(Bram, Alberto, Daniel)

This morning we were investigating the odd PLL oscillations that prevented us from increasing the servo gain. We found a very narrow resonance around 287.56kHz with a Q>100. We saw this resonance before, but it seemed to have had a much lower Q. It is unclear why this resonance has become so much more pronounced in the past few days. Our 100kHz low pass filter was designed to take care of this, but is obviously not sufficient to suppress a high Q resonance. Reducing the ugf to 6.5kHz avoids the instability, but will require that the cavity locking servo runs at a lower ugf too.

We then successfully locked the cavity. Looking at the I and Q signals it seemed that the RF phase was poorly adjusted. We first switched the delay line phase shifter to external, so we can use the medm screen to adjust the phase. Secondly, we minimized the Q signal by going from 135º (255 steps) to 85º (160 steps). We left the delay line in external mode. Meaning, it is important to restore the EPICS settings after a reboot.

While we were out there we also fixed the missing RF level readbacks from the second wavefront sensor demodulator board. They were connected to the wrong connector of the ASC demodulator concentrator.