I switched the polarity for EOM actuator on RCAV loop. Now the beat signal is comparable to what we had before.
The beat measurement from the previous entry (with new TTFSS) was higher than what we had with the old FSS. It was because of the wrong polarity of the EOM actuator. The error signal can be flipped at 2 points in RCAV servo. The first point is the phase flip button controlled by the 35.5 MHz LO card. This switch can be activated on the medm screen. The second point is the switch on the FAST path on TTFSS (a square box with +/- in the cartoon diagram). This one can be switched manually. To choose the right polarity for FAST actuator and EOM actuator, we pick certain polarities at point 1 and 2 that allow us to lock the cavity with FAST actuator. Then try to invert both switches, and see if we can lock it better (higher bandwidth.)
LO signal ---> [pol switch 1] ---------------> Fast act---> [pol switch 2]
|----------> EOM act
I tried 10kHz and 1kHz input range on the LO for PLL for beat measurement. I measured the calibration factor for 10kHz to be 7.1e3 Hz/V, but I did not measure the calibration factor for 1kHz input range, I just estimated it to be 0.71e3 Hz/V. The results look fine.
The setup is similar to what we have had before.
I use 14dBm attenuator, so the power to LO is ~ 8 dBm.
Power to the periscope: 1mW
Fast gain: 600
Common gain: 383
C3:PSL-FSS_PHCON(ADJ phase): 5.3659
C3:PSL-FSS_PHFLIP(phase flip): 0
FAST polarity switch on TTFSS: (-)
Currently I use C3:PSL-FSS_SLOWDC to control SLOWDC value on the laser. The knob is a bit sensitive, so it is hard to control.
Next, I will check the UGF of RCAV servo, we expect to see UGF around 0.5 MHz. Once I verify the UGF, I'll measure the RIN-> frequency noise coefficient.