today we performed some test for BS blending strategy in order to find one proper combination of parameters (blending filters and gain of the low frequency brench), to see if the interaction with PR tx loop could be reduced on the basis of what we understood in the past months.

None of the actions seemed to have a big impact on the interaction at 1.2 Hz. the most significant results are reported in fig.1 and fig.2 where the error signal spectra of PRtx and the correction spectra of BStx are shown respectively.

the legend speaks for itself:

• standard blending, higher LF gain (1 instead of 0.67). this didn't cause any improvement or major modification, besides an expected worsening of the correction (blue);
• the blending with higher crossing frequency (higher order with poles at 3.22 Hz), with higher LF gain, where the 1.2 Hz got damped, but an instability at around 2 Hz started to arise (red), together with a major worsening of the correction spectra. The situation was recovered when the gain was restored to its nominal value (yellow).
• the major effect on the loop was obtained switching to the older blending (first order, poles 1 Hz), where the 1.2 Hz oscillation improved a bit (purple), together with other region of the spectra. On the other side, the 400 mHz oscillation got excited. Indeed in the first place we moved to the 2Hz blending in order to get rid of this instability. Changing the gain didn't help as well but caused only the increase of the 1.2Hz.

in conclusion, working only on the blending didn't help to solve the problem.  the only knob left is to work on the control filters of the two loops.

we left the itf locked in LN3 with the standard working configuration of BStx.