Amplitude Modulation Suppression in FM Systems

1024 J 042 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1 9 5 8 Before launching into the modulation theory of limiters, it is first necessary to point out some of the inadequacies of some current concepts. It might be surmised that, if the carrier output voltage is plotted as a function of the input voltage, the amount of limiting is given by the slope of this curve. In Appendix B it is shown that this is not necessarily true. In particular, it is demonstrated that the limiting characteristic can be altered without altering the carrier transfer function. A common "clipper" limiter and its dual are shown in Figs. 1 (a) and 1(b). The customary explanation of limiting by the circuit in Fig. 1(a) is accomplished with the aid of Fig. 2. The diodes are assumed ideal in the sense that the back resistance is infinite and the forward resistance is zero. The bias current / is assumed to come from a constant current source and to bias both diodes in the forward or low-resistance direction. The input signal voltage e shown in Fig. 2(a) is a sine-wave amplitudemodulated carrier. The output voltage across Rj follows the input voltage until the input voltage reaches the clipping level A, Fig. 2(b). At this point, diode Di switches to its back resistance and further increase in input voltage does not appear in the output, since the back resistance of Di is infinite. Now, when the input voltage is reduced again to value A, DX switches to its forward resistance and the output voltage is again a replica of the input.