Effects of Tuned Circuits upon a Frequency Modulated Signal

Prior investigations indicated that the frequency modulated receiver would always respond to the signal having the largest amplitude. Thus, selective circuits would be required to pick out a desired signal existing simultaneously with a number of other signals. The first item considered in this paper is that the signal carrier is tuned to the steep side of the resonance curve. It is found that in this case conversion from frequency modulation into amplitude modulation can be effected. It is required that the amplitude and phase characteristic of the circuit be linear with respect to frequency over the whole frequency interval occupied by the modulated signal. In order to derive a faithful audio signal reproduction from the complete detection process, a phase modulated signal must be transformed into frequency modulation first. Next, the case is considered in which the signal carrier is tuned to the peak of the resonance curve. The single tuned circuit is taken up first and two methods of solution are presented. It is found that with certain values of the modulation index, the tuned circuit may cause very serious nonlinear distortion of the output. For large values of the modulation index the resulting variation in amplitude and frequency can be determined statically, while for very small values of the modulation index the effect of the tuned circuit is exactly analogous to that encountered with amplitude modulation.