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# Amplitude modulation

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### Amplitude modulation

1. 1. Amplitude 1 Modulation By: Durgesh Suthar En. No: 11115027
2. 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. 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. 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. 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. 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. 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. 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(2fct)  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. 11. Full AM modulation ( DSB-LC) 11 1 The carrier signal is sc (t )  Ac cos( c t ) where  c  2f c 2 In the same way, a modulating signal (information signal) can also be expressed as sm (t )  Am cos  m t
12. 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. 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. 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(2fct)  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. 15. Double-Sideband Suppressed-Carrier 15 AM  Figure : An example of message, carrier, and DSB-SC modulated signals
16. 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. 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. 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. 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.