I have checked the MC1 satellite box and made a bunch of changes. For now, the glitches coming from the satellite box is gone. I quickly tested the MC1 damping and the IMC locking. The IMC was locked as usual. I still have some cleaning up but will work on them today and tomorrow.

Attachment 1: Result

The noise level of the satellite box was tested with the suspension simulator (i.e., five pair of the LED and PD in a plastic box).

Each plot shows the ASD of the sensor outputs 1) before the modification, 2) after the change, and 3) with the satellite box disconnected (i.e., the noise from the PD whitening filter in the SUS rack).

Before the modification, these five signals showed significant (~0.9) correlation each other, indicating that the noise source is common. After the modification, the spectra are lowered down to the noise level of the whitening filters, and there is no correlation observed anymore. EXCEPT FOR the LR sensor: It seems that the LR has additional noise issue somewhere in the downstream. This is a separate issue.

Attachment 2: Photo of the satellite box before the modification

The thermal environment in the box is terrible. They are too hot to touch. You can see that the flat ribbon cable was burned. The amps, buffers, and regulators generate much heat.

Attachment 3: Where the board was modified

- (upper left corner) Every time I touched C51, the diode output went to zero. So C51 was replaced with WIMA 10uF (50V) cap.

- (lower left area) I found a clear indication of the glitch coming from the PD bias path (U3C). So I first replaced another 10uF (C50) with WIMA 10uF (50V). This did not change the glitch. So I replaced U3 (LT1125). This U3 had unused opamp which had railed to the supply voltage. Pins 14 and 15 of U3 were shorted to ground.

- (lower right corner) Similarly to U3, U6 also had two opamps which are railed due to no termination. U6 was replaced, and Pins 11, 12, 14, and 15 were shorted to ground.

- (middle right) During the course of the search, I suspected that the LR glitch comes from U5. So U5 was replaced to the new chip, but this had no effect.

Attachment 4: Thermal degradation of the internal ribbon cable

Because of the heat, the internal ribbon cable lost the flexibility. The cable is cracked and brittle. It now exposes  some wires. This needs to be replaced. I'll work on this later this week.

Attachment 5: Thermal degradation of the board

Because of the excessive heat for those 20years, the bond between the board and the patten were degraded. In conjunction with extremely thin wire pattern, desoldering of the components (particularly LT1125s) was very difficult. I'd want to throw away this board right now if it were possible...

Attachment 6: Shorting the unused opamps

This shows how the pieces of wires were soldered to ground vias to short the unused opamps.

Attachment 7: Comparison of the noise level with the sus simulator and the actual MC1 motion

After the satellite box fix, the sensor outputs were measured with the suspension connected. This shows that the suspension is moving much more than the noise level around 1Hz. However, at the microseismic frequency there is also most no mergin. Considering the use of the adaptive feedforward, we need to lower the noise of the satellite box as well as the noise of the whitening filters.

=> Use better chips (no LT1125, no current buffers), use low noise resistors, better thermal environment.