Envelope detector

An envelope detector, also called envelope or incoherent demodulator is a demodulator for amplitude modulated signals. This was the first wireless radio receivers, the detector receiver, realized in communications technology.

Technical Realization

An envelope detector is implemented in the simplest case by means of a diode as a rectifier for the input signal. This can happen only one polarity of the frequency received signal, so that only the upper half of the high frequency vibrations remains. This is followed by a low pass filter to remove the high-frequency carrier signal. As a result, there arises again the original modulation signal, such as speech, music or Morse code. The signal ( red curve) is only a DC voltage ( the central level of the high frequency) is superimposed, which may be removed by a subsequent coupling capacitor.

The function of the diode can be illustrated by the following thought experiment: Would the received signal directly applied to the low-pass, so positive and negative amplitude fluctuations would cancel each other ( see picture). The result would be 0 - you would not hear anything.

Developed in 1906 by HHC Dunwoody Crystal detector for radio receiver (rectifier from a natural mineral ) in combination with a downstream capacitor as a low pass filter replaced the coherer from purely as a detector for surrounding high-frequency signals, thereby sounding ( modulation ) reception was possible. This simple circuit has some disadvantages:

  • Signal amplitudes below the threshold voltage of several 100 mV can not be rectified
  • Signals with higher amplitudes are distorted reproduced, because the threshold voltage is not a fixed fraction of the same dish duck voltage
  • The signal source (usually a resonant circuit ) will be charged, which is why reduce quality factor and resonance voltage.

Improved circuits

These disadvantages can be avoided by active components that require additional energy, but relieve the signal source:

  • At the cathode rectifier, an amplifier provides the energy needed while obviating the disturbing threshold voltage.
  • With a peak voltage rectifier amplifies a comparator

The input voltage so that the threshold voltage is no longer of weight. Therefore amplitudes may already be rectified distortion of a few millivolts. The maximum frequency is approximately 10 MHz.

Often while the superimposed DC voltage signal of the demodulator for gain control and display the field strength used. The envelope detection of the intermediate frequency has today been replaced mostly by the synchronous demodulation and partly by fully digital signal processing of the RF input signal.

Envelope detectors can be found, inter alia, where a low-frequency useful signal must be of a higher-frequency carrier signal isolated (eg synthesizers and analyzers ). These detectors are today carried out by means of a digital signal processor integrated circuit.

Envelope detection was used at the time of the analog telephone network in order to evaluate the non-audible counts.

Envelope detectors are only suitable for AM with a modulation depth m <1. The reason for this is that in double-sideband AM the envelope with vector addition - as it were by accident - exactly corresponds to the modulating low frequency. Lacks a sideband or even, as in the SSB, the carrier frequency is too low, the envelope demodulation generates unacceptable distortion, while a synchronous still provides a clear signal.