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Home » GATE Study Material » Electronics and Telecommunications » Radio Receiver Technology » AM demodulation using a diode detector

AM demodulation using a diode detector

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AM demodulation using a diode detector

Radio receiver amplitude modulation (AM) demodulation

- overview or tutorial of the basics of amplitude modulation ( AM ) demodulation or detection using a diode detector or demodulator.

One of the advantages of amplitude modulation (AM) is that it is cheap and easy to build a demodulator circuit for a radio receiver. The simplicity AM radio receivers AM is one of the reasons why AM has remained in service for broadcasting for so long. One of the key factors of this is the simplicity of the receiver AM demodulator.

A number of methods can be used to demodulate AM, but the simplest is a diode detector. It operates by detecting the envelope of the incoming signal. It achieves this by simply rectifying the signal. Current is allowed to flow through the diode in only one direction, giving either the positive or negative half of the envelope at the output. If the detector is to be used only for detection it does not matter which half of the envelope is used, either will work equally well. Only when the detector is also used to supply the automatic gain control (AGC) circuitry will the polarity of the diode matter.

The AM detector or demodulator includes a capacitor at the output. Its purpose is to remove any radio frequency components of the signal at the output. The value is chosen so that it does not affect the audio base-band signal. There is also a leakage path to enable the capacitor to discharge, but this may be provided by the circuit into which the demodulator is connected.

A simple diode detector or demodulator for AM signals

A simple diode detector (demodulator) for AM signals

This type of detector or demodulator is called a linear envelope detector because the output is proportional to the input envelope. Unfortunately the diodes used can introduce appreciable levels of harmonic distortion unless modulation levels are kept low. As a result these detectors can never provide a signal suitable for high quality applications.

Additionally these detectors ( demodulators ) are susceptible to the effects of selective fading experienced on short wave broadcast transmissions. Here the ionospheric propagation may be such that certain small bands of the signal are removed. Under normal circumstances signals received via the ionosphere reach the receiver via a number of different paths. The overall signal is a combination of the signals received via each path and as a result they will combine with each other, sometimes constructively to increase the overall signal level and sometimes destructively to reduce it. It is found that when the path lengths are considerably different this combination process can mean that small portions of the signal are reduced in strength. An AM signal consists of a carrier with two sidebands.

Spectrum of an amplitude modulated (AM) signal

Spectrum of an amplitude modulated (AM) signal

If the section of the signal that is removed falls in one of the sidebands, it will change the tone of the received signal. However if carrier is removed or even reduced in strength, the signal will appear to be over modulated, and severe distortion will result. This is a comparatively common occurrence on the short waves, and means that diode detectors are not suitable for high quality reception. Synchronous demodulation ( detection ) is far superior.

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