With Taaki Yokozawa, Takafumi Ushiba

After replacing the IMC CMS, we needed to retune the system-side offset compensation parameters because the analog circuit characteristics changed. In particular, we adjusted the offsets associated with IMC CMS gain changes.

Procedure

1. Closed the IR laser shutter.
2. Set gains to 24 dB (IN1) and 7 dB (IN2), then checked the offset of the signal before the filter (K1:IMC_SERVO_MIXER_DAQ_OUT_DQ): about 0.498376 V.
3. Opened SERVO_IN1_GAIN_OFS_COMP.
4. Decreased the IN1 gain step by step and, for each step, tuned the corresponding value in SERVO_IN1_GAIN_OFS_COMP so that the measured offset remained at 0.498376 V. This was carried out from 24 dB down to −32 dB. During the process, a script was written to partially automate the adjustments.
5. For IN2, first disconnected the CARM CMS input so no signal would be sent to the IMC CMS from CARM. Then passed only IN2 and adjusted the corresponding entry (7 dB) in SERVO_IN2_GAIN_OFS_COMP while monitoring the same pre-filter signal.

Result

• IN1: Offsets were adjusted for all gains ≤ 24 dB.
• IN2: Offset was adjusted for the 7 dB setting.

Reference signal: K1:IMC_SERVO_MIXER_DAQ_OUT_DQ