Vi. Frequency Modulation

In the Quarterly Progress Report of October 15, 1957, page 52, we pointed out that we had devised a feedforward-across-the-limiter technique that could be arranged to suppress whichever of two cochannel FM signals is undesired. A simple experimental model of this system was built and tested (1). Before summarizing the experimental results, we shall present a resume of the appropriate theoretical predictions. The simplest realization of a feedforward system is shown in the block diagram of Fig. VIla. The limiter and amplifier in the lower signal path are, in effect, equivalent to one idealized narrow-band limiter. The upper signal path contains two narrow-band limiters. Each idealized limiter filter is assumed to have one i-f bandwidth. Limiters are inserted in each of the two signal paths in order to ensure that the properties of the output signal will be independent of the input signal level, as long as the resultant input signal amplitude exceeds a certain threshold value. A system that is identical in its effect with the one shown in Fig. VIla, but which requires one less limiter, is shown in Fig. VI-lb. The arrangement in Fig. VIlb is the more desirable for practical realization. It is easy to show that if the first limiter in Fig. VI-1b is not narrow-band, the combination of the second-limiter and amplifier outputs in phase opposition cannot effect the type of cancellation that is necessary for achieving beneficial changes in the interference conditions at the output.