An envelope detector is a device which is used to demodulate AM signals. The original message is decoded from the envelope of the modulated signal.
Definition of the Envelope
Any AM or FM signal can be written in the following form
- x(t) = R(t)cos(ωt+φ(t))
In the case of AM, φ(t), the phase component of the signal, is constant and can be ignored, so all the information in the signal is in R(t), which is called the envelope of the signal. Given that an AM signal is given by the equation with m(t) representing the original audio frequency message,
- x(t) = (C + m(t))cos(ωt)
R(t) is then equal to C + m(t). So, if the envelope of the AM signal can be extracted, the original message can be recovered.
Diode Detector
The simplest form of envelope detector is the diode detector. To construct a diode detector, simply connect a diode between the input and output of a circuit, and connect a resistor and capacitor in parallel from the output of the circuit to the ground. If the resistor and capacitor are correctly chosen, the output of this circuit should approximate a voltage-shifted version of the original signal. A simple filter can then be applied to filter out the DC component.
Precision detector
An envelope detector can also be constructed to use a precision rectifier feeding into a low-pass filter.
Drawbacks
The envelope detector has several drawbacks
- The input to the detector must be band-pass filtered around the desired signal, or else the detector will simultaneously demodulate several signals. The filtering can be done with a tunable filter or, more practically, a superheterodyne receiver
- It is more susceptible to noise than a product detector
- If the signal is overmodulated, distortion will occur
Most of these drawbacks are relatively minor and are usually acceptable tradeoffs for the simplicity and low-cost of using an envelope detector.