1. 1 st semester 1434-1435 Chapter4: Continuous-wave Modulation King Saud University College of Applied studies and Community Service 1301CT By: Nour Alhariqi 1 nalhareqi-2013

2. Communications System Model 2 nalhareqi-2013

3. Introduction to Modulation of Analog Signal The purpose of a communication system is to deliver the input message from the source in a recognizable form to the destination over the channel. The channel is either low-pass or band-pass. A low-pass channel has a bandwidth with frequencies between 0 and. A band-pass channel has a bandwidth with frequency between 1 and 2 3 nalhareqi-2013

4. Introduction Cont. The baseband signals produced by various information sources are not suitable for direct transmission over band-pass channels. A baseband signal or low frequency signal is a signal whose spectrum is positioned close to dc ( ω =0). Examples of baseband signals include speech signals whose spectrum occupies the frequency band 0 to 3.5 kHz and video signals whose spectrum occupies the frequency band 0 to 4.3 kHz. 4 nalhareqi-2013

5. Introduction Cont. To perform the transmission, the sender needs to modify the baseband signal into a form that is suitable for transmission over the channel. The adaptation of the baseband signal to the transmission channel called modulation. It can be viewed as a way to change message into suitable form for transmission. At the receiver, the received signal must pass through a reverse process called demodulation in order to reconstruct the baseband signal. 5 nalhareqi-2013

6. Modulation In general there are two main classes of modulation: continuous-wave modulation (CW) and pulse modulation. In CW modulation, the analog baseband signal is transmitted using a high frequency signal, called carrier signal. So, the modulation process involve two waveforms: the baseband signal (called a modulating signal) and the carrier signal which is sinusoid signal. 6 nalhareqi-2013

7. Modulation The modulating signal is used to change a parameter of the carrier signal ( amplitude, frequency, or phase), the modified carrier signal is called the modulated signal. A consequence of modulation is shifting the range of frequencies contained in the message signal into another frequency range suitable for transmission over the channel, centered onto the carrier frequency. 7 nalhareqi-2013

8. CW Modulation The CW modulation classified to : AM, FM, and PM When the amplitude of the carrier is varied in accordance with the modulating signal, we have Amplitude Modulation (AM). When the frequency of the carrier is varied in accordance with the modulating signal, we have Frequency Modulation (FM). When the phase of the carrier is varied in accordance with the modulating signal, we have Phase Modulation (PM). 8 nalhareqi-2013

9. CW Modulation nalhareqi-2013 9

10. Demodulation As we know, the receiver recreate the original message signal from a degraded version of the transmitted signal after propagation through the channel. This re-construction is a accomplished by using a demodulation. Which is the reverse of the modulation process used in the sender 10 nalhareqi-2013

11. Demodulation nalhareqi-2013 11 However, owing to the unavoidable presence of the noise and distortion in the received signal, we find that the receiver cannot recreate the original message signal exactly. The resulting degradation in the overall system performance is influenced by the type of modulation scheme used. Specifically, we find that some modulation schemes are less sensitive to the effects of noise and distortion than others

12. Analog Communication System There are two types of communication systems: Analog communication system and digital communication system. The CW modulation is used in the analog communication system. In this system, the transmitter consists of a modulator and the receiver consists of demodulator 12 nalhareqi-2013 modulator demodulator Baseband signal modulating signal modulated signal modulated signal carrier signal Baseband signal

13. Benefits of Modulation nalhareqi-2013 13 In addition to facilitating transmission, there are some important reasons for modulation: Ease of Radiation Multiplexing Reduce Noise and Interference

14. Ease of Radiation If the communication channel consists of free space (radio channel), then antennas are needed to radiate and receive the signal. Efficient electromagnetic radiation requires antennas whose dimensions are at least 1/10 of the wavelength of the signal being radiated. 14 nalhareqi-2013

15. Ease of Radiation nalhareqi-2013 15 Many baseband signals, including audio signals, have frequency components down to 100 Hz or lower. For these signals, antennas about 300 km long will be necessary if the signal is radiated directly. If modulation is used to impress the baseband signal on a high-frequency carrier, say at 100 MHz, then antennas need be in order of 30 meters.

16. Multiplexing Consider a case of several senders want to send at the same time. If they send the baseband signals directly, all signals at same frequencies from different senders would be mixed up. The solution of allowing one sender to send at time is wasteful because the channel’s bandwidth my be much larger than that of the signal. 16 nalhareqi-2013

17. Multiplexing nalhareqi-2013 17 One way to solve this problem is to use modulation. modulation is used to translate different signals to different frequency range by assign the carriers to different frequencies that are sufficiently far apart of each other. This way of multiplexing called the frequency division multiplexing (FDMA). At the received side, a tunable bandpass filter is used to select the desired signal

18. Reduce Noise and Interference The effect of noise and interference cannot be eliminated in a communication system. A brute-force method for combating noise and interference is to increase the signal power until it overwhelms the contaminations. But increasing power is costly and my damage equipments. 18 nalhareqi-2013

19. Reduce Noise and Interference nalhareqi-2013 19 Other way to minimize the effects of noise and interference is using modulation. certain types of modulation schemes have valuable property of suppressing both noise and interference. These schemes generally require a transmission bandwidth much larger than the bandwidth of the modulating signal.

20. Amplitude Modulation (AM) nalhareqi-2013 20 As the name suggests, in AM, the modulating signal varies the amplitude of the carrier sinusoid wave. The instantaneous value of the carrier amplitude changes in accordance with the amplitude and frequency variations of the modulating signal. The carrier frequency and phase remain constant during the modulation process, just its amplitude varies in accordance with the modulating signal.

21. Amplitude Modulation (AM) nalhareqi-2013 21

22. Types of Amplitude Modulation (AM) nalhareqi-2013 22 There are several different ways of amplitude modulating the carrier signal by the baseband signal ( modulating signal), each of which results in different spectral characteristics for the transmitted signal (modulated signal): Double-sideband suppressed carrier (DSB-SC) modulation Amplitude modulation (AM). Single-sideband modulation (SSB). Vestigial-sideband modulation (VSB). Each of these schemes has its own distinct advantages, disadvantages, and practical applications.

23. Double-sideband suppressed carrier (DSB-SC) modulation Modulation Spectrum Upper and Lower Sidebands Bandwidth Example Demodualtion nalhareqi-2013 23

24. Modulation nalhareqi-2013 24 The modulating signal (information or baseband signal) The carrier signal is The modulated signal

25. Modulation nalhareqi-2013 25

26. Modulation nalhareqi-2013 26 The DSB-SC modulated signal undergoes a phase reversal whenever the message signal m(t) cross zero. Consequently, the envelop of a DSB-SC modulated signal is different from the message signal Envelop phase reversal

27. Spectrum nalhareqi-2013 27 The process of DSB-SC modulation shifts the spectrum of the modulating signal m(t) to the left and the right by ω c ( carrier frequency). if then

28. Spectrum nalhareqi-2013 28

29. Upper and Lower Sidebands nalhareqi-2013 29 The modulated signal spectrum centered at ω c is composed of two parts: a portion that lies above ω c, known as the upper sideband (USB), and a portion that lies below ω c, known as the lower sideband (LSB). Similarly, the spectrum centered at - ω c has upper and lower sidebands. Hence, this is a modulation scheme with double sidebands

30. Upper and Lower Sidebands nalhareqi-2013 30 ω c +ω M ω c - ω M

31. Bandwidth nalhareqi-2013 31 If the bandwidth of the modulating signal is B Hz, then the bandwidth of the modulated signal is 2B Hz. 2B 2πB2πB

32. Bandwidth nalhareqi-2013 32 In order to avoid the overlap of the spectra centered at ω c and –ω c, the ω c should be ≥ ω M. If ω c < ω M, the LSB of ω c will overlap with LSB of -ω c Which make it impossible to get back the m(t) from the modulated signal m(t) cos (ω c t).

33. Example nalhareqi-2013 33 Suppose that the modulating signal m(t) is a sinusoid of the form And the carrier signal is : Determine The DSB-SC signal S DSB (t) (modulated signal) The upper and lower sidebands The bandwidth Sketch the spectrum of the m(t) and S DSB (t)

34. Solution nalhareqi-2013 34 The DSB-SC signal By using the relation The S DSB (t) is

35. Solution nalhareqi-2013 35 The upper and lower sidebands USB = 10 6 +10 3 =1001KHz LSB = 10 6 -10 3 =999KHz The bandwidth of S DSB (t) BW = 2 * 10 3 =2000 Hz = 2 KHz

36. Solution nalhareqi-2013 36 Sketch the spectrum of the m(t) and S DSB (t) M( ω ) 10 3 -10 3 S DSB ( ω ) f f 1000

37. Demodulation nalhareqi-2013 37 The DSP-SC demodulation consists of multiplication of the incoming modulated signal m(t) cos ( ω c t ) by a carrier cos ( ω c t) followed by a low pass filter. The receiver need to generate a local carrier that has the same frequency and the same phase of the carrier used for modulation. DemodulationModulation

38. Demodulation nalhareqi-2013 38 The multiplication will shift the modulated signal spectrum to the left and to the right by ω c and multiply it by one-half. Use a low pass filter to get the desired baseband spectrum and suppress the unwanted spectrum at + ω c and - ω c

39. nalhareqi-2013 39 modulated signal spectrum ωcωc -ωc-ωc 2ωc2ωc -2ω c After multiplication filter

40. Demodulation nalhareqi-2013 40 This method of recovering the baseband signal is called synchronous detection or coherent detection, because the receiver uses a carrier of exactly the same frequency and phase as the carrier used for modulation

41. Standard Amplitude Modulation (AM) nalhareqi-2013 41

42. Standard Amplitude Modulation (AM) nalhareqi-2013 42 In the DSP-SC demodulation, a receiver must generate a local carrier in frequency and phase synchronism with the carrier used for modulation. This call for a sophisticated receiver and could be quit costly. The other alternative is for the transmitter to transmit a carrier A c cos (ω c t) along with the modulated signal m(t) cos (ω c t). so there is no need to generate a carrier at the receiver and this is the idea of the standard AM.

43. Modulation nalhareqi-2013 43 The modulating signal (information or baseband signal) The carrier signal is The modulated signal

44. Modulation nalhareqi-2013 44

45. Modulation – case 1 nalhareqi-2013 45 When ( is nonnegative) for all value of t. The Envelop has the same shape as m(t). Hence, we can recover m(t) from this envelop. At the receiver, the detection is extremely simple and inexpensive operation, which doesn’t require generation of a local carrier for demodulation.

46. Modulation – case 2 nalhareqi-2013 46 The condition is not satisfied. The Envelop shape is not as m(t). So, we can’t recover m(t) from this envelop. We can’t build a simple receiver.

47. Modulation Index nalhareqi-2013 47 Let m p the peak amplitude of m(t) and A is the carrier amplitude. We define modulation index as: When : this mean that for all t and the S AM (t) (modulated signal) can be demodulated by the envelop detector. When (overmodulation): this mean that A + m(t) is not ≥ 0 for all t and the option of envelop detection is no longer viable.

48. Demodulation nalhareqi-2013 48 In the standard AM modulation, the modulation index should be in order to demodulate the received modulated signal by using an envelop detector.

49. Spectrum nalhareqi-2013 49 The spectrum of the modulated signal S AM (t) is the same as that of m(t) cos ( ω c t) plus tow additional impulses at + ω c and - ω c. If then

50. Spectrum nalhareqi-2013 50 USB LSB

51. Spectrum nalhareqi-2013 51 The modulated signal spectrum centered at ω c is composed of three parts: An impulse at the carrier frequency ω c The upper sideband (USB), a portion that lies above ω c whose highest frequency component is at ω c + ω M The lower sideband (LSB), a portion that lies below ω c whose lowest frequency component is at ω c - ω M The bandwidth of the modulated waveform is twice the information signal bandwidth.

52. Transmission Power nalhareqi-2013 52 The advantage of envelop detection in the standard AM has its price. Recall, the transmission power and the channel bandwidth are the two primary communication resources and should be used efficiently. The transmission of the carrier wave represent a waste of power. In the standard AM, only a fraction of the total transmitted power is actually for m(t).

53. Single-Sideband (SSB) Modulation nalhareqi-2013 53

54. Introduction nalhareqi-2013 54 Standard AM and DSB-SC modulations has two sidebands: LSB and USB. These two methods waste the channel bandwidth because they both require a transmission bandwidth equal to twice the message bandwidth.

55. Introduction nalhareqi-2013 55 Note that the USB and the LSB are symmetric about the carrier frequency. Hence, given the spectra of either sideband, we can determine the other. So, the transmission of either sideband is sufficient to reconstruct the message signal m(t) at the receiver. Thus, the bandwidth of the transmitted signal ( modulated signal) will be the bandwidth of the modulating signal ( baseband signal)

56. Single-Sideband (SSB) Modulation nalhareqi-2013 56 In single-sideband (SSB) modulation just only one sideband is transmitted.

57. Generation of SSB Signals nalhareqi-2013 57 One way to generate an SSB signal is to: generate a DSB signal first, and then suppress one of its sidebands by filtering ( band-pass filter designed to pass one of the sidebands of the modulated signal)

58. Generation of SSB Signals nalhareqi-2013 58 The filter must have sharp cutoff characteristics to eliminate the undesired sideband

59. Demodulation nalhareqi-2013 59 Demodulation of SSB signals can be achieved easily by using the coherent detector as used in the DSB demodulation, that is, by multiplying by a local carrier and passing the resulting signal through a low-pass filter.

60. Demodulation nalhareqi-2013 60

61. Vestigial-Sideband (VSB) modulation nalhareqi-2013 61

62. Vestigial-Sideband (VSB) modulation nalhareqi-2013 62 Vestigial-Sideband (VSB) modulation is a compromise between SSB and DSB modulations. In this modulation scheme, one sideband is passed almost completely, whereas just a trace, or vestige, of the other sideband is retained. The typical bandwidth required to transmit a VSB signal is about 1.25 that of SSB. VSB is used for transmission of the video signal in commercial television broadcasting.

63. Generation of VSB Signals nalhareqi-2013 63 A VSB signal can be generated by passing a DSB signal through a sideband-shaping filter [VSB filter].

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65. Demodulation of VSB nalhareqi-2013 65 The m(t) can be recovered by synchronous or coherent demodulation, that is, by multiplying by a local carrier and passing the resulting signal through a low-pass filter.

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67. ANGLE MODULATION nalhareqi-2013 67

68. Introduction nalhareqi-2013 68 Another class of modulation methods are frequency and phase modulation which referred to as angle- modulation methods. In frequency-modulation (FM), the frequency of a carrier wave is changed by the message signal. In phase-modulation (PM), the phase of the carrier is changed according to the variations in the message signal.

69. Introduction nalhareqi-2013 69 For all AM modulation schemes Modulated spectrum is the frequency translated message spectrum Transmission BW never exceeds twice the message BW In Angle modulation „ Modulated spectrum is not a translated copy of the message spectrum. „Transmission BW is usually much greater than twice the message BW. The major benefit of the FM and PM modulation is their high degree of noise immunity.

70. Introduction nalhareqi-2013 70 Consider a sinusoid, A c cos ( ω c t+ φ ) where A c is the (constant) amplitude, ω c is the (constant) frequency and φ is the initial phase. In the AM modulation, the condition that A c be a constant is relaxing and the amplitude become a function of the message signal m(t). the frequency and the phase remain constant and don’t change or effect by the m(t).

71. Introduction nalhareqi-2013 71 In the FM and PM modulation, A c is a constant but ω c t+ φ, instead of being constants it will be a function of m(t). We must extend the concept of a sinusoid to a generalized function whose frequency vary with time

72. Generalized Sinusoidal Signal nalhareqi-2013 72 Let us consider a generalized sinusoidal signal given as A c cos ( θ (t) ) where the θ (t) is the instantaneous angle and is a function of t. The generalized angle for the conventional sinusoid A c cos ( ω c t+ φ ) is θ (t) = ω c t+ φ

73. Angle Modulated signal nalhareqi-2013 73 So for angle modulation, the modulated carrier represented by S angle_mod (t) = A c cos ( θ (t) ) where A c is a constant amplitude and θ (t) is a function of the message signal m(t). We define the instantaneous radian frequency of the angle modulated wave ω i (t) as:

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75. Phase Modulation (PM) nalhareqi-2013 75 In phase modulation the angle is varied linearly with the message signal m(t) as θ (t) = ω c t + k p m(t) where k p is the phase deviation or sensitivity constant. Thus the phase modulated signal is defined as: S PM (t) = A c cos ( ω c t + k p m(t) ) The instantaneous radian frequency of S PM (t) is

76. Example nalhareqi-2013 76 If the message signal m(t) = a cos ( ω m t) is used to phase modulate the carrier A c cos ( ω c t) Find the PM modulated signal S PM (t) = A c cos ( ω c t + a k p cos ( ω m t) )

77. Frequency Modulation (FM) nalhareqi-2013 77 In frequency modulation the angle is varied linearly with the integral of message signal m(t) as where k f is the frequency deviation or sensitivity constant. Thus the frequency modulated signal is defined as: The instantaneous radian frequency of S FM (t) is

78. Example nalhareqi-2013 78 If the message signal m(t) = a cos ( ω m t) is used to phase modulate the carrier A c cos ( ω c t) Find the FM modulated signal

79. Example nalhareqi-2013 79 In a frequency modulation process m(t) be a periodic triangular wave with m max (t) =1 and m min (t)=-1 the carrier frequency is 100 kHz k f = 10 4 Hz/volt find the maximum and minimum values of the instantaneous frequency f i_max (t) = 100 *10 3 + 10 4 =110 kHz f i_min (t) = 100 *10 3 - 10 4 =90 kHz

80. The Relationship Between FM and PM nalhareqi-2013 80 There is a close relation between FM and PM modulations. An FM modulated wave can be generated by first integrating the message signal m(t) with respect to time t and thus using the resulting signal as the input to a phase modulation. A PM modulated wave can be generated by first differentiating m(t) with respect to time t and then using the resulting signal as the input to a frequency modulator

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82. Bandwidth nalhareqi-2013 82 The FM modulated wave is not band-limited. It has an infinite bandwidth and is not related to the modulating signal spectrum in any simple way, as was the case in AM modulation. Although the theoretical bandwidth of an FM wave is infinite, the most of the modulated signal power resides in a finite bandwidth. There are two distinct possibilities in terms of bandwidth: narrow-band FM and wide-band FM.

83. Single-Tone Frequency Modulation nalhareqi-2013 83 Consider a sinusoidal modulating signal defined as m(t) = A m cos( 2 π f m t) So, the instantaneous frequency (in Hertz) of the FM signal is f i (t) =f c + k f A m cos( 2 π f m t) = f c + ∆f cos( 2 π f m t) where ∆f is called the frequency deviation given by ∆f =k f A m The resultant FM signal is is the modulation index

84. Single-Tone Frequency Modulation nalhareqi-2013 84 The frequency deviation factor indicates the amount of frequency change in the FM signal from the carrier frequency f c on either side of it. Thus FM signal will have the frequency components between (fc - ∆f ) to (fc +∆f ). The modulation index, β represents the phase deviation of the FM signal and is measured in radians. Depending on the value of β, FM signal can be classified into two types: 1. Narrow band FM ( β << 1) and 2. Wide band FM ( β >> 1).

85. nalhareqi-2013 85