Rana, Craig

Today we calibrated the CARM servo board spectrum into Hz.

First, we drove a strong line in the MC2 suspensions at 71.1 Hz, and measured the transfer function from REFL_SERVO_ERR_OUT_DQ (we assume counts from the common mode servo board error signal) to IMC_F_OUT_DQ (calibrated into kHz).
We got 3.1e-7 kHz/cts, or 0.5 Hz/V, at 71.1 Hz, assuming an ADC of 2^16 cts / 40 V. 
Assuming a CARM pole of 0.5 Hz, this gives a CARM optical gain at DC = 284 V/Hz.

We've also measured the CARM OLG, giving a UGF of 10 kHz.  
By extrapolating the CARM OLG with a 1/f^2 line, we can approximately calibrate CARM into Hz of frequency noise incident on the interferometer:


where 
 is the CARM OLG, 
 is the common mode boost (z=4kHz,p=40Hz), 
 is the REFL9 sensing chain,
 is the CARM plant, and 
 is the common mode board test1 point.

The calibrated spectrum is pretty flat, quickly reaching an RMS of 2 Hz at high frequencies.  This RMS frequency noise seems too high to me, and the spectrum doesn't have the shape I expected.