Sheila, Daniel, Stefan
I tuned up the beat note alignment this morning.
With the arm locked we have 88uW total on the COMM_A_LF detector, 42uW from the arm and 47uW from the SHG. This gives us a beatnote of 28mV pp, with the PFD RF power readback saying -35dBm.
The Comm PLL locks for a few seconds and runs out of range (due to fluctuations at the microseism frequency). One solution would be to feedback to the ETM from the green laser at the microseim frequency ( right now we only have very low frequency feedback to HEPI), this would probably be nice in the end, because the PLL would lock nicely and we need to figure out out how to do this for HIFOXY anyway. Another solution would be to engage COMM with the PLL partially locking, which does still give a useable error signal, which would be quicker to implement.
We remembered that we can just use an ezcaservo script to feed the COMM_PLL_CTRLMON signal back to offset of the the mode cleaner VCO,
ezcaservo -s 0 -g -1 -f 0.3 -r H1:ALS-C_COMM_PLL_CTRLMON H1:IMC-VCO_TUNEOFSWe then proceeded to measure the open loop transfer function of the PLL servo. The settings were:
The unity gain frequency is at 55 kHz with a phase margin of 40º.
With the same configurations as above I measured the power spectrum out of the PFD IMON while the PLL was locked.