How to Find Percentage Of Modulation
The modulation index can be determined by measuring the actual values of the modulation voltage and the carrier voltage and computing the ratio. However, it is more common to compute the modulation index from measurements taken on the composite modulated wave itself.
When the AM signal is displayed on an oscilloscope, the modulation index can be computed from Vmax and Vmin, as shown in Fig. 3-5. The peak value of the modulating signal Vm is one-half the difference of the peak and trough values:
As shown in Fig. 3-5, Vmax is the peak value of the signal during modulation, and Vmin is the lowest value, or trough, of the modulated wave. The Vmax is one-half the peak-to-peak value of the AM signal, or Vmax(p2p)/2. Subtracting Vmin from Vmax produces the peakto-peak value of the modulating signal. One-half of that, of course, is simply the peak value. The peak value of the carrier signal Vc is the average of the Vmax and Vmin values:
The modulation index is
The values for Vmax( p2p) and Vmin( p2p) can be read directly from an oscilloscope screen and plugged directly into the formula to compute the modulation index. The amount, or depth, of AM is more commonly expressed as the percentage of modulation rather than as a fractional value. In Example 3-1, the percentage of modulation is 100 x m, or 66.2 percent. The maximum amount of modulation without signal distortion, of course, is 100 percent, where Vc and Vm are equal. At this time, Vmin = 0 and Vmax = 2Vm, where Vm is the peak value of the modulating signal.
Example 3-1 Suppose that on an AM signal, the Vmax(p2p) value read from the graticule on the oscilloscope screen is 5.9 divisions and Vmin( p–p) is 1.2 divisions.