The coupling from frame motion to beat signal is now significantly reduced after we fixed the cable wiring .

      The plot below shows the acceleration measured on the frame and the corresponding beat signals. I did not calibrate the signal since this is just a qualitative check.  I tried to excite the frame so that the signals from the frame motion were comparable (blue and cyan). After we fixed the cables wiring, the beat signal (pink) are less sensitive to the frame vibration than before (red).

      Note that the set up were similar in both measurements. The input range for PLL was 5kHz, with gain 200.

I have not measured the beat signal after we fixed the cable yet. It will be interesting to see if the beat has been improved or not.