The Down and Dirty on How Amplitude Demodulation Works

The process of detection or demodulation consists in recovering the original modulating voltage from the modulated carrier voltage. Thus detection is a process of reverse modulation. The modulation frequency terms which were translated to higher part of the frequency spectrum through the modulation process are translated back to their original positions in the frequency spectrum through the demodulation process.

The detection process is accomplished by mixing the carrier with the sideband components carrying the intelligence, in a non-linear device. This mixing process results in sum and difference frequency terms. Each sideband beats with the carrier to produce the difference term which is nothing but the original modulation frequency term. In so far as the mixing operation is concerned, the detection process is similar to the modulation process. Accordingly similar circuits may be used for both modulation and detection purpose except for certain differences in the output circuits for selection etc. Thus, in modulation process, the carrier voltage and the audio frequency modulating voltage are mixed in the modulated amplifier to produce the sum and the difference frequency terms and the output circuit is tuned carrier frequency so that it responds to the carrier and the two sideband terms and rejects all other frequency terms. In demodulation, on the other hand, each sideband term mixes with the carrier to produce the difference frequency terms which constitute the original modulating frequency terms. The output circuit of the detector is selected to reject all other frequency terms except the desired modulation frequency terms.

Detectors are of two types namely Square Law Detector and Linear Diode detector. The square law detector utilizes the non linear portion of the dynamic current-voltage characteristics of the diode. Its difference from the linear diode detector is that in this case the applied input carrier voltage is of small magnitude and hence, is restricted to the excessively non linear portion of the dynamic characteristic, where as in linear diode detector, a large amplitude modulated carrier voltage is applied to the diode and most of the operation takes place over the linear region of the characteristic. The diode is biased positively to shift the zero signals operating point to the small current non linear region of the dynamic current-voltage characteristic. The capacitor-resistor combination constitutes to load.

Linear diode detectors are extremely popularly used in commercial radio receivers. However a linear diode detector for satisfactory operation requires modulated carrier voltage of five volts or more. With such high carrier voltage, the cut-in voltage of the diode may be neglected and the operation may be considered to be taking place over essentially linear region of the dynamic current voltage characteristic of the diode. With small carrier voltage, operation takes place over the excessively non linear region of the diode characteristic giving square law detection with associated distortion.

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