Transient Response of an FM Receiver

T HIS paper develops various formulas for the response of an FM receiver to signal or noise input voltages of arbitrary form. The principal object in view is to obtain a more complete understanding of how an FM receiver responds to transient voltages, such as those arising from ignition interference, but the more general aspects of the theory have other applications as well. In particular, general formulas are given for the response of a linear circuit to an applied voltage, or current, of variable frequency. The Fourier transforms, or frequency spectra, of the response, and the envelope thereof, are determined. Two examples are given: (1) the audio response of an FM receiver to a very large impulse and (2) the response, including harmonic distortion, to a sinusoidal signal wave. The element of an FM receiver that demands most discussion is the balanced frequency detector. The greater part of the paper accordingly deals with that important element. The general problem can be stated as follows: A limiter and frequency detector are transmitting a steady unmodulated carrier wave to an audio output circuit. At time, I = 0, frequency modulation of arbitrary form is applied to the carrier (either by signal modulation or a superposed noise transient). What is the audio output voltage that results? FREQUENCY DETECTOR