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Percentage of ModulationAmplitude Modulation Fundamentals97m
to compute the modulation index from measurements taken on the composite modulatedwave itself. When the AM signal is displayed on an oscilloscope, the modulation indexcan be computed from and , as shown in Fig. 3-5. The peak value of the mod-ulating signal is one-half the difference of the peak and trough values:As shown in Fig. 3-5, is the peak value of the signal during modulation, and is the lowest value, or trough, of the modulated wave. The is one-half the peak-to-peakvalue of the AM signal, or Subtracting from produces the peak-to-peak value of the modulating signal. One-half of that, of course, is simply the peakvalue.The peak value of the carrier signal is the average of the and values: