Sheila, Kiwamu
We newly put an elliptic low pass in ETMY_L3 in order to reduce the number of saturation events.
Today, we noticed that ETMY frequently saturated at the bottom stage. Looking at the spectra and time series, we found that high frequency components above 1 kHz was a culprit. This is related to the activity that Evan et al worked on last night (alog 20585) trying to get a better phase margin. We decided to install a roll off in order to mitigate the issue. We put an elliptic whose cut off is at 950 Hz with a 20 dB rejection in the stop band and 3 dB ripple in the pass band. Since the filter banks in ETMY_L3_LOCK was full, we put it in DRIVEALIGN instead. It is now in FM7 and the ISC_LOCK turns it on in the ETMY_TRANSITION state. This costed a 2.6 deg phase loss at 40 Hz which should be OK according to Evan's open loop measurement.
As a result, it reduced the DAC counts in RMS by a factor of between 2 and 3. This seemed to help reducing the saturation events so far. See the attached. We could have lower the cut off frequency more, but since it is going to be limited by frequency components below a couple of Hz anyway, we leave it as it is.
We did some more DARM filter cleanup:
- The 200 Hz pole in LSC-DARM suscomp was moved to 1000 Hz.
- In EX L3 lock, there is now a 200 Hz / 1000 Hz p/z. It is turned on from ALS DIFF onwards.
- In EY L3 lock, the 1000 Hz / 200 Hz p/z was removed.
- The above ELP950 was removed from the L3 drivealign and put into L3 lock.