Some flight controllers such as the Pixhawk autopilot only support PPM (pulse position modulation) compatible receivers. If you have a PWM (pulse width modulated) receiver, you will need a PPM encoder such as this one for it to work with a PPM-only FC.
Input Channel : 8 PWM channel
Output channel : 1 PPM channel
PWM signal cable length (mm) : 60
PPM signal cable length (mm) : 90
PPM Encoder dimensions : As per below