An envelope detector is a simple electronic circuit used to extract the envelope of an amplitude-modulated (AM) signal. The envelope of an AM signal represents the variations in its amplitude over time, which carry the original information or modulation.
The AM signal consists of a high-frequency carrier wave that is modulated in amplitude by a lower-frequency message signal (the information signal). This modulation causes the carrier's amplitude to vary with the message signal's amplitude.
The envelope detector works by rectifying and smoothing the AM signal to obtain the envelope. Here's a step-by-step explanation of how it extracts the envelope:
Rectification: The first stage of the envelope detector is typically a diode (a semiconductor device that allows current to flow in only one direction). The AM signal is passed through the diode, which effectively removes the negative half-cycles of the signal, leaving only the positive half-cycles.
Smoothing: After rectification, the output of the diode contains the positive half-cycles of the AM signal, but it still has some ripple and fast variations. To smooth out these variations and get a more continuous envelope, a low-pass filter (LPF) is used. The LPF can be a simple resistor-capacitor (RC) circuit or an active filter, depending on the application.
In an RC filter, the resistor and capacitor work together to create a time constant that determines how fast the capacitor charges and discharges. The larger the time constant, the slower the variations are smoothed out, and vice versa.
In an active filter, additional amplification and filtering stages are used to achieve better performance and control over the smoothing process.
Envelope Output: The output of the smoothing stage is the extracted envelope of the AM signal. This envelope waveform represents the original message signal that was used to modulate the carrier.
It's important to note that the envelope detector is a simple and inexpensive way to extract the envelope of an AM signal, but it has some limitations. One limitation is that it can introduce some distortion and inaccuracies in the extracted envelope, especially in the presence of noise or interference. More advanced detection techniques, such as synchronous detectors or digital signal processing methods, are used in modern communication systems to achieve better accuracy and performance in extracting the envelope of an AM signal.