Today we verified the best comprise in terms of OLTF for the power at 18W and 12W with different values for the IMC_REFL_DC power.

For this, we measured the OLTF after varying the REFL power with the IPC1 rotator from remote. 

We had the following results at 18 W:

• with REFL at 0.85W -> OLTF 142 kHz and 9 deg phase margin;
• with REFL at 0.73W -> OLTF 114 kHz and 19 deg phase margin;
• with REFL at 0.68W -> OLTF 96 kHz and 24 deg phase margin;

From 0.73 of IMC_REFL_DC we descrease the power of IMC_TRA from 18W to 12W and remeasured the OLTF (the corresponding value of IMC_REFL_DC at 12W is 0.64W):

• with REFL at 0.64W -> OLTF at 65 kHz and 28 deg phase margin;
• with REFL at 0.6W -> OLTF at 53 kHz and 18.3 deg phase margin.

Analyzing the data, we decided to choose the best compromise for the REFL_DC value, which is 0.73W of REFL for 18W, corresponding to 0.64W of REFL for 12W. Figs. 1 and 2 show respectively the OLTF for these two configurations. 

For the 12W OLTF there is apparently some noise structure very close to the UGF (also present for 18W but farther away from the UGF). Looking closely at it (Fig. 3), we can see that it corresponds to some excitation lines with unknown origin. This doesn't seem to generate control issues.