-There is no beat note if the tank circuit on the 1550 EOM is removed.
-If the tank circuit for the 775 is removed the same 120 Hz oscillations are observed. The PDH error amplitude is also very small, as expected.
-I am going to remake a tank circuit and plan to operate OFC1 at 70 MHz (~20 MHz difference, should be filtered out by the low pass in the laser lock box).
Maybe it's fine, but a 20 MHz beat notes seems dangerously close to our nominal readout frequency. However, this is still ~100x the single cavity pole so it should be filtered plenty.