The phase reference or synchronization (trigger signal) sensor is an inductive (eddy current) proximity sensor used for triggering the phase reference once per revolution. It measures the distance between a target glued on the shaft and detects its presence once the target is in range. Once per revolution, the target crosses the sensor and a pulse is sent to the acquisition unit. Trigger signal is used as a phase reference for all the measurements that require correlation of data with the position of rotor.