Summary:

Somebody asked for a better L2P from ETMX L1 stage, so I made an attempt at making one.

With new top mass damping that was described in alog 16895 without OL damping, the measurement was done, filter was made, nothing was tricky, no hand tuning, no nonsense unstable poles, it just works.

But of course when OL damping is on, this nice L2P decoupling is broken (because the nice thing was made without OL damping).

I can probably make similarly nice decoupling filter for when the OL damping is on, but the question is exactly when such nice decoupling is necessary. OL damping on? Off? Both?

The ETMX is left with old configuration.

Details:

EX L1 drivealign L2L to oplev P, and drivealign L2P to oplev P were measured, and the latter was divided by the former. In both of these measurements, uniform noise from 0.1 to 1Hz was used and the amplitude was set so each coil outputs several thousands counts or so.

The resulting inversion function between 0.1 and 1.14Hz was fitted using happyvectfit, discarding any bad coherence data. No tuning was done except for selecting the measurement data and setting the fit order to 8.

The resulting L2P filter looks much nicer, Qs are lower,  and in general it makes sense more than the ancient filter that was eventually abandoned (first attachment).

The second attachment shows the step response of ETM oplev (blue) when a large length drive is applied to the L1 stage (brown).

Left panel shows that the step response was about an order of magnitude smaller with the new L2P filter than the old flat gain filter that is used these days.

Right panel shows that when the OL damp is on, this nice reduction is gone, both the flat filter and the new filter are about the same, they are larger than "new" in the left panel.

Note that I had to make a new oldamp filter for this, as the old OLdamp filter is incompatible with the new M0 pit damping.