Amplitude modulation


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Amplitude modulation

  1. 1. 2009-CPE-03UCE&T BZU MULTAN
  2. 2. Amplitude Modulation
  3. 3. What is Modulation 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 CSULB May 22, 2006 3
  4. 4. About Modulation 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. CSULB May 22, 2006 4
  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. AMPLITUDE MODULATION (AM) 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  We will describe these methods, which are called (a) Double sideband, suppressed-carrier AM (DSB-SC AM) (b) Single-sideband AM (SSB AM) Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: 6
  7. 7. Amplitude Modulation The condition for envelope detection of the AM signal for all t [ A  m(t )]  0 If m(t )  0 and A=0 also satisfy the above condition Let be the peak amplitude of m p (t ) m(t ) m(t   m p (t ) This condition)is equivalent to A  m p (t ) The min. carrier amplitude required for envelope detection is m p (t )
  8. 8. Modulation index m (t ) / A  p The modulation index 0   1
  9. 9. Modulation Index of AM Signal CSULB May 22, 2006 10
  10. 10. Modulation Index of AM Signal CSULB May 22, 2006 11
  11. 11. Double-Sideband Suppressed-Carrier AM 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 Figure 3.1  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 Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: 12
  12. 12. Double-Sideband Suppressed-Carrier AM  Figure 3.1 An example of message, carrier, and DSB-SC modulated signals Oh-Jin Kwon, EE dept., Sejong Univ., Seoul, Korea: 13
  13. 13. 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 Typically, a 3dB improvement in signal-to-noise ratio is achieved as a result of SSBSC
  14. 14. Single-Sideband AM . A method, illustrated in Figure 3.16, 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 Figure 3.16 Generation of a single- signal sideband AM signal by filtering one of the sidebands of a DSB-SC AM signal. ., 15
  15. 15. Sideband and carrier power Carrier term does not carry information, and hence the carrier power is wasted  AM (t )  A cos ct  m(t ) cos ct  carrier  sidebands The carrier power Pcis the mean sq. value of A cos c twhich is A2 / 2 The sideband power Ps is the mean sq. value of m(t ) cos c t which is m 2 (t ) / 2
  16. 16. Power Efficiency The power efficiency m2 (t ) Ps   100% Pc  Ps A  m (t ) 2 2 For the special case of tone modulation m(t )  A cos mt m (t )  A / 2 2 2 Hence Ps   2 A2 / 2 100%   2 100% Pc  Ps A  A / 2 2 2 2   1, m ax  33 %
  17. 17. Quadrature AM Two carriers generated at the same frequency but 90º out of phase with each other allow transmission of two separate signals This approach is known as Quadrature AM (QUAM or QAM) Recovery of the two signals is accomplished by synchronous detection by two balanced modulators
  18. 18. Quadrature Operation
  19. 19. Advantages/disadvantagesAdvantages of Amplitude Modulation, AMThere 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 modulationAmplitude 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.