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An AM envelope varies between \( 15 \ \mathrm{V} \) and \( 5 \ \mathrm{V} \). The modulation index is

  • A.0.33
  • B.3.0
  • C.0.67
  • D.0.5

Answer: D

From the envelope extremes:

\[ \begin{aligned} \mu &= \frac{V_{max}-V_{min}}{V_{max}+V_{min}} \\ &= \frac{10}{20} = 0.5 \end{aligned} \]

so 0.5, or fifty per cent modulation. 0.33 is the ratio of the minimum to the maximum. 3.0 is the ratio of maximum to minimum, which is greater than one and so cannot be an index for an undistorted signal. 0.67 comes from dividing the difference by the maximum alone. Reading the index off an oscilloscope trace this way is the standard bench measurement.

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