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.
Table of contents |
2 Diode Detector 3 Precision detector 4 Drawbacks |
Definition of the Envelope
Any AM or FM signal can be written in the following form
- x(t) = R(t)cos(ωt+φ(t))
- x(t) = (C + m(t))cos(ωt)
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
Most of these drawbacks are relatively minor and are usually acceptable tradeoffs for the simplicity and low-cost of using an envelope detector.