Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

334 FREQUENCY MODULATION, Volume IThis will be true for all modulation indexes D/\y. The effect of increasingthe modulation index is to produce more peaks and dips in output withno change in the limits. Figure 17 shows these two limits as dotted lines,for x = 0.5, and modulation indexes of 5 and 10. The output that wouldbe obtained with no interference is also shown as a cosine wave of unitamplitude. Figure 18 shows how the number of peaks increases when themodulation index increases to 30. The limiting curves are the same asbefore. The two signals have the common center frequency at 2ir\d = 90Fig. 18—Common-channel interference.degrees and 270 degrees so they do not beat together there.As the modulatedsignal deviates toward the end of the swing, the frequency differenceis large, and the peaks come more and more rapidly.Effect of Detuning Interfering Signal. If the interfering signal is detunedby an amount equal to one-half the deviation of the desired signal, theeffect is to move the frequency at which zero beat occurs to that point.