I tried to replace IMC CMS to increase the PZT actuator range and reducing the glitches when increasing laser power by adjusting offset voltage at the input gain stage.
However, I found that it is difficult to adjust the offset due to the input offset is larger than the adjustable range, so I gave up replacing the CMs this time.
Detail will be posted tomorrow.
During the work, I accepted the SDFs shown in fig1 and fig2 in the down.snap to remember the epics value changes during my work.
The CMS circuit diagram can be found in document JGW-D1503567-v5.
The work has the following two purposes:
1. Increasing the gain of the FAST path by a factor of 10, which allows us to turn off the 0.1 gain at the SLOW path and increase the actuator range of the laser PZT by a factor of 10.
2. Adjusting the offsets at the input gain stages reduces glitches that can sometimes cause lock loss when increasing the laser power during gain switching.
Note that the detailed modifications for purpose 1 are reported in klog35396.
For purpose 2, trimmers have been installed on U10, U13, U14, U60, U61 and U65 (see the modification log in JGW-S1809466).
The following strategies are adapted for adjusting offsets at the input gain stages.
The input signals (Vin) are converted from differential to single-ended by U2, U3, U4 and U5, and U10.
During this process, offsets (Vofs_in) are added to the signals.
The gain stage then increases (or decreases) these signals according to the corresponding stage gain (gx) and adds some offsets intrinsic to the gain stage (Vofs_gx), where gx is a gain of x at the input gain stage).
These signals are monitored at the MIXER DAQ channels, which include the offsets (Vofs_ mon) at the monitoring circuit.
Therefore, the final outputs that we observe can be written as the following equations:
Vout = gx (V_in + Vofs_in) + Vofs_gx + Vofs_mon
Considering the condition when IMC is locked, Vin + Vofs_in ~ 0 thanks to the feedback loop, so what we would like to achieve is Vofs_gx ~ 0 for all gx and Vin + Vofs_in ~ 0 without signals.
Note that, since the trimmer is only installed for gx = 16 dB and 8 dB, not all gains are adjusted, but this is acceptable because Vofs_gx becomes small when gx is small.
To achieve the above, I adjusted the gain state using the following procedure:
1. connecting the signals from RFPD (IN1) and CARM CMS (IN2), closing all laser shutters in the process.
2. Measuring the Vout when the input switch is opened and closed with a gain setting of 0 dB.
3. Adjust the offsets of U10 so that Vout does not change when the input switches are opened or closed. This adjustment achieves the condition of Vin + Vofs_in ~ 0 without signals.
4. Measuring Vout when changing gx from 0 dB to 8 (16) dB and adjusting the offset of U14 (U13) so that Vout does not change when the gains are changed. This adjustment achieves the condition Vofs_gx ~ 0 for gx = 8 (16) dB.
Offset adjustment can be done for IN1 (signals from RFPD), but failed for IN2 since the input offset without signals is very large and the adjustable range is insufficient.
So, I decided not to replace the CMS at this time.
