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Lò vi sóng RF và hệ thống không dây P9

Modulation and Demodulation Modulation is a technique of imposing information (analog or digital) contained in a lower frequency signal onto a higher frequency signal. The lower frequency is called the modulating signal, the higher frequency signal is called the carrier, and the output signal is called the modulated signal. The benefits of the modulation process are many, such as enabling communication systems to transmit many baseband channels simultaneously at different carrier frequencies without their interfering with each other | RF and Microwave Wireless Systems. Kai Chang Copyright 2000 John Wiley Sons Inc. ISBNs 0-471-35199-7 Hardback 0-471-22432-4 Electronic CHAPTER NINE Modulation and Demodulation 9.1 INTRODUCTION Modulation is a technique of imposing information analog or digital contained in a lower frequency signal onto a higher frequency signal. The lower frequency is called the modulating signal the higher frequency signal is called the carrier and the output signal is called the modulated signal. The benefits of the modulation process are many such as enabling communication systems to transmit many baseband channels simultaneously at different carrier frequencies without their interfering with each other. One example is that many users can use the same long-distance telephone line simultaneously without creating a jumbled mess or interference. The modulation technique also allows the system to operate at a higher frequency where the antenna is smaller. Some form of modulation is always needed in an RF system to translate a baseband signal e.g. audio video data from its original frequency bandwidth to a specified RF frequency spectrum. Some simple modulation can be achieved by direct modulation through the control of the bias to the active device. A more common method is the use of an external modulator at the output of the oscillator or amplifier. Figure 9.1 explains the concept of modulation. There are many modulation techniques for example AM FM amplitude shift keying ASK frequency shift keying FSK phase shift keying PSK biphase shift keying BPSK quadriphase shift keying QPSK 8-phase shift keying 8-PSK 16-phase shift keying 16-PSK minimum shift keying MSK and quadrature amplitude modulation QAM . AM and FM are classified as analog modulation techniques and the others are digital modulation techniques. After modulation the signal is amplified and radiated to free space by an antenna. The signal is then picked up by a receiver antenna at some distance away and is then 274 9.2 .