Practical RF Handbook, 4th Edition

8

Modulation and demodulation

Modulation is the process of impressing information to be transmitted onto an RF ‘carrier’ wave, in such a way that it can be retrieved again in more or less undistorted form at the receiver. Figure 8.1a shows how information is transmitted by CW (continuous wave) using the Morse code, once widely used on the HF band (1.6–30 MHz) for commercial marine traffic and still used by amateurs for world-wide DX-ing on a few watts. Broadcasting on the long, medium and short wavebands uses AM (amplitude modulation) (Figure 8.1b). The amplitude of the RF carrier wave changes to reflect the instantaneous value of the modulating baseband waveform, e.g. speech or music. The baseband signal is limited to 4.5 kHz bandwidth, restricting ...