Demodulation

Demodulation is the process of recovering the signal intelligence from a modulated carrier wave. This process, also called detection, is the reverse process of modulation.

Need For Demodulation

The wireless signal consists of radio frequency (high frequency) carrier wave modulated by audio frequency (low frequency). The diaphragm of a telephone receiver or a loud speaker cannot vibrate with high frequency. Moreover, this frequency is beyond the audible range of human ear. So, it is necessary to separate the audio frequencies from radio- frequency carrier waves.

Types of Demodulation

Corresponding to different types of modulation processes, there are different types of demodulation.

AM detector

For demodulation, the modulated wave is fed to a circuit. This circuit is called detector. The output of the detector must have the same variations as the signal that modulated the carrier. If the variation in the output of the detector is not the same, then we say that the signal has been distorted.

Operation

The detector rectifies the input signal. So, either the top or the bottom half of the wave is removed. A suitable filter removes the radio frequency of the rectified wave. A signal is produced which follows the envelope of one-half of the wave.

Amplitude demodulation process

Importance of rectification in demodulation

The modulation enveloped on the positive and negative sides are 180^o out of phase. If rectification is not done, then after filtering, the positive and negative envelopes would cancel each other. So, the original modulating signal cannot be recovered. Thus, rectification is necessary in demodulation.

Note:

Since both the halves have the variations of intelligence, either of them can be removed except in case of certain applications.

Important characteristics of a detector

(i) Linearity

(ii) Sensitivity

(iii) Signal handling capacity

Linearity is determined by how accurately the output of the detector follows the input signal. If the variation in the amplitude of output is proportional to the input amplitude, the detector is said to be linear.

The sensitivity is a measure of how much the input signal is delivered as useful output.

The signal handling is a measure of the signal amplitudes that a detector can accept without distortion.