There appears to be a gain change somewhere in the IMC cavity control sensing chain from where it was last February. See the attached pdf. The first two pages show the measured MC2 M2 loop gain against the model. The first is before correcting the cavity sensing gain, the second is after. The 3rd and 4th pages are structured the same way but for the MC2 M3 loop gain. After adjusting the gain by this 1.36 factor, the measurements agree much better.

The gain of 1.36 was calculated by measuring the difference in the gain between the model and the measurements in the M3 loop gain. The model had about 1.36 times more gain than the measurements. So in the model I reduced the gain of the signal entering the M3 locking filter by this much. This scales the M2 loop as well since the M2 locking filter input is the output of the M3 locking filter (distributed hierarchy). The result is much better agreement between the modeled loop gain and measured loop gain for both M2 and M3.

The data in the attached pdf are from measurements done by Jeff Kissel and myself from last Friday in log 7255. The measurements in that log and this one are plotted against a model developed by Jeff and discussed in DCC doc G1300111. Note that the measurements agree well with the model in G1300111. The cavity locking loops have changed a bit since G1300111 was posted last February, but that is taken into account in the updated model for these current measurements (/ligo/svncommon/SusSVN/sus/trunk/HSTS/Common/FilterDesign/IMCmodel_H1_MC2_M2M3hierarchicalfeedback_26July2013.m). So it is not clear to me where this gain change may have come from.