Sudarshan, Kiwamu,

This is just a brief update of what we did with the ISS during the maintenance window today.

Yesterday, Sudarshan and Matt came up with another idea of a quick hack in order to increase the gain margin of the ISS 2nd loop. The idea is to modify the unused filter path of the ISS 2nd loop servo box to obtain more poles and zeros without making a change in the main servo filers. We pulled out the servo box out of the PSL rack, soldered some components on the board in the EE shop and put the box back to the rack. The above plot is the open loop transfer funciton of the 2nd loop after the modification. As shown, the slope of the open loop is now steeper than it used to be, indicating a more gain margin. We will post more details about the circuit modeling, today's mofication and noise model.

The open loop measurement was done with a PSL power of 2.4 W. PD1-4 were used as in-loop sensors, the second loop gain was at the maximum of 40 dB, the boost and integrator engaged,  and the additional gain was also engaged. The raw data is attached -- the first file is the magnitude in dB and the second for the phase in degrees.