To compare with future PRMI locking, I measured spectra of POPDC and feedback signal. I also measured openloop transfer function of half-PRC locking.
Beam spot motion was at ~ 2.4 Hz, not 3.3 Hz.
Below is uncalibrated spectra of POPDC and LSC feedback signal (C1:LSC-PRM_OUT).
Below is openloop transfer function of the half-PRC locking loop. UGF is ~ 120 Hz and phase margin is ~ 45 deg. This agrees with the expected curve.
Data was taken when half-PRC was locked using REFL11_I as error signal and actuating on PRM.
For comparison, POPDC when PRMI was locked in July 2012: elog #6954 and PRCL openloop transfer function: elog #6950.
Peak in the spectra of POPDC and feedback signal was at ~ 3.3 Hz in July 2012 PRMI, but it is now at ~ 2.4 Hz in half-PRC. The peak also got broader.
Is it because of the change in the resonant frequency of the BS-PRM stack? How much the load on BS-PRM changed?
Or is it because of the change in the resonant frequency of PR2/PR3?
Phase margin is less now because of gain boost ~ 5 Hz and resonant gain at 24 Hz.