Q11Principle of Electronic Communication

Envelope Detection of an AM Signal

An envelope detector recovers the original message signal from an AM waveform using only a diode, a capacitor, and a resistor in parallel. The incoming AM signal is first passed through the diode, which conducts only during the positive half-cycles of the carrier, charging the capacitor rapidly to nearly the peak voltage of each RF cycle. Between successive RF peaks, the capacitor discharges slowly through the parallel resistor R (chosen so that the RC time constant is much longer than the carrier period but much shorter than the period of the message signal), so the capacitor voltage closely tracks and follows the slowly-varying envelope (peak amplitude) of the AM waveform rather than following the rapid carrier oscillations themselves.

This reconstructed envelope is essentially the original message signal (plus a DC offset corresponding to the carrier amplitude), and a subsequent DC-blocking capacitor removes this offset, leaving the recovered message signal. This simple envelope detector works correctly only for standard (double-sideband, large-carrier) AM with a sufficiently large carrier and modulation index below 100% to avoid diagonal clipping distortion, and its simplicity (requiring no local oscillator or synchronous demodulation) is precisely why AM broadcast radio receivers have historically used this low-cost detection method.

The diode used must have a low forward voltage drop (germanium or Schottky diodes are commonly preferred over silicon for this reason) to avoid clipping small-amplitude portions of the envelope, and the RC time constant selection is critical: too large a time constant causes the detector to fail to track rapid decreases in the envelope (diagonal clipping distortion, where the discharge curve cannot keep pace with the falling envelope), while too small a time constant lets excessive RF ripple pass through onto the recovered output, requiring an additional low-pass filter stage to smooth it out.