envelopedemodulaatioon

Envelopedemodulaatioon, or envelope demodulation, is a demodulation technique used in amplitude-modulated AM communications to recover the original modulating signal from the carrier. In practice it extracts the slowly varying envelope of the AM waveform, which encodes the information.

Principle: For a carrier Ac cos(ωc t) modulated by a baseband signal m(t), the transmitted signal can be written s(t) = Ac [1 + m(t)] cos(ωc t). The envelope of s(t) is Ac [1 + m(t)]. An envelope detector uses a rectifier to capture the envelope and a low-pass filter or RC network to smooth the carrier remnants, producing a recovered signal proportional to m(t).

Implementation and design considerations: A simple envelope detector uses a diode, a capacitor, and a resistor. The RC time constant is chosen so that the carrier period is much shorter than RC and RC is shorter than the modulation period (1/fc << RC << 1/fm). This allows fast charging at peaks and gradual discharging between peaks. Non-idealities such as diode drop, noise, and overmodulation can cause distortion.

Limitations and variants: Envelope demodulation requires a carrier; it does not work for suppressed-carrier schemes unless a carrier recovery stage is used. Variants include peak detectors, synchronous (coherent) demodulation, and active envelope detectors using op-amps for improved fidelity.

Applications and history: The envelope detector is a classic component in AM radio receivers and other amplitude-modulated communication systems. It played a central role in early radio and remains a simple, cost-effective demodulation method for suitable signals.