1. © 2010 The McGraw-Hill Companies Communication Systems, 5e Chapter 4: Linear CW Modulation A. Bruce Carlson Paul B. Crilly (W.J. Song and J. H. Cho)

2. Why modulation? 1. Antenna size 2. Fractional Bandwidth < 10% 3. Wideband noise reduction 4. Freq. Assignment 5. Multiplexing © 2010 The McGraw-Hill Companies

3. Chapter 4: Linear CW Modulation  Bandpass signals and systems  Double-sideband amplitude modulation (DSB)  Modulation and transmitters  Single-sideband (SSB) amplitude modulation  Frequency conversion and demodulation  cf. Nonlinear modulation in Ch. 5

4. © 2010 The McGraw-Hill Companies Bandpass signals and systems

5. © 2010 The McGraw-Hill Companies Bandpass signal (a) Spectrum; Waveform

6. © 2010 The McGraw-Hill Companies Bandpass signals

7. © 2010 The McGraw-Hill Companies (a)Rotating phasor; (b) Phasor diagram with rotation suppressed Figure 4.1-3

8. © 2010 The McGraw-Hill Companies Quadrature-carrier representation:

9. © 2010 The McGraw-Hill Companies Envelope-phase representation

10. © 2010 The McGraw-Hill Companies Modulation to create a BP signal Modulation is the translation of a band limited LP signal to a band pass signal with some center frequency message spectrum bandpass spectrum

11. © 2010 The McGraw-Hill Companies Analog message conventions

12. © 2010 The McGraw-Hill Companies Bandwidth  Message bandwidth:  Transmission bandwidth:

13. © 2010 The McGraw-Hill Companies Fractional bandwidth (a) Relevant to band pass signals (c) For practical systems we require

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16. How is bandwidth defined?  Absolute bandwidth: 100% of energy is within some frequency range  3 dB, or half power bandwidth: frequency range where magnitude reduction is less than -3 dB  Noise equivalent bandwidth (see chapter 9)  Occupied bandwidth: FCC definition where frequency range that contains 99% of energy  Relative power bandwidth: frequency range where magnitude rolloff is less than a given level of dB (e.g. -40 dB)

17. © 2010 The McGraw-Hill Companies Types of linear CW modulation  Conventional/Standard Amplitude Modulation (AM)  Suppressed carrier double sideband (SC-DSB, DSB-SC,or DSB)  Single sideband (USSB and LSSB)  Vestigial sideband (VSB) Vestigial 퇴화한, 흔적으로 남아 있는

18. © 2010 The McGraw-Hill Companies AM, DSB, SSB, and VSB are subclasses of amplitude modulation because the message alters the carrier’s amplitude. However, ‘AM’ quite often means conventional/standard AM = DSB-WC.

19. © 2010 The McGraw-Hill Companies 4.2 Double-sideband amplitude modulation  Conventional AM  Suppressed carrier DSB, (SCDSB) or simply DSB

20. © 2010 The McGraw-Hill Companies Conventional AM, or simply AM Overmodulation: phase reversal & envelop distortion bandwidth for (Baseband cut-off Freq.)

21. © 2010 The McGraw-Hill Companies AM waveforms (a) Message; (b) AM wave with  1 (overmodulation)

22. © 2010 The McGraw-Hill Companies Spectrum of AM signals

23. © 2010 The McGraw-Hill Companies AM Spectrum Note: (a) carrier impulse at (b) redundant sidebands: the lower and upper sidebands carry the same information  increased bandwidth

24. © 2010 The McGraw-Hill Companies AM Power 4

25. © 2010 The McGraw-Hill Companies AM Systems  Relatively simple transmitter and receiver hardware  The first voice modulation system  More than half power goes into carrier, but carrier carries no information  inefficient  Not suited for messages with low frequency content

26. © 2010 The McGraw-Hill Companies Suppressed Carrier DSB (DSB- SC) Signals

27. © 2010 The McGraw-Hill Companies DSB waveforms

28. © 2010 The McGraw-Hill Companies DSB systems  Suppressed carrier: typically -40→ -60 dB  All power goes into sidebands  more efficient than AM  Demodulation is more complicated than AM; requires synchronization  Transmitter hardware is more complex than AM.  Well suited to transmission of messages with low frequency or DC content

29. © 2010 The McGraw-Hill Companies 4.3 Modulation and transmitters  AM (DSB-WC)  DSB (DSB-SC)

30. © 2010 The McGraw-Hill Companies AM transmitters  Implemented using a nonlinear element or some nonlinear portion of a circuit.  Often done where the message is superimposed on one of the active device’s terminals (i.e. the base/gate or collector/drain)  Because of nonlinear elements, may require a tank circuit to remove other of band components

31. Multiplication ? © 2010 The McGraw-Hill Companies filter Nonlinear Device

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34. AM modulator example (a) the concept, (b) practical circuit. Note in (b) how the message and carrier source are superimposed onto the gate circuitry. tank circuit

35. © 2010 The McGraw-Hill Companies DSB transmitters  Due to technological limitations, practical DSB systems are rarely implemented via ordinary multiplers.  Balanced modulators  Ring modulators  Other nonlinear devices

36. © 2010 The McGraw-Hill Companies Balanced modulator concept for DSB generation * *Only for purposes of illustrating the balanced modulator concept. Practical DSB modulators are implemented using nonlinear devices such as diode arrays

37. © 2010 The McGraw-Hill Companies Ring modulator for DSB generation

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39. 4.4 Single-sideband (SSB) modulation  If we have a DSB signal with symmetrical sidebands, we can suppress one of the sidebands without loss of information.  Therefore, we reduce transmission bandwidth from  Double the number of users on a channel

40. © 2010 The McGraw-Hill Companies Types of SSB  Lower sideband: LSSB or LSB  Upper sideband: USSB or USB  The decision to choose one over the other is dictated by: Convention or prior assignment Technological considerations  Neither USSB or LSSB is inherently better than the other

41. © 2010 The McGraw-Hill Companies SSB signals

42. © 2010 The McGraw-Hill Companies SSB spectra (a) Generation of SSB from DSB using filter method, (b) USSB, (c) LSSB

43. © 2010 The McGraw-Hill Companies SSB generation  Filter method: use a high-Q filter to suppress one of the sidebands.  Phase methods: shift sidebands using a phase shift method to cancel one of them out Phase shift Weavers, weaver modulator

44. © 2010 The McGraw-Hill Companies Weaver’s SSB modulator

45. © 2010 The McGraw-Hill Companies VSB  SSB method but with a trace of the other sideband left  Practical SSB systems with imperfect filters are VSB  VSB allows for messages with low frequency or DC content

46. © 2010 The McGraw-Hill Companies 4.5 Frequency conversion and demodulation  Modulation: translate message to some carrier frequency  Frequency translation: move a signal from one carrier frequency to another  Demodulation: move modulated signal back to baseband Synchronous or product detectors: phase recovery Envelope detectors : no phase recovery

47. © 2010 The McGraw-Hill Companies Basic hetrodyne frequency converter

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49. Frequency conversion via hetrodyning (multiplication) We use a filter to select a particular component cf. homodyne receiver, super heterodyne receiver

50. © 2010 The McGraw-Hill Companies Frequency translation example Q. Convert 7 MHz USSB signal into a 50 MHz LSSB signal via hetrodyning.

51. © 2010 The McGraw-Hill Companies Synchronous detection example LPF Demodulation by translation to baseband

52. © 2010 The McGraw-Hill Companies LPF Synchronous detection when there is an error in the local oscillator (LO)

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56. Mandatory that the local oscillator and its phase match the carrier frequency Synchronization error continued

57. © 2010 The McGraw-Hill Companies Envelope detection  Suitable for AM signals or signals with a carrier  Does not require synchronization  Simple hardware: diode, resistor, capacitor  Will work with suppressed carrier modulation systems if the receiver inserts a carrier

58. © 2010 The McGraw-Hill Companies Envelope detection (a) Circuit; (b) Waveforms

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60. Envelope

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65. VSB+C

66. 용어정리  AM Verbatimly, it can be any amplitude modulation scheme. However, by convention, it means DSB-WC (with carrier), a subclass of amplitude modulation Often called ‘standard AM’.  DSB It is an amplitude modulation. DSB-SC (suppressed carrier) © 2010 The McGraw-Hill Companies