Discus about amplitude modulation and demodulation, Baseband and carrier communication, the baseband signal, double sideband suppressed carrier amplitude modulation(DSB-SC)

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2. Amplitude Modulation
Ref.: Lathi – Chap. 4 (4.1-4.5)
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3. Baseband and Carrier Communication
 Modulation is a process that causes a shift in the
range of frequencies in a signal.
 Baseband Communications are communication
systems that do not use modulation.
 Carrier Communication are communication
systems that use modulation.
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4. Baseband Signals
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5. Baseband/Carrier Communication
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6. Amplitude Modulation
 Amplitude modulation is a type of carrier
communication
 Types of amplitude modulation:
1. DSB-SC
2. AM (DSB+C)
3. SSB-SC
4. SSB+C
5. QAM
6. VSB
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7. DSB-SC
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8. Double-sideband Suppressed Carrier
Amplitude Modulation (DSB-SC)
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9. EEE 309 Apratim Roy 9
DSB-SC

10. DSB-SC (Why DSB? Why SC?)
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11. DSB-SC Demodulation
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12. EEE 309 Apratim Roy 12
DSB-SC Demodulation (Coherent
Detection)

13. Difference between Modulator & Demodulator
 Difference between modulator and demodulator lies in input
signal and output filter.
 For modulator, input is message signal and output of
multiplier is passed through a band pass filter (centered at fc).
 For demodulator, input is DSB-SC signal and output of
multiplier is passed through a low pass filter.
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14. Example of DSB-SC Signal
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15. Example of DSB-SC Signal
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16. AM Modulators
 Multiplier Modulators
 Non-linear Modulators
 Switching Modulators
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17. Multiplier Modulators
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18. Non-linear Modulators
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19. Non-linear Modulators
 This is known as a balanced modulator (single balanced modulator)
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20. Switching Modulators
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21. Switching Modulators
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22. Switching Modulators
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23. Switching Modulators
 The lesson of the example is:
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24. Diode-bridge Switch (Switching Modulator)
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25. Ring Modulator (Switching Modulator)
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26. Ring (Switching) Modulator
 Here, wo(t)=2w(t)-1
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27. Home Task
 Try examples 4.2 and 4.3
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28. Amplitude Modulation (AM)
 Problem with DSB-SC Modulation :
 Alternative of DSB-SC :
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29. AM (DSB+C)
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30. Amplitude Modulation (AM)
 Alternative of DSB-SC = AM
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31. Amplitude Modulation (AM)
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32. Amplitude Modulation (AM)
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33. EEE 309 Apratim Roy 33
Envelope Detection for AM

34. Envelope Detection for AM
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35. Envelope Detection for AM
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36. Home Task
 Try example 4.4
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37. Problem with Envelope Detection
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38. Problem with Envelope Detection=Low Power
Efficiency
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39. Problem with Envelope Detection=Low Power
Efficiency
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40. Demodulation of AM Signals
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41. Rectifier Detector
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42. Envelope Detector
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43. Limitations of DSB-SC and AM
 Both DSB-SC and AM have two sidebands (LSB/USB).
 So the modulated bandwidth is 2B Hz when the
message bandwidth is B Hz. This leads to lower
spectral efficiency.
 There are two ways to improve spectral efficiency:
1. SSB
2. QAM
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44. Bandwidth Efficient AM (SSB and QAM)
 SSB (single sideband AM) removes LSB or USB
and uses a bandwidth of B instead of 2B Hz for one
message.
 QAM (quadrature AM) sends two (instead of one)
messages over the same bandwidth of 2B Hz.
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45. SSB
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46. SSB-SC AM
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47. SSB-SC AM
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48. SSB-SC AM
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49. Example of SSB-SC AM
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50. SSB Modulation
 Three methods:
1. Phase shifting
2. Selective filtering
3. Weaver method
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51. Selective Filtering (SSB Modulation)
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52. Phase-shift method (SSB Modulation)
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53. Phase-shift method (SSB Modulation)
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54. Hilbert Transform
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55. SSB-SC Demodulation
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56. Home Task
 Study the detection of
SSB with a carrier (SSB+C)
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57. QAM
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58. Quadrature Amplitude Modulation (QAM)
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59. Quadrature Amplitude Modulation (QAM)
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60. Limitation of QAM
 A
Therefore, QAM is susceptible to cross talk or co-channel interference.
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61. VSB
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62. Vestigial Sideband Amplitude
Modulation (VSB-AM)
 Generation of SSB signals is difficult (Limitation of SSB).
 Generation of DSB signals is simpler but requires twice the
bandwidth (Limitation of DSB).
 Compromise between SSB and DSB:  VSB.
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63. VSB
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64. VSB
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65. VSB
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66. Example of VSB Output Filter
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67. VSB in Broadcast TV
 TV baseband signal has B=4.5 MHz.
 For DSB, modulated bandwidth 2B will be 9 MHz (too large).
 For SSB, modulated bandwidth will be 4.5 MHz (but SSB generation
is difficult and expensive for TV signal).
 VSB for TV signal has modulated bandwidth of 6 MHz (<9 but >4.5).
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68. VSB in Broadcast TV
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69. Amplitude Modulation
(AM)
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