Summary

The modulation phase of POP17 was optimised using SRMI.

Detail

For a long time, we haven't tuned the demodulation phase of POP17. We optimised it by checking the POP17I and POP17Q signals when SRMI was locked using them (POP17I for SRCL and POP17Q for MICH).

We injected the excitation signal at 50 Hz, which would be outside of SRCL control band, at K1:LSC-SRCL1_EXC, and measured the ASD of SRCL_IN2 and MICH_IN2. We should not see any signal in MICH, i.e. POP17Q, so we changed the demodulation phase of POP17 and minimised the excess noise in MICH_IN2. Fig. 1 shows the results of the optimisation; the injected noise in MICH_IN2 was minimised when the modulation phase was set to -141 deg (by default it was -137 deg).