Before, when we tried to lock the cavity, it seemed that the gain was too high, and the power output
from RefCav(C3:PSL-FSS_RCTRANSPD) fluctuated when both PC and Fast feedback were connected, and the data was measured when only Fast feedback was on.
So I try to change RF power to reduce the modulation index (hence, the gain.)
The value before was 6.5V, This time I tried 5.5 and 5.8 V.
At each RF power setting, I lock the cavity with
1) Fast feedback alone, and
2) both Fast and PC feedback
and measure the transmitted beam's RIN. The gain for each setup was readjust (see the values for each setup below)
so that the transmitted beam power was most stable as seen on an oscilloscope, then the data was taken.
The results are plotted below. It turns out that
when the PC feedback is connect, RIN becomes noisier. I don't no where
it comes from. It might be that the setting still has too high gain, or RFAM from broadband EOM due to misalignment,
or broken opamp in FSS PC path.
I'm still not sure why there are jumps at 100 Hz and 1k Hz. The sr785 is set at 4 different spans
which are, 0-100Hz, 0-1kHz,0-10kHz, and 0- 100kHz. Each span contains 800 FFT lines. The auto range is on.
The setting are [this will be updated soon]
Fast feedback: on
PC feedback: off
3 RF 5.8
4 RF 5.8