I injected 40 - 50 Hz white noise to MNP test filter bank. This is the same method with klog32983
Fig.1. IX about 45 Hz
Fig.2. IY about 44 Hz
Fig.3. EX about 44.5 Hz
Fig.4. EY about 44.4 Hz (cannot be seen in silent case)
I will check the performance of this filter after locking the interferometer
(We cannot see the peak in EY, so I didn't perform it)
I implemented those filters in P4 NBDAMP
Fig.1. and Fig.2. showed the transfer function for IY and EX.
Fig.3. showed the spectrum of DARM in the state of the INCREASING_LAS_POWER (red)with and (blue)without damping filters.
Actually, the damping of peaks can be succeeded by checking the spectrum of DARM and DHARD_P, but the spectrum around the peaks significantly increased.
So, we need to perform more narrow band filter and/or reducing the gain.
Started the oscillation and OMC DC PDs saturated after ~10 min.
It seems like DHARD_Y has better SNR than DHARD_P to see peaks around 45 Hz. Interestingly, POS_SPOL also sees this motion (could be due to ITM birefringence). See attached. These channels would be useful for noise subtraction as well (sadly, they are 256 Hz DQ channels). See attached for noise subtration result using DHARD_Y with simple bandpass filters around these peaks.
Fig.1. - Fig.3. showed the filter performance of each suspension (IX, IY and EX)
Actually, when we performed the NB damp (P4) to each suspension, the peak itself can be damped, but still increased the sideband and no improvement of the sensitivity can be found(Fig.4. and Fig.5.).
