Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

COMMON- AND ADJACENT-CHANNEL INTERFERENCE 341The amplitude of any desired combination tone can be determined bychoosing the appropriate values of r and s and by computing the desiredC-function. Since the C-function cannot exceed unity for a given combinationtone, it is evident that if D i»[x i or jx 2 the distortion will be reducedwith increasing modulation index.ConclusionsFrequency-modulated radio broadcasting offers the advantage of improvednoise reduction when compared with the usual amplitude-modulationsystems. There is less interference between stations operating onthe same frequency than for the corresponding case of amplitude modulation,and less power is required to cover a given area.A difficulty arose occasionally in the 42-50 megacycle frequencymodulation band because long-distance transmission could be observedfrom frequency modulation broadcast stations during hot weather orbefore a storm. It sometimes happened that such an interfering stationbecame stronger than a desired station in the same channel for shortintervals.When this happened there was a small amount of noise andthe programs suddenly were interchanged.This change often lasted forseveral seconds but sometimes was limited to a word or two or a few notesof music. If the proposed new frequency modulation stations are allcompleted, this interference may occur again. When the interfering stationhas nearly the same carrier frequency as the desired station this effectis called common-channel interference.If the two carrier frequencies areseparated by the width of one channel the result is called adjacent-channelinterference.The simplest case of frequency modulation interference occurs whentwo modulated carriers, having nearly the same frequency, beat togetherto produce a resultant signal. As the two voltages alternately reinforceand cancel each other, the result is a heterodyne envelope consisting ofa series of broad peaks and sharp dips. Each time the two interferingvoltages cancel each other to produce a hole in the envelope, there is arapid phase shift of the resultant voltage.Since the audio output from afrequency-modulation receiver is proportional to the rate of change ofthe phase of this resultant, the rapid phase shift produces a distorted audiooutput, which becomes more and more like an impulse as the interferingcarrier voltage becomes nearly equal to the desired carrier voltage.When the two amplitudes of the interfering voltages are kept constantbut the frequency of one is changed, the result is common- or adjacentchannelinterference depending upon what range of frequencies the swingsof the modulated carrier cover. The beat-note produced by this interferenceconsists of a series of sharp peaks and dips of noise and is super-