J. Kissel, R. Savage, S. Karki, K. Izumi

After considering the front-end precision issues found in the DELTAL_EXTERNAL calibration yesterday by Stefan and Evan (LHO aLOG 20700), we've agreed to move the pole frequency which reconstructs the integrator (a pole at 0 [Hz]) in FM1 of the L1 (UIM) ETMY control signal chain from what was quickly installed yesterday -- 0.001 [Hz] -- up to 0.01 [Hz] to gain another factor of ten high-passing of the control singal, such that we don't suffer from floating point single precision issues. Rick and Sudarshan will follow up shortly, repeating the study on the correct whitening filter we should use.

Note: The means that the DARM calibration is invalid below ~ 0.5 [Hz]. If you need to have a calibrated DARM spectrum below there, then make sure you (offline) compensate for this.

The history is as follows: 
- In ER7, we ran with this integrator compensator OFF, accepting that the front-end calibration below ~1 Hz was incorrect. LHO aLOG 17528, or LHO aLOG 18248
- A few days ago, Kiwamu updated all of the digital control filters to make that actual suspension filter chain using an automated script, forgetting about this dynamic range issue. LHO aLOG 20617
- Yesterday, Stefan Evan and Sudarshan rediscovered the issue, and moved the pole frequency up to 0.001 [Hz], and added more stages of whitening on the DELTAL_CTRL signal. LHO aLOG 20700

Today on the calibration call, Joe reminded us that he moves the compensation filter's pole up to 0.1 [Hz]. After comparing our options (in course 1 mHz, 10 mHz, 100 mHz increments), we decided that 10 mHz caused the least loss of phase in the 1 - 10 [Hz] region (where we need to worry about accurately reconstructing the PUM / UIM and PUM / TST cross-overs), for the most amount of high-passing of the large DC control signal.