H1 SUS RM1 and RM2 Damping Loop Open Loop Gain / Loop Suppression After Gain Drop -- Assuming Loops are SISO...

J. Kissel, O. Patane, E. Capote

After we dropped the RM damping loop gains (see LHO:84590), we remeasured the open loop gain transfer function (OLG TF, or just G) and loop suppression TF [ 1 / (1+G) ], in order to see / confirm that the frequency dependence in the 1-3 Hz region changes, and hopefully ''reducing'' it back to its 2022 response and regaining phase margin / reducing loop suppression magnitude.

The magnitude of the OLG TF decreased by the expect value.
The magntidue of the loop suppression also reduced back to virtually identical values.
While yes, the phase margins in the OLG TFs agree with the loop suppression, the frequency dependence of the phase of the OLG TF did *not* restore to original frequency dependence.
I again suspect the change in cross-coupling transfer functions -- namely the influence of P on the L loop and the influence of L on the P loop has changed because the underlying mechanical system has changed in this frequency region. 
Said differently, we only ever look at the on-diagonal OLG TFs. If we look, e.g. at the change in IN1 P / IN2 L (in the presence of L EXC), we might be able to get a better story. 

Oli's retaking the damped plant transfer functions [ P / (1+G) ], which will be the final metric to see if this gain adjust is ''good enough.''
Elenna will be taking before vs. after DC centering loop open loop gain transfer functions to also confirm if / whether this is ''good enough.''