[Gabriele, Sheila, Louis]
We changed the OMC lock dither frequency to 4190 Hz.
As mentioned in LHO:76582, we moved the OMC Lock dither frequency from 4100 Hz to 4101 Hz and saw the 4Hz bump in the OMC lock control loop error signal, H1:OMC-LSC_I_OUT, move to 5Hz as the ramp progressed.
This confirms that the 4Hz bump was due to mixing between the frequency of the OMC lock dither (nominally, 4100 Hz) and the 4096 Hz. It's not completely clear why 4096 Hz is special now but we note that it's equal to 16384 Hz / 4. This bump wasn't present in O4a so why is here now?
In any case, moving the lock dither frequency up to 4190Hz should take care of this issue. We expect the bump to move up to 94Hz, well above the UGF of the OMC lock loop where it won't dominate the error signal RMS.
We also had to change a band pass filter in OMC-LSC_PD_IN, to center around 4190 Hz instead of 4100 Hz. The old band pass is FM7, the new is FM8. If we stick to 4190 Hz dither, we need to update guardian.
The bump at 4 Hz in the OMC error signal is gone.
And indeed the bump moved to 94 Hz