Servo - Fixed Wing Attitude Controller

Hi everyone,

I am trying to build a transfer function model of this FW Attitude Controller diagram.


The problem is that I have not found how the mixer block works. Does anybody know where I could find how the command u is calculated?
For example, in this code, https://github.com/PX4/PX4-Autopilot/blob/master/src/modules/fw_att_control/ecl_pitch_controller.cpp the output parameter seems to be _last_output, which is constrained between -1 and 1, but I imagine that the command u is the value of width pulse (expressed in ms or us), so how is this calculated?

Thanks in advance, regards.