I took a function generator and an oscilloscope from filter cavity clean room to modulate EOM and acquire data in ATC.

The RF PD is moved to OPO reflection to acquire PDH signal. A minicircuit zx05-1l-s+ is used for RF signal demodulation. 10dBm 40MHz signal is sent to New focus 4003 EOM to get a phase modulation around 0.07~0.21rad. 3dBm signal is used as LO. We got error signal as attached figure. Some glitches are found and we will try to understand it better tomorrow.

One possible reason is that I should put a DC block between RF PD and mixer. Note that demodulation phase is not optimized yet.