Rca1948FrequencyModu.. - The New Jersey Antique Radio Club

VARIATION OF BANDWIDTH 81S(l)Fig. 15—Variation of phase angle.x = 1,the frequency varies linearly from one extreme of the frequencyexcursion to the other, while for sinusoidal modulation the frequency isnear the extremes of frequency a greater portion of the time. Asmight be expected, more of the energy in the spectrum is near the endsof the swing for sine-wave modulation than for triangular modulation.Bandwidth Required for Triangular ModulationIf the bandwidth is defined as the extremes of frequency beyondwhich none of the side-frequency amplitudes are greater than 1 percent of the carrier amplitude that would be obtained if the modulationwere removed, the variation of bandwidth with modulation index canbe computed from the equations for the side-frequency amplitudes.Curve A of Figure 17 shows how the bandwidth increases as the repetitionrate is increased. For a more precise definition of bandwidth,either curve B or curve C can be used.If x is reduced to 0.1, the signal becomes a series of triangularpulses with blank spaces between. Most of the sideband energy willoccur near the frequency which the carrier wave has between pulses,but the pulses will cause energy to be distributed on both sides of thisfrequency. Figure 18 shows the spectrum for a modulation index of 10.The amplitudes decrease much more slowly than in the case of triangularor sinusoidal modulation.ConclusionsWhen a carrier wave is modulated in frequency, an infinite numberof side frequencies is produced. As the modulation index is changed,