When CC1 is locked and the filter cavity is detuned, the CCFC error signal only shows an offset. This field should be identical with CC1 error signal if offset is not considered. Before the optimization of the mixer, we checked the histogram of this offset. From the attached figure, we could see that this histogram has some problems (no data located in the center). It could come from an oscillation of this signal.

We should recheck it after the optimization of the mixer.