Amplitude modulation
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1) Modulation involves changing characteristics of a high-frequency carrier signal according to an information signal. This allows signal transmission over long distances and multiple signals over the same channel. 2) The main modulation types are amplitude modulation (AM), which changes amplitude; frequency modulation (FM), which changes frequency; and phase modulation (PM), which changes phase. 3) AM is the simplest form and varies the carrier amplitude by the information signal. It has advantages of simplicity but is inefficient in power and bandwidth usage, and susceptible to noise.
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Therefore The full AM signal may be
written as
s(t ) Ac (1 m cos( mt )) cos( c t )
cos A cos B 1 / 2[cos(A B) cos(A B)]
mAc
mAc
s(t ) Ac (cos ct )
cos( c m )t
cos( c m )t
2
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Amplitude modulation
- 1. Amplitude 1 Modulation By: Durgesh Suthar En. No: 11115027
- 2. What is Modulation 2 Modulation In the modulation process, some characteristic of a high-frequency carrier signal (bandpass), is changed according to the instantaneous amplitude of the information (baseband) signal. Why Modulation is used Suitable for signal transmission (distance…etc) Multiple signals transmitted on the same channel Capacitive or inductive devices require high frequency AC input (carrier) to operate. Stability and noise rejection
- 3. About Modulation 3 Application Examples broadcasting of both audio and video signals. Mobile radio communications, such as cell phone. • Basic modulation types – Amplitude Modulation: changes the amplitude. – Frequency Modulation: changes the frequency. – Phase Modulation: changes the phase.
- 4. Modulation Theory 4 A sine wave is represented as follows c(t)= Ac cos(2πfct +φ(t) Here Ac, fc and φ(t) all represent parameters that can be modulated in the carrier waveform in order to carry information. The modulation schèmes are known as : Ac -> Amplitude Modulation fc -> Frequency Modulation Φ(t) -> Phase Modulation
- 5. Basic Amplitude Modulation Amplitude Modulation is the simplest and earliest form of transmitters The information signal varies the instantaneous amplitude of the carrier
- 6. Benefits of Modulation 6 Modulation can shift the spectral content of a message signal into a band which is better suited to the channel Antennas only efficiently radiate and admit signals whose wavelength is similar to their physical aperture. Hence, to transmit and receive, say, voice, by radio we need to shift the voice signal to a much higher frequency band.
- 7. 7 Modulation permits the use of multiplexing Multiplexing means allowing simultaneous communication by multiple users on the same channel. For instance, the radio frequency spectrum must be shared and modulation allows users to separate themselves into bands.
- 8. AMPLITUDE MODULATION (AM)8 In amplitude modulation, the message signal m(t) is impressed on the amplitude of the carrier signal c(t) = Accos(2fct) This results in a sinusoidal signal whose amplitude is a function of the message signal m(t) There are several different ways of amplitude modulating the carrier signal by m(t) Each results in different spectral characteristics for the transmitted signal Mainly these methods are used for AM: (a) Double Sideband with Large carrier AM (DSB-LC AM) (b) Double sideband, suppressed-carrier AM (DSB-SC AM) (c) Single-sideband AM (SSB AM) (d) Vestigial Sideband (VSB) modulation
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- 11. Full AM modulation ( DSB-LC) 11 1 The carrier signal is sc (t ) Ac cos( c t ) where c 2f c 2 In the same way, a modulating signal (information signal) can also be expressed as sm (t ) Am cos m t
- 12. 13 3 The amplitude-modulated wave can be expressed as s(t ) Ac sm (t )cos(c t ) 4 By substitution s (t ) Ac Am cos( mt )cos( c t ) 5 The modulation index. Am m Ac
- 13. 13 6 Therefore The full AM signal may be written as s(t ) Ac (1 m cos( mt )) cos( c t ) cos A cos B 1 / 2[cos(A B) cos(A B)] mAc mAc s(t ) Ac (cos ct ) cos( c m )t cos( c m )t 2 2
- 14. Double-Sideband Suppressed-Carrier AM 14 A double-sideband, suppressed-carrier (DSB-SC) AM signal is obtained by multiplying the message signal m(t) with the carrier signal c(t) = Accos(2fct) Amplitude-modulated signal u (t ) m(t )c(t ) Ac m(t ) cos(2 f c t ) An example of the message signal m(t), the carrier c(t), and the modulated signal u (t) are shown in fig in next slide. This figure shows that a relatively slowly varying message signal m(t) is changed into a rapidly varying modulated signal u(t), and due to its rapid changes with time, it contains higher frequency components At the same time, the modulated signal retains the main characteristics of the message signal; therefore, it can be used to retrieve the message signal at the receiver
- 15. Double-Sideband Suppressed-Carrier 15 AM Figure : An example of message, carrier, and DSB-SC modulated signals
- 16. 16 Single-Sideband AM The two sidebands of an AM signal are mirror images of one another As a result, one of the sidebands is redundant Using single-sideband suppressed-carrier transmission results in reduced bandwidth and therefore twice as many signals may be transmitted in the same spectrum allotment
- 17. Single-Sideband AM A method, illustrated in 17 . Figure, generates a DSB-SC AM signal and then employs a filter that selects either the upper sideband or the lower sideband of the double-sideband AM signal Figure : Generation of a singlesideband AM signal by filtering one of the sidebands of a DSB-SC AM signal.
- 18. Sideband and carrier power 18 Carrier term does not carry information, and hence the carrier power is wasted AM (t ) A cos ct m(t ) cos ct carrier sidebands Pc is the mean sq. value of A cos c t which is A2 / 2 The sideband power P is the mean sq. value s 2 of m(t ) cos c t which is m (t ) / 2 The carrier power
- 19. Advantages/disadvantages 20 Advantages of Amplitude Modulation, AM There are several advantages of amplitude modulation, and some of these reasons have meant that it is still in widespread use today: It is simple to implement it can be demodulated using a circuit consisting of very few components AM receivers are very cheap as no specialized components are needed. Disadvantages of amplitude modulation Amplitude modulation is a very basic form of modulation, and although its simplicity is one of its major advantages, other more sophisticated systems provide a number of advantages. Accordingly it is worth looking at some of the disadvantages of amplitude modulation. It is not efficient in terms of its power usage It is not efficient in terms of its use of bandwidth, requiring a bandwidth equal to twice that of the highest audio frequency It is prone to high levels of noise because most noise is amplitude based and obviously AM detectors are sensitive to it.