(Kiwamu, Stefan)

IMC demodulation phase rotated again for the 4th time...

After the IMC sideband sweep measurement this morning we noticed that the IMC WFS were no longer stable, and the IMC locking was a bit shaky. Sure enough we found that  the RF phase rotated again, with most of the signal back in the I-phase (we use the Q-phase for feed-back.)

The 1st attached picture shows the I signal along the x-axis, and the Q signal along the y-axis. Also note that the size of the signal is not much smaller than in the picture from alog 7181 (and we currently has a slightly reduced PSL input power due to the installed 10% and 30% BS for the FI isolation measurement).

For now we again adjusted the phase of both length and angle sensors.

The new delay setting is in the 2rd attached picture (the old one is in alog 7119 - note that in alog 7119 we used the I-phase for feed-back). Picture 3 shows the I signal along the x-axis, and the Q signal along the y-axis after the LO delay change of 9.75nsec (~85deg).

The new WFS phase settings are the same as back in alog 7119 :

H1:IMC-WFS_A_SEG1_PHASE_R = -185
H1:IMC-WFS_A_SEG2_PHASE_R = -112
H1:IMC-WFS_A_SEG3_PHASE_R = -185
H1:IMC-WFS_A_SEG4_PHASE_R = -185
H1:IMC-WFS_B_SEG1_PHASE_R = -80
H1:IMC-WFS_B_SEG2_PHASE_R = -132
H1:IMC-WFS_B_SEG3_PHASE_R = -66.5
H1:IMC-WFS_B_SEG4_PHASE_R = -173

Before I changed them, the phases were

H1:IMC-WFS_A_SEG1_PHASE_R = -90
H1:IMC-WFS_A_SEG2_PHASE_R = -17
H1:IMC-WFS_A_SEG3_PHASE_R = -27
H1:IMC-WFS_A_SEG4_PHASE_R = -22
H1:IMC-WFS_B_SEG1_PHASE_R = +15
H1:IMC-WFS_B_SEG2_PHASE_R = -34
H1:IMC-WFS_B_SEG3_PHASE_R = +28.5
H1:IMC-WFS_B_SEG4_PHASE_R = -81

In conclusion - we are back in the same "RF phase state" as in elog 7119.