Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

VARIATION OF BANDWIDTH 77Pig. 9—Limits for side-frequency Fig. 10—Spectrum for rectangularamplitudes.modulation.kmost of the energy of the spectrum isconcentrated about the frequenciesthat correspond to the two limits of the deviation.Bandwidth Required for Rectangular ModulationEquation 18 shows that there is an infinite number of side frequenciesin the spectrum of a frequency-modulated wave with rectangularmodulation. As shown by Figures 9 and 10, the amplitudesof these side frequencies decrease uniformly beyond the limits ofthe deviations. If the edges of the band are denned as the points correspondingto a limiting amplitude of 0.0 IS, the bandwidth can becomputed directly from (18). For the case of square-wave modulation,x = 0.5, and the increase in bandwidth with decreasing modulationindex will be as shown by Figure 11. If a more strict definition ofbandwidth is required, curve B shows the width for the limitingamplitude 0.00LE 1 . Curve A is an accurate enough limit for mostpractical cases.90