How does an envelope detector recover the modulating signal from an AM waveform, and what modulation-index-related limitation does it have?

How does an envelope detector recover the modulating signal from an AM waveform, and what modulation-index-related limitation does it have?

• A. Rectifier followed by low-pass filter; requires the modulation index to be moderate to avoid distortion (overmodulation causes envelope distortion)
• B. Heterodyne mixer; uses high-pass filter; modulation index not a factor
• C. Sample-and-hold circuit; digital processing; modulation index irrelevant
• D. Transformer isolation; no filtering; amplitude unaffected

Answer: (A)

An envelope detector recovers the modulating signal by grabbing the slowly varying envelope of the AM waveform. The diode rectifies the RF signal, producing a waveform whose peaks outline the envelope, and the following RC network filters out the rapid carrier oscillations. What finally appears across the filter is a voltage that tracks the modulation, provided the envelope remains proportional to the modulating signal. The modulation-index limitation comes from overmodulation. If the modulation index is too large, the envelope can become distorted or distorted enough that the detector can’t follow it correctly—the peaks may clip or invert, and the recovered signal no longer matches the original modulating signal. With moderate modulation, the envelope stays faithful to the modulating signal, so the detector works well; with overmodulation, accuracy is lost.

An envelope detector recovers the modulating signal by grabbing the slowly varying envelope of the AM waveform. The diode rectifies the RF signal, producing a waveform whose peaks outline the envelope, and the following RC network filters out the rapid carrier oscillations. What finally appears across the filter is a voltage that tracks the modulation, provided the envelope remains proportional to the modulating signal.

The modulation-index limitation comes from overmodulation. If the modulation index is too large, the envelope can become distorted or distorted enough that the detector can’t follow it correctly—the peaks may clip or invert, and the recovered signal no longer matches the original modulating signal. With moderate modulation, the envelope stays faithful to the modulating signal, so the detector works well; with overmodulation, accuracy is lost.