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Signal Encoding Techniques (modulation and encoding) Analog data to analog signal (AM, FM, PM) Digital data to analog signal (ASK, FSK, BPSK, QAM) Analog data to digital signal (PCM, DM) Digital data to digital signal (line codes) DCTC, By Ya Bao 1 Analog Signals DCTC, By Ya Bao 2 Digital Signals DCTC, By Ya 3 Analog and Digital Transmission AMPLITUDE MODULATION DCTC, By Ya Bao 5 Modulation The process by which some characteristics of a carrier wave is varied in accordance with an informationbearing signal. Continuous-wave modulation Amplitude modulation Frequency modulation AM modulation family Amplitude modulation (AM) Double sideband-suppressed carrier (DSB-SC) Single sideband (SSB) Vestigial sideband (VSB) DCTC, By Ya Bao 6 AMPLITUDE MODULATION Carrier wave: is a waveform (usually sinusoidal) that is modulated (modified) with an input signal for the purpose of conveying information. This carrier wave is usually a much higher frequency than the input signal. 1. DEFINING AM A carrier wave whose amplitude is varied in proportion to the instantaneous amplitude of a modulating voltage GENERATING THE AM nonlinear device: diode or transistor biased in its nonlinear region 2. DCTC, By Ya Bao 7 DCTC, By Ya Bao 8 3. ANALYSIS OF THE AM WAVE vc Vc sin 2f c t m m v Vc sin 2f c t Vc cos 2 ( f c f s )t Vc cos 2 ( f c f s )t 2 2 DCTC, By Ya Bao 9 DCTC, By Ya Bao 10 4. Different Carriers and AM Carriers are spaced at 20 kHz, beginning at 100kHz. Each carrier is modulated by a signal with 5kHz bandwidth. Is there interference from sideband overlap? DCTC, By Ya Bao 11 5. MODULATION INDEX AND SIGNAL POWER Vm Vmax Vmin m Vc Vmax Vmin DCTC, By Ya Bao 12 Moduiation Index and Power 2 carr 2 V 2 c (Vc / 2 ) V Pc R R 2R PLSB PUSB 2 Vc m 2R 4 2 C 2 2 2 PT m 2( 1) PC V m m PT (1 ) Pc (1 ) 2R 2 2 PT m 1 Pc 2 DCTC, By Ya Bao 13 Current Calculations 2 IT m 1 Ic 2 Example A carrier of 1000 W is modulated with a resulting modulation index of 0. 8. What is the total power? What is the carrier power if the total power is 1000 W and the modulation index is 0.95? DCTC, By Ya Bao 14 6.2 Double Sideband Suppressed Carrier (DSBSC) When the carrier is reduced, this is called doublesideband suppressed-carrier AM, or DSB-SC. If the carrier could somehow be removed or reduced, the transmitted signal would consist of two informationbearing sidebands, and the total transmitted power would be information DCTC, By Ya Bao 15 6.3 Single-Sideband (SSB) suppressing the carrier and one of the sidebands DCTC, By Ya Bao 16 DCTC, By Ya Bao 17 6.4 Filtering the SSB LSB or USB Dual Conversion: up-converting the modulating frequency twice and selecting the upper or lower sideband for transmission. DCTC, By Ya Bao 18 AM: Features and Drawbacks: the AM signal is greatly affected by noise impossible to determine absolutely the original signal level conventional AM is not efficient in the use of transmitter power AM is useful where a simple, low-cost receiver and detector is desired DCTC, By Ya Bao 19 Angle Modulation DCTC, By Ya Bao 20 ANGLE MODULATION: The intelligence of the modulating signal can be conveyed by varying the frequency or phase of the carrier signal. When this is the case, we have angle modulation, which can be subdivided into two categories: frequency modulation (FM), and phase modulation (PM). DCTC, By Ya Bao 21 Frequency Modulation. The carrier's instantaneous frequency deviation from its unmodulated value varies in proportion to the instantaneous amplitude of the modulating signal. eFM Ac sin(ct m f sin mt ) Phase Modulation. The carrier's instantaneous phase deviation from its unmodulated value varies as a function of the instantaneous amplitude of the modulating signal; ePM Ac sin(ct m sin mt ) DCTC, By Ya Bao 22 FIGURE 4-1 The FM and PM waveforms for sine-wave modulation: (a) carrier wave; (b) modulation wave; (c) FM wave; (d) PM wave. (Note: The derivative of the modulating sine wave is the cosine wave shown by the dotted lines. The PM wave appears to be frequency modulated by the cosine wave.) DCTC, By Ya Bao 23 MODULATION INDEX modulation index for an FM signal mf fm δ = maximum frequency deviation of the carrier caused by the amplitude of the modulating signal fm = frequency of the modulating signal DCTC, By Ya Bao 24 FREQUENCY ANALYSIS OF THE FM WAVE eFM Ac J 0 sin ct Ac {J 1 (m f )[sin(c m )t sin(c m )t ]} Ac {J 2 (m f )[sin(c 2m )t sin(c 2m )t ]} Ac {J 3 (m f )[sin(c 3m )t sin(c 3 m )t ]} ..., etc where: eFm = the instantaneous amplitude of the modulated FM wave Ac = the peak amplitude of the carrier Jn = solution to the nth order Bessel function for a modulation index mf. mf = FM modulation index, Δf/fm DCTC, By Ya Bao 25 DCTC, By Ya Bao 26 Spectral components of a carrier of frequency, fc, frequency modulated by a sine wave with frequency fm DCTC, By Ya Bao 27 FM signal characters • The FM wave is comprised of an infinite number of sideband components • bandwidth of an FM signal must be wider than that of an AM signal • As the modulation index increases from mf = 0, the spectral energy shifts from the carrier frequency to an increasing number of significant sidebands. • Jn(mf) coefficients, decrease in value with increasing order, n. • negative Jn(mf) coefficients imply a 1800 phase inversion. DCTC, By Ya Bao 28 Carrier Frequency Eigenvalues in some cases the carrier frequency component, JO, and the various sidebands, Jn go to zero amplitudes at specific values of m. These values are called eigenvalues. DCTC, By Ya Bao 29 Bandwidth Requirements for FM The higher the modulation index, the greater the required system bandwidth BW 2(n f m ) where n is the highest number of significant (least 1%, or -40 dB; (20 log 1/100 ), of the voltage of the unmodulated carrier) sideband components and fm is the highest modulation frequency. Carson's Rule BW 2( f m ) 2 f m (1 m f ) DCTC, By Ya Bao 30 Amplitude versus frequency spectrum for various modulation indices (fm fixed, & varying): (a) mf = 0.25; (b) mf = 1; (c) mf = 2; (d) mf = 5; (e) mf = 10. DCTC, By Ya Bao 31 Warren Hioki Telecommunications, Fourth Edition DCTC, By Ya Bao 32 Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. FIGURE 4-6 Commercial FM broadcast band. DCTC, By Ya Bao 33 Commercial FM broadcast band • The maximum permissible carrier deviation, δ, is ±75 kHz • Modulating frequencies (voice or music) is ranging from 50 Hz to 15 kHz • The modulation index can range from as low as 5 for fm = 15 kHz (75 kHz/15 kHz) to as high as 1500 for fm = 50 Hz (75 kHz/50 Hz). • The ±75-kHz carrier deviation results in an FM bandwidth requirement of 150 kHz for the receiver. • A 25-kHz guard band above and below the upper and lower FM sidebands. • Total bandwidth of one channel is 200Hz. DCTC, By Ya Bao 34 Narrowband FM (NBFM) NBFM uses low modulation index values, with a much smaller range of modulation index across all values of the modulating signal. An NBFM system restricts the modulating signal to the minimum acceptable value, which is 300 Hz to 3 KHz for intelligible voice. 10 to 15 kHz of spectrum. Used in police, fire, and Taxi radios, GSM, amateur radio, etc. DCTC, By Ya Bao 35 POWER IN THE FM WAVE power of the unmodulated carrier Vcrms PT R For a modulated carrier 2 PT PJ 0 PJ1 PJ 2 PJ 3 ... PJ n 2 J0 V R DCTC, By Ya Bao 2 J1 2V R 2 J2 2V R 2 J3 2V R ... 2 Jn 2V R 36 FM NOISE Increased bandwidth of an FM – to enhance the signalto-noise ratio (SNR). Advantages of FM over AM. To take this advantage, large mf is necessary– high order sidebands are important – wider bandwidth is required. Phase Analysis of FM Noise VN sin Vc 1 where α = the maximum phase deviation of the carrier frequency caused by the noise VN = noise voltage Vc= carrier voltage DCTC, By Ya Bao 37 Phasor addition of noise on an FM signal’s carrier frequency causes a phase shift, whose maximum value is . Warren Hioki Telecommunications, Fourth Edition DCTC, By Ya Bao 38 Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey 07458 All rights reserved. The ratio of carrier voltage to noise voltage, Vc Vn is the SNR (voltage) Vc SNR VN 1 sin SNR 1 α represents the equivalent modulation index produced by the noise. N fm SNRFM DCTC, By Ya Bao N 39 • The effect of noise on an FM carrier signal is directly proportional to the modulation frequency fm. • Increasing fm, degrades the SNR N fm Voice, data, and music contain many frequencies, which are distributed throughout the given modulation passband. Therefore, the SNR is not uniform throughout. To maintain a flat SNR, some techniques are employed. DCTC, By Ya Bao 40