envelopedetector

An envelope detector is a demodulation circuit used to recover the modulating signal from an amplitude-modulated carrier. In its simplest form it consists of a diode rectifier followed by an RC network that filters high-frequency components and yields the slowly varying envelope as the output voltage.

Principle of operation: The diode conducts on the peaks of the carrier waveform, charging the capacitor to the peak value. Between peaks, the capacitor discharges through the resistor. The result is a voltage that tracks the envelope of the modulated signal. The discharge rate is determined by the time constant τ = RC. For faithful recovery, the time constant should be longer than the carrier period but short enough to follow the modulation, so T_c << τ << T_m.

Circuit considerations and variants: A passive envelope detector uses a diode to rectify and an RC network to filter. Variants include active envelope detectors that employ transistors or operational amplifiers to reduce the effects of diode forward drop and to improve linearity. Schottky diodes are often used to minimize voltage loss. Some designs employ multiple stages or peak-detect configurations to improve performance for high-frequency or heavily modulated signals.

Applications and limitations: Envelope detectors are widely used to demodulate amplitude-modulated signals in traditional AM radio receivers, and they appear in measurement and communications equipment where simple, low-cost demodulation is sufficient. Limitations include distortion from diode drops and nonlinearity, sensitivity to temperature, and poor performance with very small modulation indices or high-frequency content beyond the RC filter’s ability to follow. Proper design of the RC time constant and, in active variants, careful component choice, mitigate some of these issues.