EnvelopeDetektor

An envelope detector is an electronic circuit used to extract the slowly varying envelope of an amplitude-modulated AM signal. It is commonly employed to demodulate AM radio, where the information resides in the carrier’s envelope.

A typical envelope detector consists of a diode rectifier followed by an RC network. The diode clips the negative half-cycles of the incoming AM signal, charging a storage capacitor. Between peaks, the capacitor discharges through a load resistor, causing the output to follow the envelope. The circuit effectively downconverts the high-frequency carrier to a lower-frequency signal that represents the modulation. The time constant RC must be chosen carefully: 1/ωc << RC << 1/ωm, where ωc is the carrier angular frequency and ωm is the highest modulation frequency. If RC is too large, the detector cannot follow rapid envelope changes; if too small, the carrier is not adequately filtered and the output contains carrier ripple.

Design considerations include the forward voltage drop of the diode, which can distort low-level envelopes, and the input and output impedances that affect fidelity and loading. Variants use low-drop diodes, germanium or Schottky diodes, or active rectifiers (op-amp-based) to improve linearity and sensitivity. Some designs incorporate a limiter stage beforehand to remove amplitude variations not related to modulation.

Applications extend beyond traditional AM radio to any system requiring envelope detection or peak detection of amplitude variations, including instrumentation, audio processing, and certain ultrasound or pulsed-signal measurements. While highly effective for AM demodulation, envelope detectors are not suitable for demodulating frequency-modulated signals, which require different detection methods.