The principle of converting FM noise of the local oscillator into AM noise of the receiver due to the discrimination effect in agile radar is discussed. The phase-shifting discrimination and its effect on the receiver noise in the radar with a fast AFC subsystem are analysed and formulated. The methods to minimize the extra FM noise of LO and the discrimination effect are proposed so that the additional receiver noise can be effectively repressed. The practical results applied to several radars have proved these methods to be successful.