Envelope detector
`
`An envelope detector (sometimes called a peak detector) is an electronic circuit that takes a (relatively)
`high-frequency amplitude modulated signal as input and provides an output, which is the demodulated
`envelope of the original signal.
`
`Circuit operation
`
`The capacitor in the circuit above stores charge on the rising edge
`and releases it slowly through the resistor when the input signal
`amplitude falls. The diode in series rectifies the incoming signal,
`allowing current flow only when the positive input terminal is at a
`higher potential than the negative input terminal.
`
`A signal and its envelope marked
`with red
`
`General considerations
`
`Most practical envelope detectors use either half-wave
`or full-wave rectification of the signal to convert the
`AC audio 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 instrument.
`Reducing the filter cutoff frequency gives a smoother
`output, but decreases the high frequency response.
`Therefore, practical designs must reach a compromise.
`
`Definition of the envelope
`
`Any AM or FM signal
`following form
`
` can be written in the
`
`A simple envelope demodulator circuit.
`
`In the case of AM, φ(t) (the phase component of the
`signal) is constant and can be ignored. In AM, the
`carrier frequency
` is also constant. Thus, all the information in the AM signal is in R(t). R(t) is called the
`envelope of the signal. Hence an AM signal is given by the function
`
`A signal in blue and the magnitude of its analytic
`signal in red, showing the envelope effect
`
`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.
`
`

`

` has a constant envelope R(t) = R and can be ignored. However,
`In the case of FM, the transmitted
`many FM receivers measure the envelope anyway for received signal strength indication.
`
`Diode detector
`
`The simplest form of envelope detector is the diode detector which is shown above. A diode detector is
`simply a diode between 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.
`
`Precision detector
`
`An envelope detector can also be constructed using 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(i.e. modulation index > 1), 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.
`
`Demodulation of signals
`
`An envelope detector can be used to demodulate a 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 baseband signal.
`
`Audio
`
`An envelope detector is sometimes referred to as an envelope follower in musical environments. It is still
`used to detect the amplitude variations 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-wah or
`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.
`
`Both expanders and compressors use the envelope's output voltage to control the gain of an amplifier. Auto-
`wah uses the voltage to control the cutoff frequency of a filter. The voltage-controlled filter of an analog
`synthesizer is a similar circuit.
`
`

`

`Modern envelope followers can be implemented:
`
`1. directly as electronic hardware,
`2. or as software using either a digital signal processor (DSP) or
`3. on a general purpose CPU.
`
`See also
`Analytic signal
`Attack-decay-sustain-release envelope
`
`External links
`Envelope detector (https://web.archive.org/web/20071105091112/http://seniord.ee.iastate.ed
`u/SSOL/RADAR/prjpln99/detector3.html)
`Envelope and envelope recovery (http://www.ele.uri.edu/Courses/ele436/labs/expA4.pdf)
`
`Retrieved from "https://en.wikipedia.org/w/index.php?title=Envelope_detector&oldid=1147053811"
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