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1 Frequency Modulation ANGLE MODULATION: The intelligence of the modulating signal can be conveyed by varying the frequency or phase of the carrier signal.

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Presentation on theme: "1 Frequency Modulation ANGLE MODULATION: The intelligence of the modulating signal can be conveyed by varying the frequency or phase of the carrier signal."— Presentation transcript:

1 1 Frequency Modulation 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).

2 2 Frequency Modulation. The carrier's instantaneous frequency deviation from its unmodulated value varies in proportion to the instantaneous amplitude of the modulating signal. Phase Modulation. The carrier's instantaneous phase deviation from its unmodulated value varies as a function of the instantaneous amplitude of the modulating signal;

3 3 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.) Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

4 4 MODULATION INDEX modulation index for an FM signal δ = maximum frequency deviation of the carrier caused by the amplitude of the modulating signal f m = frequency of the modulating signal

5 5 FREQUENCY ANALYSIS OF THE FM WAVE Where:e Fm = the instantaneous amplitude of the modulated FM wave Ac = the peak amplitude of the carrier J n = solution to the nth order Bessel function for a modulation index m f. m f = FM modulation index, Δf/f m

6 6 FIGURE 4-3 Spectral components of a carrier of frequency, f c, frequency modulated by a sine wave with frequency f m. (Source: James Martin, Telecommunications and the Computer, 2nd ed. [Englewood Cliffs, N.J.: Prentice-Hall, 1976], p Reprinted with permission from the publisher.) Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

7 7 Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

8 8 FM signal characters The FM wave is comprised of an infinite number of sideband components As the modulation index increases from m f = 0, the spectral energy shifts from the carrier frequency to an increasing number of significant sidebands. J n (m f ) coefficients, decrease in value with increasing order, n. negative Jn(mf) coefficients imply a 180' phase inversion. The carrier component, J o, and various sidebands, J n, go to zero amplitude at specific values of modulation index, m f.

9 9 Carrier Frequency Eigenvalues in some cases the carrier frequency component, J O, and the various sidebands, J n go to zero amplitudes at specific values of m. These values are called eigenvalues.

10 10 Bandwidth Requirements for FM The higher the modulation index, the greater the required system bandwidth where n is the highest number of significant sideband components and f m is the highest modulation frequency. Carson's Rule

11 11 FIGURE 4-5 Amplitude versus frequency spectrum for various modulation indices (f m fixed, & varying): (a) m f = 0.25; (b) m f = 1; (c) m f = 2; (d) m f = 5; (e) m f = 10. Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

12 12 Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

13 13 Broadcast FM extends from 88 to 108 MHz is divided into 100 channels Channels range from 88.1 MHz, where N = 201, to MHz, where N = 300 N=5(f-47.9) whereN = the FM broadcast channel number f = the frequency in MHz Guard Band: A range of frequences separating transmitted channels in which no signals should be transmitted.

14 14 FIGURE 4-6 Commercial FM broadcast band. Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

15 15 Commercial FM broadcast band The maximum permissible carrier deviation, δ, is ±75 kHz Modulating frequencies is ranging from 50 Hz to 15 kHz The modulation index can range from as low as 5 for f m = 15 kHz (75 kHz/15 kHz) to as high as 1500 for f m = 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.

16 16 Narrowband FM 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.

17 17 BA1404 HI-FI Stereo FM Transmitter Kit Price: $24.95 Be "On Air" with your own radio station

18 BA1404 based transmitter is an exciting product that will broadcast high quality stereo signal in 88MHz - 108MHz FM band. It can be connected to any type of stereo audio source such as iPod, Computer, Laptop, CD Player, Walkman, Television, Satellite Receiver, Tape Deck or other stereo system to transmit stereo sound with excellent clarity throughout your home, office, yard or camp ground. Add 500mW FM / VHF Transmitter Amplifier / Booster for even longer range. 18

19 19 POWER IN THE FM WAVE power of the unmodulated carrier For a modulated carrier

20 20 FM NOISE increased bandwidth of an FM -- enhance the signal-to- noise ratio (SNR). Advantages of FM over AM. To take this advvantage, large m f is necessary– high order sidebands are important – wider bandwidth is required. Phasor Analysis of FM Noise where α = the maximum phase deviation of the carrier frequency caused by the noise V N = noise voltage V c = carrier voltage

21 21 FIGURE 4-7 Phasor addition of noise on an FM signal’s carrier frequency causes a phase shift, whose maximum value is . Warren Hioki Telecommunications, Fourth Edition Copyright ©2001 by Prentice-Hall, Inc. Upper Saddle River, New Jersey All rights reserved.

22 22 The ratio of carrier voltage to noise voltage, is the SNR (voltage) α represents the equivalent modulation index produced by the noise.

23 23 The effect of noise on an FM carrier signal is directly proportional to the modulation frequency f m. Increasing f m, degrades the 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, following techniques are employed.

24 24 Pre-emphasis and De-emphasis Pre-emphasis: boost the signal levels of the higher modulating frequencies before the modulation process to maintain a uniform SNR.—high-pass filter De-emphasis: FM receiver brings back pre- emphasized signals to their original amplitudes Low-pass filter. Direct FM: the modulation signal is used to directly change the carrier signal's frequency or phase Indirect FM: the modulation signal is used to change the phase of the carrier signal, which indirectly changes its frequency.


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