Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

1400 FREQUENCY MODULATION, Volume IAt an audio frequency of 2500 cycles the resultant argument(Deviation divided by modulating frequency) comes within the rangeof the usual Bessel tabulations, and the sidebands of the multiplexmodulation can be determined. Thus with a 2500-cycle tone at 60kilocycles deviation and the 20,000-cyele tone at 15 kilocycles deviationthe amplitudes can be plotted as in Figure 6. The first chart gives theside bands of the audio tone only, and the second the side bands for thecombination of both tones. In both cases all side bands and side frequenciesare plotted as positive quantities, but due account of sign ofthe coefficients has been taken where several terms of the expansionare involved for a particular frequency. In these charts any coefficientgreater than 0.1 per cent was included.It can be seen that the bandwidth in Figure 6 is somewhat greaterthan in Figure 5, but that the side bands outside the 200-kilocyclechannel have less than 1 per cent amplitude.JoJ.0.86J2SOKC 40 30T 120 j10 - Fc + 10T0.35_LJz20 SO 40Fig. 3—Sideband chart of 20-kc signal at 15-kc deviation.By comparing Figure 4 with Figures 5 and 6, it may be seen thatthe low aural frequencies cause a greater side band extension than thehigher aural frequencies. For the higher frequencies, the product ofthe Bessel coefficientsbands toreduces the amplitude of the higher order sidea degree where the bandwidth of consequence becomes lessthan for a single modulating tone.The 60-kilocycle deviation for aural signals was used in the multiplexcalculations rather than 75 kilocycles, in order to keep the signalwithin the 200-kilocycle band width. As can be readily seen fromFigure 5, maintaining the aural deviation at 75 kilocycles and usingthe 15-kilocycle facsimile deviation would produce side bands outsidethe 200-kilocycle channel. Reducing the facsimile deviation to 10 kilocycleswould lower the amplitude of the out-of-channel side bands to64 per cent of their former value. This reduction would be insufficientto clear up the out-of-channel radiation, and furthermore would handicapthe facsimile in signal-to-noise ratio. Thus Figure 5 shows the firstpractical reason for reducing the aural deviation to 60 kilocycles whenmultiplexing with facsimile.