We fixed the ground loop problem in ACAV setup and got rid of most of the harmonic lines in beat signal.
The ground loop problem in our current setup comes from the common ground of the two Marconis. The first one is for 14.75 MHz EOM. Its signal is split, and sent to demodulate the signal from ACAV RFPD. The second Marconi is for driving the AOM around 80 MHz for frequency lock. By using a transformer between the mixer (for PDH demodulation) and the LO signal from the first Marconi, we broke the ground loop, hence got rid of the harmonic lines in the signal.