- Envelope detector
An envelope detector is an electronic circuit that takes a high-frequency signal as input, and provides an output which is the "envelope" of the original signal. The capacitor in the circuit stores up charge on the rising edge, and releases it slowly through the resistor when the signal falls. The diode in series ensures current does not flow backward to the input to the circuit.
Most practical envelope detectors use either half-wave or full-wave
rectificationof the signal to convert the ACaudio input into a pulsed DC signal. Filtering is then used to smooth the final result. This filtering is rarely perfect and some "ripple" is likely to remain on the envelope follower output, particularly for low frequency inputs such as notes from a bass guitar. More filtering gives a smoother result, but decreases the responsiveness of the design, so real-world solutions are a compromise.
Definition of the envelope
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. Hence an AM signal is given by the equation : with "m"("t") representing the original audio frequency message, "C" the carrier amplitude and "R"("t") equal to "C" + "m"("t"). So, if the envelope of the AM signal can be extracted, the original message can be recovered.
The simplest form of envelope detector is the
diode detectorwhich is shown above. A diode detector is simply a diodebetween the input and output of a circuit, connected to 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 (baseband) signal. A simple filter can then be applied to filter out the DC component.
An envelope detector can also be constructed to use a
precision rectifierfeeding into a low-pass filter.
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
* If the signal is overmodulated, distortion will occurMost of these drawbacks are relatively minor and are usually acceptable tradeoffs for the simplicity and low cost of using an envelope detector.
Demodulation of Signals
An envelope detector can be used to
demodulatea previously modulated signal by removing all high frequency components of the signal. The capacitor and resistor form a low-pass filter to filter out the carrier frequency. Such a device is often used to demodulate AM radio signals because the envelope of the modulated signal is equivalent to the basebandsignal.
An envelope detector is sometimes referred to as an envelope follower in
musical environments. It is still used to detect the amplitudevariations of an incoming signal to produce a control signal that resembles those variations. However, in this case the input signal is made up of audible frequencies.
Envelope detectors are often a component of other circuits, such as a compressor or an
auto-wahor envelope-followed filter. In these circuits, the envelope follower is part of what is known as the "side chain", a circuit which describes some characteristic of the input, in this case its volume.
expandersand compressors use the envelope's output voltage to control of the gain of an amplifier. Auto-wah uses the voltage to control the cutoff frequencyof a filter. The voltage-controlled filterof an analog synthesizeris a similar circuit.
Modern envelope followers can be implemented:
# directly as electronic
# indirectly using DSPs
# completely virtually in
* Attack Decay Sustain Release envelope
* [http://seniord.ee.iastate.edu/SSOL/RADAR/prjpln99/detector3.html Envelope detector]
* [http://www.ele.uri.edu/Courses/ele436/labs/ExpA4.pdf Envelope and envelope recovery]
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