Aso, Yuheng, Michael

There is some indication that the CC PLL has a weak lock, so we tried to make an open loop transfer function measurement. Unlike the cavity servos however, the CC PLL servo doesn't have a perturb or monitor ports.

Takahashi-san gave us an old generic summing amplifier (from the original TAMA end mirror) which can be inserted into the CC PLL feedback path to inject swept sine or noise. It has IN, OUT, ADD which function like EPS1, EPS2, PERTURB IN on the cavity servos, and then monitor ports for the input and output signal. There are four sets of amplifiers named after IN/END PITCH YAW.

We checked the frequency response of the summing amplifier. With nothing in ADD, the IN MON, OUT MON and OUT signals should all be the same as IN. We tried first 3 Hz 1Vpp, which was what the function generator was set to. OUT and OUT MON could reproduce the signal but IN MON was attenuated. When the signal was a ramp, IN MON also had some integration, so the summing amplifier is not so useful for low frequency (fig 1, 2). But we don't really care so much about low frequency to check the PLL fast loop. We then tried 10 kHz 1Vpp which was reproduced in all outputs so it seems fine for higher frequency (fig 3). 60 kHz 0.2 Vpp was sent to ADD and it seems to have the proper response for the output and monitor channels (fig 4).

After testing we set up as follows. CC PLL FAST outputs the fast correction signal to the laser PZT. We connected this to IN, then connected OUT to the laser PZT. ADD, IN MON and OUTMON go to Source, Ch2, Ch1 on the SR785 spectrum analyzer (fig 5). The PLL was locked and then we checked the transfer function up to 100 kHz for swept sine. The excitation frequency that could be applied before unlock was quite small, about 10 mV. But we didn't see anything really meaningful. We checked in FFT mode using noise excitation but couldn't get meaningful coherence anywhere in the full span of the spectrum analyzer. I should more carefully check the relevant frequency range and actuation strength of the CC PLL fast signal. The SR785 only goes up to 100 kHz. The other spectrum analyzers we use to monitor the PLL can look at up to 2 GHz but are only single channel.

When we locked the PLL, I compared the time traces with what I saw previously to show the CC glitch problem (fig 6, from before). Only to see that the CC glitches did not appear (fig 7)... I don't have any idea what changed.