Marc, Michael

We looked at a measurement of the CC PLL phase noise to see if there were any visible glitches. This measurement works as outlined previously and in Yuhang's thesis 4.25-4.31. To reiterate, we can inject a local oscillator and the relevant PLL beat signal into a mixer. There is a 2*f component rejected by a low pass filter 1.9 MHz. If the LO and the signal are the same frquency it can be shown that the output is an average DC offset (which can be set to zero via LO/signal relative phase) and the PLL phase noise assuming a sufficiently clean LO. The spectrum is monitored on the spectrum analyzer and the amplitude is calibrated by offsetting the LO by some small frequency inside the LPF passband to bring back the 2*f oscillation.

We took a measurement of the CC_PLL_MON channel (7 MHz) combined with DAC3 DDS3 (what is normally ppol LO) also at 7 MHz. The calibration factor K_mix * A_1 * A_2 / 2 comes from the peak to peak amplitude when sending mixer out to the oscilloscope with LO = 7.000 1 MHz -> 2.80 mV. We also took low and high frequency PSD (have not postprocessed yet). During this process we could not see any glitches. Actually, this measurement really isn't necessary in and of itself, it was mostly just some curiosity I did for the KAGRA filter cavity project, but it is where the CC glitch noise problem first became apparent.

The previous measurement of this phase noise used ppol LO as the local oscillator, but now that we have removed the 78 MHz EOM we have a spare DDS output channel, so we can try this phase noise measurement again while applying squeezing. We confirmed and locked the CC2 loop, which measn that we are generating squeezing, but somehow we did not see any glitches anymore in the CC2 error signal.  While it would be fortunate that this noise source has disappeared, it would be better to know where it came from. 

However, after some time GRMC and MZ refused to lock. MZ became very misaligned and there was no error signal from GRMC reflection PD. Perhaps in our switching of local oscillator cables the MZ PZT got kicked - the alignment and PDH error signal of GRMC was rmostly recovered just by a yaw adjustment of the MZ non-PZT mirror, and then I optimized the PDH phase (1.11 V peak to peak PDH signal at 110 degrees in DAC2 DDS2). The GRMC mode matching was recovered to 592 + 22.4 + 20.8 + 8.8 -> 91.9%. However, GRMC and MZ still do not lock. I don't know what is wrong but I guess it has something to do with DDS signal connection and or on/off status.