I made sure that the gain setup for ACAV's servo was not too high and caused instability in the loop. The beat was not improved.
Previously from this entry, we see that noise at the error point, ACAV loop, increases with the ACAV servo gain.In general, it should decrease, since larger signal from actuator can suppress more noise. Frank told me that it might start oscillating somewhere and should check that the gain setting we use still make sense (increase gain, lower noise at error point,) so I added more data from gain 0(minimum) and 0.5 to show that the noise is actually going down as gain increase at these gain setup, and the beat signal still remain the same.
Above, noise at error point during close loop operation.
Above, noise at error point projection to frequency noise. From gain 0 to 1.5 noises at error point still lie below the measured beat.
Since the error signal measured above might not be very clear, I repeated the experiment by adding a 20 dB attenuator at the UPDH input to make sure that the signal from the mixer is not too large and cause any osscilation. However, there is no improvement in beat measurement. I think the PLL readout part is the most suspicious one right now.