1. Calibrating offset :
I measured the shift in the beat frequency while scanning through the offset. Offset stepped by 50 resulted in 1MHz shift of the beat frequency.
2. Anti-whitening filter for beatbox output:
I made an anti-whitening filter for the beatbox output in the ALS_BEATX_FINE_I module by inverting the whitening filters that Jamie had installed in the beatbox earlier (elog). I have kept the old anti-whitening filter in the module as well for the time-being because the new anti-whitening filter was not as good as the old one in stabilizing the servo (large error signals and unstable ALS).
3. Beat frequency scan for 3FSR:
With ALS loop enabled, I did an offset sweep corresponding to 3FSR (FSR = c/2L = 3.7MHz). The loop doesn't seem to be stable enough to reduce the arm fluctuation to get a resonance for IR. Time series of scan is shown below:
4. No-loop and in-loop spectrum:
I measured the spectrum of the error signal (C1:ALS-BEATX_FINE_I_IN1) with ALS loop enabled and disabled. To suppress the peaks at 3.2Hz and 16.5Hz, I turned ON the corresponding filters. I have recorded the error signal spectrum with only 16.5Hz res gain filter turned ON. Turning ON res gain 3.2Hz filter kicked ETM.
Spectrum of error signal shown below:
1. What is wrong with the new anti-whiteing filter?
2. Why would the res gain filters kick ETM and show no noise suppression?