limits the amount of subtraction we can get now with the SRCL FF. I also tried to retune the SRCL FF with a new measurement, but I could not improve the subtraction. The reason is that I need to take a new measurement of the SRCLFF to DARM path with the new high-pass and notch. I did not have time to do it today because violin modes were damped and we needed to go back to Observe. But it should be an easy fix (10 minutes measurement + retuning + 10 minues test)

However, looking at the coherence between DARM and SRCL, I don't see a lot of it, so maybe we're fine with this reduced level of SRCL subtraction.

The new SRCLFF combination to be used is FM1 FM2 FM3 (all together). ISC-LOCK has been updated and reloaded