Daniel, Stefan, Sheila

Today we spent some time carefully double checking the calibration of the ALS COMM signal. 

The first screenshot is a repeat of the measurement from alog 10930, locking with the additive offset and then moving the offset over resonance. The current calibration is 0.127, this is loaded in LSC_REFLBIAS_GAIN so that REFLBIAS_OUT is in units of Hz. 

We moved the pole in the additive offset path from 0Hz to 1mHz to prevent the integrator from running away. 

We also changed the gain on the common mode board in1 to 11dB (to avoid saturations in the additive offset path), common comp is on, in2 gain is 6dB and the REFL DC BIAS gain is 50. 

in this configuration we measured the gain in the additive offset path, and got a 3Hz ugf. 

We are now using the ALS-C_REFL_DC_BIAS_OUT calibrated because this is a DQ channel. This calibration has a gain of 0.g and a zero at 3Hz, the ugf of the additive offset path.

Using this measurement we get an rms of 50-200Hz depending on the time of the measurement.  The variation is below 1 Hz, and seems to change with the seismic noise.