Lò vi sóng RF và hệ thống không dây P9

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 AMPLITUDE MODULATION AND DEMODULATION 275 FIGURE 9.1 Different modulation schemes: (a) direct modulation; (b) external modulation. amplified, downconverted, and demodulated to recover the original baseband signal (information). 9.2 AMPLITUDE MODULATION AND DEMODULATION Amplitude and frequency modulation techniques are classified as analog modula-tion. They are old techniques, having been used for many years since the invention of the radio. Analog modulation uses the baseband signal (modulating signal) tovary one of three variables: amplitude Ac, frequency oc; or phase y. The carrier signal is given by vcðtÞ ¼ Ac sinðoct þ yÞ ð9:1Þ The amplitude variation is AM, the frequency variation is FM, and the phase variation is PM. Phase modulation and FM are very similar processes and can be referred to as angle modulation. The unique feature of AM is that the message of the modulated carrier has the same shape as the message waveform. Figure 9.2 illustrates the carrier, modulating, and modulated signals. For simplicity, let a single audio tone be a modulating signal vðtÞ ¼ Am sin omt ð9:2Þ 276 MODULATION AND DEMODULATION FIGURE 9.2 Signals in AM. Although a sine wave is assumed, a more complex wave can be considered to be the sum of a set of pure sine waves. The modulated signal can be written as v0 ðtÞ ¼ ðAc þ Am sin omtÞ sin oct ¼ Ac 1 þ Ac sin omt sin oct ¼ Acð1 þ m sin omtÞ sin oct ð9:3Þ where Am peak value of modulating signal Ac peak value of unmodulated carrier signal where m is the modulation index, which sometimes is expressed in percentage as the percent of modulation. To preserve information without distortion would require m to be 1 or less than 100%. Figure 9.3 shows three cases of modulation: under-modulation ðm < 100%Þ, 100% modulation, and overmodulation ðm > 100%Þ. Using a trigonometric identity, Eq. (9.3) can be rewritten as v0 ðtÞ ¼ Ac sin oct þ 1 ðmAcÞcosðoc omÞt 1 ðmAcÞcosðoc þ omÞt ð9:4Þ 9.2 AMPLITUDE MODULATION AND DEMODULATION 277 FIGURE 9.3 Degrees of modulatioin: (a) undermodulation; (b) 100% modulation; (c) overmodulation. The modulated signal contains the carrier signal ðocÞ, the upper sideband signal ðoc þ omÞ, and the lower sideband signal ðoc omÞ. This is quite similar to the output of a mixer. A nonlinear device can be used to accomplish the amplitude modulation. Figure 9.4 shows examples using a modulated amplifier and a balanced diode modulator. 278 MODULATION AND DEMODULATION The demodulation can be achieved by using an envelope detector (described in Chapter 4) as a demodulator to recover the message [1]. Example 9.1 In an AM broadcast system, the total transmitted power is 2000W. Assuming that the percent of modulation is 100%, calculate the transmitted power at the carrier frequency and at the upper and lower sidebands. Solution From Equation (9.4) PT ¼ Pc þ 1 m2Pc þ 1 m2Pc ¼ 2000 W FIGURE 9.4 Amplitude modulation using (a) a modulated amplifier and (b) a balanced modulator. ... - tailieumienphi.vn