Chpt.6 Analogue Modulation-FM

Chpt.6 Analogue Modulation-FM - Chapter 6 Analog Modulation...

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1 Chapter 6 Analog Modulation and Demodulation (Part 2. Frequency Modulation)
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2 Frequency Modulation We have seen that information can be transmitted by varying the amplitude of the carrier signal in AM. Next we will see that information can also be transmitted by varying the frequency of the carrier signal. Frequency modulation (FM) is a special case of angle modulation. It is E. H. Armstrong who managed to demonstrate that a FM radio is clearer and more robust than an AM radio in middle 1930s. FM can be broadly divided into narrowband FM and wideband FM. Wideband FM can bring great improvement in signal to noise ratio if the signal is stronger than a certain threshold. Narrow band FM is usually used as an intermediate stage of the wideband FM. FM has been widely used in radio, TV and analogue mobile systems (the so-called 1G systems).
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3 Frequency Modulation For an AM signal s AM ( t ) = s ( t )cos(2 π f c t ), the amplitude varies according to information. An frequency modulated signal can be represented in the time domain by: where k is a proportional constant, the amplitude remains constant but frequency varies according to information. Here we define the Instantaneous Frequency of the signal s FM ( t ) as: Note that the above definition is slightly different from the textbook since we will always use Hz as unit. () cos ( () ) cos{2 [ () ]} t FM c st A t A f t k s t d t Ψπ −∞ == + 1( ) Hz 2 c dt ft f k dt Ψ +
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4 The time domain representation of the FM waveform with the single tone modulating signal is shown below. s ( t ) s FM ( t ) FM with Single Tone Modulating Signal
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5 Note that the instantaneous frequency of the of FM signal is continuously varying, but the amplitude of the waveform is always the same. This means that the power of an FM signal is not a function of the information signal. Advantages - Constant amplitude means that efficient non-linear power amplifiers can be used. - Better signal to noise ratio can be achieved (compared with AM) if bandwidth is sufficiently high. Disadvantages: - Usually occupy more bandwidth than AM. - More complicated hardware. Advantages and Disadvantages of FM
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6 FM with Single Tone Modulating Signal FM modulation is a non-linear process. Therefore it is usually very difficult to analyze an FM signal. A common approach to the analysis of an AM signal is assuming that the signal is a single tone, i.e., the signal is of the form s ( t ) = A m cos(2 π f m t ) This is only a special case, but it tell us very useful insights. For example, later we will see that the bandwidth of an FM signal s FM ( t ) is related to the maximum frequency of the information signal s ( t ). We can normally let f m above be maximum frequency of s ( t ) and then use it to estimate the FM bandwidth. Note that such a treatment is only an approximation. Nevertheless, it is very useful in practice.
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Chpt.6 Analogue Modulation-FM - Chapter 6 Analog Modulation...

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