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 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 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;
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 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
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 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 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
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.
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 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.
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19 POWER IN THE FM WAVE power of the unmodulated carrier For a modulated carrier
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
22 The ratio of carrier voltage to noise voltage, is the SNR (voltage) α represents the equivalent modulation index produced by the noise.
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 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.