Abstract: This chapter discusses the principles of modulation. The process of modulation is that of modifying some feature of an otherwise regularly recurring quantity in accordance with the signal. Transmission then takes place by means of the modulated high-frequency wave. The high-frequency unmodulated wave is referred to as the carrier wave; after modulation, it is called a modulated carrier. If the carrier frequency is maintained constant and its amplitude is varied in accordance with the modulating signal, the carrier is amplitude modulated. If, on the other hand, the amplitude of the carrier is kept constant and its frequency is varied in accordance with the modulating signal, the carrier is frequency (or angle) modulated. In amplitude modulation, the amplitude of a high-frequency wave is varied in accordance with the signal to be transmitted. This variation may, in the most simple case (which is, in fact, a quite complicated case), consist of an on and off condition of the carrier, representing the transmission of a code such as Morse. In both amplitude and frequency modulation, the carrier wave is continuously present. There are the methods of modulation where the carrier wave is made discontinuous in the form of a train of rectangular pulses. The modulation process is then to vary some characteristic of the pulses in accordance with the modulating signal.
Publication Year: 1988
Publication Date: 1988-01-01
Language: en
Type: book-chapter
Indexed In: ['crossref']
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