# Gain tuning

This morning we have reduced the gain of the ELEC_1712_01 rampauto to be better centered within the stability area.

plot 1 gives the OLTF got today with respect to the one got on 22/02/2018

plot 2 gives the Nichols plot, we check the gain is well centered wrt the stability area

plot 3 gives the position of the potentiometer which is used to set the gain of the OLTF (set to 9 o'clock)

-> this should help in removing the peaks seen at 158 kHz and 366 kHz by Julia and others

# Notch tuning

Two notches at 1* IMC_FSR and 2*IMC_FSR are implemented in the rampauto to remove the spoiling contribution at those frequency on PSTAB.

Upon arrival we found the notches tuned to 1.02 MHz and 2.07 MHz instead of the 1.045129 MHz and 2.09 MHz theoretical ones.

We adjust it to 1.046 MHz and 2.09 MHz

plot 3 & 4 give the TF of the notch once adjusted

# Gain tuning, next step

we have asked to reduce to power incident on the PSTAB PDs by a factor at least 2, in order to be in a "safer" region.

The idea is to check whether this could reduce the loud glitches which were visible on the faulty PD2 (46399), and also reduce the ageing of the PDs (assuming that ageing could be responsible to the glitches seen of PD2)

When this will be done one needs to compensate the overall gain increase. This can be done by adjusting the gain of the rampauto with the gain potentiometer (see plot3). To fine tune it either we need to measure the OLTF or just feed the perturb input with a 10 kHz line and check to recover the same amplitude of this line once read on the PSTAB error signal as the one got before the optical gain change.