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Communication Systems

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46 diapositives · 693 Views · Oct 12, 2014

Communication Systems. Prof. Chungming Kuo. Chapter 6. Double Sideband and Single Sideband (cont.). Double Sideband and Single Sideband. This module will provide an introduction to amplitude modulation by considering double sideband ( DSB ) and single sideband ( SSB ).

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  1. Communication Systems Prof. Chungming Kuo

  2. Chapter 6 Double Sideband and Single Sideband (cont.)

  3. Double Sideband and Single Sideband • This module will provide an introduction to amplitude modulation by considering doublesideband (DSB) and single sideband (SSB). • Historically, these were not the earliest forms of amplitude modulation employed on a wide scale, but they relate very closely to concepts developed in the module frequency conversion.

  4. Double Sideband and Single Sideband (cont.) • Hence, they will be covered before conventional amplitude modulation with a large carrier is introduced. The commercial AM broadcast system employs the latter system.

  5. Amplitude Modulation Forms • Conventional Amplitude Modulation (Sometimes called AM-LC for AM with “large carrier.” When we use AM without any modifier, it will be assumed to mean conventional AM.)

  6. Amplitude Modulation Forms (cont.) • Double Sideband (DSB) (Sometimes called DSB-SC, with SC representing “suppressedcarrier.” We will refer to it simply as DSB.) • Single Sideband (SSB) • Vestigial Sideband (VSB)

  7. Essential Trigonometric Identities

  8. Essential Trigonometric Identities (cont.)

  9. Notation

  10. Continuous Spectrum Signal

  11. Discrete Spectrum Signal

  12. Balanced Modulator

  13. Balanced Modulator EquationsContinuous Spectrum

  14. Balanced Modulator EquationsContinuous Spectrum (cont.)

  15. Balanced Modulator EquationsDiscrete Spectrum

  16. Balanced Modulator EquationsDiscrete Spectrum (cont.)

  17. Balanced Modulator EquationsDiscrete Spectrum (cont.)

  18. Example 1 • Lowest frequency is 1 MHz-15 kHz = 985 kHz • A continuous-spectrum signal has components from near dc to 15 kHz and carrier is 1 MHz. Find range of DSB frequencies and bandwidth:

  19. Example 1 (cont.) Highest frequency is 1 MHz+15 kHz = 1015 kHz • A continuous-spectrum signal has components from near dc to 15 kHz and carrier is 1 MHz. Find range of DSB frequencies and bandwidth:

  20. Example 2 LSB: 250 - 1 = 249 kHz 250 - 3 = 247 kHz 250 - 5 = 245 kHz • A discrete-spectrum signal has components at 1, 3, and 5 kHz, and carrier has a frequency of 250 kHz. List DSB frequencies and find bandwidth:

  21. Example 2 (cont.) USB: 250 + 1 = 251 kHz 250 + 3 = 253 kHz 250 + 5 = 255 kHz • A discrete-spectrum signal has components at 1, 3, and 5 kHz, and carrier has a frequency of 250 kHz. List DSB frequencies and find bandwidth:

  22. Single Sideband (SSB) • With SSB, only one of the two sidebands is transmitted. It may be the lower sideband (LSB) or the uppersideband (USB). The transmission bandwidth is: • The most common method for generating SSB is the filter method illustrated on next two slides.

  23. A DSB signal is first generated

  24. Spectral Plots for USB

  25. Spectral Plot for LSB

  26. SSB Filter Method Generator

  27. SSB Equations for Discrete-Spectrum

  28. Example 3 LSB: 985 kHz to 1 MHz USB: 1 MHz to 1.015 MHz • For system of Example 1, determine range of SSB frequencies and bandwidth for LSB and USB:

  29. Example 4 LSB: 249 kHz 247 kHz 245 kHz • For system of Example 2, list SSB frequencies and determine bandwidth for LSB and USB.

  30. Example 4 (cont.) USB: 251 kHz 253 kHz 255 kHz • For system of Example 2, list SSB frequencies and determine bandwidth for LSB and USB.

  31. Product Detection of DSB and SSB

  32. DSB Product Detection Analysis

  33. DSB Product Detection Analysis (cont.)

  34. DSB Detection Spectral Plots

  35. SSB Product Detection Analysis

  36. SSB Product Detection Analysis

  37. SSB Product Detection Analysis (cont.)

  38. SSB Detection Spectral Plots

  39. Effects of Non-Synchronization • The preceding analysis has assumed that the carrier at the receiver is locked in frequency and phase with that at the transmitter. • This condition is referred to as synchronous product detection.Next, assume the following form for the receiver carrier:

  40. Results of Mathematical Analysis • A rather detailed analysis at the receiver now yields the results below. Recall that the DSB signal was assumed as a continuous spectrum signal while the SSB signal was assumed as a discrete spectrum signal.

  41. Comments • Both signals are distorted but effects on DSB are more serious. • DSB is useful in the following situations: • Systems in which a small pilot carrier is transmitted.

  42. Comments (cont.) • DSB is useful in the following situations: • Certain complex signal processing schemes that can extract a coherent reference. • Automatic control systems of the “ac-carrier” types where reference carrier is available.

  43. Summary • The instantaneous product of a baseband signal and a carrier yields a DSB signal. • If one of the sidebands is eliminated, an SSB signal is generated. • For a baseband bandwidth W, the bandwidth of a DSB signal is 2W, and the bandwidth of an SSB signal is W.

  44. Summary (cont.) • Theoretically, both DSB and SSB signals can be demodulated by product detection. • In practice, DSB requires an exact synchronized reference while tolerable detection can be achieved with SSB without exact synchronization.