Not much got done. I installed a LP with Fc = 800Hz and -20dB attenuation between the output of LO common mode board and input of PZT driver. Because we can't bypass the PZT driver (Beckhoff needs it to talk to in-vac driver chassis) I turned on the 4Hz/400Hz compensation. OUT2 on the LO common mode board also get signal from IN2 and hooked up to IN1 so we can use both gain slide bars to give it even less gain. I can close the loop with the lowest gain setting without killing the OPO lock but non of the sign combinations (at input and slow) would lock the loop. Looking at IMON on SR785 the noise was pretty much all over the place but ~50kHz-70kHz seems to be its favorite hang-out spot.  I *think* we need a lot of gain to suppress high frequency noise but also can't have that because it will kick the OPO out of place (?). Right now the error signal comes out of IQ demod box. Will it help to install a PFD?
 

Feed forward work: I played around a little bit. Funny that I can't seem to make a different whether I get the output gain to 1, 100, or 1000. Even giving it an arbitrary offset wouldn't make a different to VCO frequency difference. The feed forward signal did show up on H1:SQZ-OPO_FREQ_OUTPUT (summing point according to the front end model).

Today I also learned that open/close the table can make >10mW difference in the Mephisto output power. I couldn't engage the EOM path until the power was stabilized.

Attachments: 1) LO config I've tried. 2) Spectrum of LO IMON