Sheila, Gabriele, Camilla, Jennie

The other day I used Elenna's template to make a PRCL excitation: 77252

We can use this to check on our PRCL to SRCL feedforward element in the LSC input matrix, where we add some of the POP A 9I signal (PRCL) to the POP A 45I signal (SRCL) to cancel the POP45 sensitivity to PRCL.

When the PRCL excitation is on, the transfer function of POP45I/POP9I is -0.028716 cnts/cnt

We want to set out input matrix such that:

5_1 = -5_3 * POP41/POP9I tf (with PRCL excitation on) to cancel the PRCL contribution to the SRCL error signal.  This means that now, our POP 9I to SRCL ERR matrix element should be 0.107 but it is now 0.06. 

EDITING to Add:

Today I re did this measurement and saw that POP45/POP9I is -0.02551 cnts/cnt, meaning that the input matrix should be 0.0946. Updating the input matrix to this did improve the PRCL to SRCL decoupling, as shown in the second attachment. 

I also tried moving the PRCL offset, this did not seem to change the POP45I/POP9I ratio much, so the PRCL to SRCL decoupling doesn't seem to need retuning each time the PRCL offset changes. 

The third screenshot shows the PRCL to DARM coupling, there are small changes (2-3dB change) with these changes in PRCL offset and PRCL to SRCL decoupling.  So this suggests that the main coupling of PRCL to DARM isn't through SRCL.  (And that the PRCL offset sn't helping much). 

Gabriele remembered that the PRCL to DARM coupling could also be through MICH, indeed the third screenshot shows that the PRCL signal is a factor of 10 smaller in POP45 Q than in I. In 66151 Anamaria tuned the phasing so that the PRCL signal in 45Q was a factor of 100 smaller than in I, so we can try to improve this by rephasing POP45.

Edited again:

Camilla and I moved the POP45 phase from 8 to 11, this reduced the PRCL to POP45Q transfer function to be about a factor of 100 below PRCL to POP45I (last screenshot blue references show the improvement from a phase of 8 to 11).  We then remeasured the SRCL to PRCL input matrix element, which should be 0.087.  This made the SRCL coherence in DARM rather bad, although the PRCL to MICH and PRCL to SRCL couplings were improved.  We decided to revert these, and wait for next week to make the changes at a time when we could immediately retune FF.  As I stepped the phase back I accidentaly moved us from 11 to 6 in one step, which unlocked the IFO.