UNIT I AMPLITUDE MODULATION

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1 UNIT I AMPLITUDE MODULATION Prepared by: S.NANDHINI, Assistant Professor, Dept. of ECE, Sri Venkateswara College of Engineering, Sriperumbudur, Tamilnadu.

2 CONTENTS Introduction to communication systems Modulation Amplitude Modulation Generation and Detection of Amplitude modulation Single sideband Vestigial sideband Comparison Hilbert Transform Pre envelope Complex envelope Superhetrodyne Reciever

3 INTRODUCTION TO COMMUNICATION SYSTEM

4 INTRODUCTION TO COMMUNICATION SYSTEMS What is Communication? Communication is the process of conveying information from one point to another What is the purpose of Communication system? The purpose of communication systems is to communicate information bearing signals from a source located at one point in space to a user destination located at another point. The three most common sources of information are: speech (or sound), video and data. Regardless of the source, the information that is transmitted and received in a communication system consists of a signal, encoding the information in some appropriate fashion

5 . MODEL OF COMMUNICATION SYSTEMS

6 INTRODUCTION TO COMMUNICATION The purpose of communication systems is to communicate information Figure depicts the general layout of a communication : An input transducer (e.g., a microphone) converts the input message into a message signal (e.g., a time varying voltage) that is transmitted over a channel by means of a transmitter which performs a very important function on communication signals by encoding the signals in some fashion making use of a carrier signal, and converted by a receiver into an output signal. An output transducer (e.g., a loudspeaker) converts the received signal into an output message (e.g.: sound).the information is contained in a socalled modulating signal that modulates a carrier signal. For example, in FM radio the modulating signal consists of speech and music, and the carrier is a sinusoidal wave of pre-determined frequency, much higher than the modulating signal frequency.

7 NOISE Noise can be defined as an unwanted signals that tends to disturb the transmission and processing of signals in communication system NOISE EXTERNAL NOISE INTERNAL NOISE Atmospheric Noise Manmade Noise Shot Noise White Noise Galactic Noise Thermal Noise

8 INFORMATION THEORY It provides quantitative measure of the information contained in message signal and allows us to determine the capacity of a communication system to transfer the information from source to destination The information theory is used for mathematical modelling and and analysis of the communication system

9 TYPES OF COMMUNICATION SYSTEM COMM SYSTEM ANALOG COMM HYBRID COMM DIGITAL COMM Analog Communication System: Analog communication is that types of communication in which the message or information signal i.e transmitted is analog in nature. This means that in analog communication the modulating signal (i.e base-band signal) is an analog signal. This analog message signal may be obtained from sources such as speech, video shooting etc. Analog signals are continuous in both time and value. Analog signals are used in many systems, although the use of analog signals has declined with the advent of cheap digital signals. All natural signals are Analog in nature.

10 TYPES OF COMMUNICATION CONT Digital Communication System In digital communication, the message signal to be transmitted is digital in nature. This means that digital communication involves the transmission of information in digital form. Digital signals are discrete in time and value. Digital signals are signals that are represented by binary numbers, "1" or "0". The 1 and 0 values can correspond to different discrete voltage values, and any signal that doesn't quite fit into the scheme just gets rounded off. Digital signals are sampled, quantized & encoded version of continuous time signals which they represent. In addition, some techniques also make the signal undergo encryption to make the system more tolerent to the channel. Advantage of digital communication over analog communication system Increased immunity to channel noise and external interference. Flexible operation of the system. A common format for the transmission of different kinds of message signal (e.g voice signal, video signal, computer data ). Improved security of communication through the use of encryption.

FREQUENCY RANGE In FM radio the modulating signal consists of speech and music, and the carrier is a sinusoidal wave of pre-determined frequency,

11 FREQUENCY RANGE In FM radio the modulating signal consists of speech and music, and the carrier is a sinusoidal wave of pre-determined frequency, much higher than the modulating signal frequency. Table summarized the frequency band allocation and typical applications in each frequency band.

12 MAJOR CLASSIFICATION OF COMMUNICATION SYSTEM BASED ON PHYSICAL STRUCTURE LINE COMMUNICATION RADIO COMMUNICATION

13 MAJOR CLASSIFICATION OF COMMUNICATION SYSTEM BASED ON THE SIGNAL SPECIFICATIONS Nature of the baseband or information signal. Nature of the transmitted signal. BASED ON THE NATURE OF THE BASEBAND SIGNAL Analog communication systems Digital communication systems BASED ON THE NATURE OF TRANSMITTED SIGNAL: The baseband signal can either be transmitted as it is without modulation, or through a carrier signal with modulation. The two systems can then be categorized as: Baseband communication system Carrier communication system Therefore, the four types of communication system categorized based on signal specification are: Analog communication systems Digital communication systems Baseband communication systems Carrier communication systems

14 MODULATION

15 What is modulation? MODULATION Modulation is performed at the transmitting end of the communication system. At the receiving end of the system we usually require the original baseband signal to be restored, this is usually accomplished by using a process known as demodulation which is the reverse process of the modulation In basic signal processing terms, we thus find that the transmitter of an analog communication system consists of a modulator and the receiver consists of a demodulator as

16 What are the reasons for modulation? If modulation is not employed however, the system designer could confront the following problems: Antenna Height Narrow Banding Poor radiation and penetration Diffraction angle Multiplexing To overcome equipment limitations To reduce noise and interferences

17 What are the Different Modulation Methods?

18 AMPLITUDE MODULATION

19 AMPLITUDE MODULATION What is Amplitude modulation? Define Amplitude modulation? A Sinusoidal carrier wave c(t) is given as Let m(t) denote the baseband signal that carries the specification of the message The source of carrier wave c(t) is physically independent of the source responsible for generating m(t) Amplitude modulation is defined as a process in which the amplitude of the carrier wave c(t) is varied about a mean value, linearly with the baseband signal m(t). An amplitude modulated wave may thus be described in its most general form as a function of time as follows

20 AMPLITUDE MODULATION From the above eqn, we find that the fourier transform of the AM wave s(t) is given by Spectrum of baseband signal Spectrum of AM wave

21 BASEBAND SIGNAL CARRIER SIGNAL & AMPLITUDE MODULATED WAVE

22 OVER MODULATION AND UNDER MODULATION. m(t) AM MODULATED WAVE s(t) s(t) Phase reversal

23 EQUATION OF AN AM WAVE The instantaneous value of modulating signal and carrier signal can be represented as given below

24 Equation of an AM Wave cont Instantaneous value of amplitude modulated signal Using the above mathematical expression for modulating and carrier signals, we can create a new mathematical expression for the complete modulated wave, as given below The instantaneous value of the amplitude modulated wave is given as

25 CALCULTION OF PERCENTAGE OF MODULATION Looking at the figure we can visualize that something unusual (distortion) will occur if Em is greater than Ec Therefore the modulating signal voltage Em must be less than the carrier voltage Ec for proper amplitude modulation This relationship between the amplitude of the modulating and carrier signals is important and it is expressed in terms of their ratio, commonly known as modulation index (m) It is also called modulation factor, modulation coefficient or the degree of modulation The m is the ratio of the modulating signal voltage to the carrier voltage The modulation index is a number lying between 0 and 1 and it is very often expressed as a percentage and called the percentage modulation

26 DEGREE OF MODULATION Calculating the modulation index using AM wave We know that with this relation we can calculate the modulation index from the modulated waveform. Hence we can write By substituting 1 st eqn in 2 nd eqn we get By diving 1 st and 3 rd eqn we get

27 FREQUENCY SPECTRUM AND BANDWIDTH OF A.M WAVE The modulated carrier has new signals at different frequencies, called side frequencies or sidebands which occur in the frequency spectrum directly above and below the carrier frequency The expression for the instantaneous value of the amplitude modulated wave Substituting the value in eqn we get Eqn 3 can be further expanded by means of the trignometric relation

28 FREQUENCY SPECTRUM AND BANDWIDTH OF A.M WAVE Looking at eqn 4 we can say that 1 st term represents unmodulated carrier and two additional terms represents two sidebands The frequency of the lower sideband (LSB) is fc fm and the frequency of the upper sideband (USB) is fc+fm BANDWIDTH OF AM WAVE We know bandwidth can be measured by subtracting lowest frequency of the signal from highest frequency of the signal For amplitude modulated wave it is given by Therefore the bandwidth required for the amplitude modulation is twice the frequency of the modulating signal

29 POWER DISTRIBUTION IN AN A.M WAVE We have seen that AM wave has three components : Unmodulated carrier Lower sideband Upper sideband Therefore the total power of AM wave is the sum of the carrier power Pc and Power in the two sidebands Pusb and Plsb. It is given as Where all three voltage represents r.m.s values and resistance R is a characteristic impedance of antenna in which the power is dissipated

30 POWER DISTRIBUTION IN AN A.M WAVE CONT... Carrier power Power in sideband Total power Modulation index in terms of Pc and Ptotal Transmission efficiency

31 VIRTUES AND LIMITATIONS OF AM MODULATION Virtue: Its greatest virtue is its simplicity of implementation What happens in the transmitter side? What happens in the receiver side? Limitations Amplitude modulation is wasteful in power Amplitude modulation is wasteful in bandwidth How to overcome these limitations?

32 DSB-FC

33 GENERATION OF AMPLITUDE MODULATION Lets describe two devices for the generation of AM waves namely Square law modulator Switching modulator Both of which will require the use of non linear element for their implementation. These two devices are well suited for low power modulation process

34 GENERATION OF DSB-FC AM SQUARE LAW MODULATOR: A Square-law modulator requires three features: a means of summing the carrier and modulating waves, a nonlinear element, and a band pass filter for extracting the desired modulation products. Semi-conductor diodes and transistors are the most common nonlinear devices used for implementing square law modulators.

35 EQUATION OF SQUARE LAW MODULATOR The transfer characteristic of the diode load transistor combination can be represented closely by a square law The input voltage v1(t) consists of the carrier wave plus the modulating wave that is Substituting the 2 nd eqn in the 1 st eqn the resulting voltage developed across the primary winding of the output transformer is given by

36 SQUARE LAW MODULATOR cont SPECTRAL RELATIONSHIP IN A SQUARELAW MODULATOR SPECTRUM OF BASEBAND SIGNAL SPECTRUM OF V2(t)

37 GENERATION OF DSB-FC AM SWITCHING MODULATOR CIRCUIT DIAGRAM Idealized Input Output Characteristic Curve

38 EQUATION OF SWITCHING MODULATOR The input voltage applied to the carrier The resulting load voltage The 2 nd eqn can be mathematically expressed as Representing this g(t) by its fourier series we have

39 The component EQUATION OF SWITCHING MODULATOR Which is the desired AM wave with amplitude sensitivity Periodic pulse train

40 DEMODULATION OR DETECTION OF AM SIGNAL SQUARE LAW DETECTOR We can remove the unwanted terms by passing this output voltage V0(t) through the band pass filter and finally we will get required AM signal. When such a device is used for the demodulation of an AM wave, we have for the input By substituting 2 nd eqn in the 1 st eqn we get

41 DEMODULATION OR DETECTION OF AM SIGNAL ENVELOPE DETECTOR Circuit diagram AM Wave input Envelope detector output

42 EQUATION OF ENVELOPE DETECTOR The charging time constant RsC is very small when compared to the carrier period 1/fc i.e., The discharging time constant R1C is very large when compared to the charging time constant i.e.,

43 LIMITATIONS IN AMPLITUDE MODULATION The carrier wave c(t) is completely independent of the information carrying signal or baseband signal m(t), which means that the transmission of the carrier wave represents a waste of power Only a fraction of the total transmitted power is affected by m(t) To overcome this the carrier component from the modulated wave resulting in Double side band suppressed carrier modulation

44 DSB-SC

45 DOUBLE SIDE BAND- SUPPRESSED CARRIER What is DSB-SC? Power saving in DSB-SC signal Equation of DSB-SC signal s(t)=c(t)m(t) From the above eqn the Fourier transform of s(t) is

46 DOUBLE SIDE BAND- SUPPRESSED CARRIER It is a form of linear modulation where the signal is generated by simply multiplying a message signal along with a carrier wave

47 DOUBLE SIDE BAND- SUPPRESSED CARRIER PHYSICAL APPEARANCE OF DSB-SC SIGNAL BASEBAND SIGNAL DSB-SC MODULATED WAVE

48 DOUBLE SIDE BAND- SUPPRESSED CARRIER FREQUENCY SPECTRUM OF DSB-SC SIGNAL SPECTRUM OF BASEBAND SIGNAL SPECTRUM OF DSB-SC MODULATED WAVE

49 GENERATION OF DSB-SC BALANCED MODULATOR What is balanced modulator? Operation of balanced modulator BALANCED MODULATOR

50 GENERATION OF DSB-SC EQUATION OF BALANCED MODULATOR The outputs of the two AM modulators may be expressed as follows and Subtracting 2 nd eqn from the 1 st eqn we obtain

51 EQUATION OF RING MODULATOR Illustrating the condition when the outer diodes are switched on and the inner diodes are switched off Illustrating the condition when the outer diodes are switched on and the inner diodes are switched off

52 GENERATION OF DSB-SC RING MODULATOR What is Ring modulator? Operation of Ring Modulator

53 EQUATION OF RING MODULATOR The square wave carrier c(t) can be represented by a fourier series as follows The ring modulator output is therefore

54 WAVEFORM ILLUSTRATING THE OPERATION OF THE RING MODULATOR FOR A SINUSOIDAL MODULATING WAVE MODULATING WAVE

55 SPECTRUM OF RING MODULATOR OUTPUT ILLUSTRATING THE SPECTRUM OF RING MODULATOR OUTPUT

56 DEMODULATION OF DSB-SC COHERENT DETECTION What is Coherent Detection? Operation of Coherent Detection COHERENT DETECTOR FOR MODULATING DSB-SC

57 EQUATION OF COHERENT DETECTION OF DSBSC WAVES The product modulator output is given as shown in the figure At the filter output we then obtain a signal given by Illustrating the spectrum of product modulator o/p with a DSBSC wave as i/p

58 DEMODULATION OF DSB-SC COSTAS RECEIVER

59 DEMODULATION OF DSB-SC SQUARING LOOP BLOCK DIAGRAM OF SQUARING LOOP

60 EQUATION OF SQUARING LOOP The square law device is characterized by the relation Therefore the DSBSC wave Applied to the input of this square law device we obtain The o/p is approximately sinusoidal as shown

61 SQUARING LOOP Cont.. Illustrating the Amplitude response of narrow band filter

62 SSB

63 SINGLE SIDE BAND What are the limitations of DSB-SC? What is SSB signal? If we consider the fact that two sidebands carry same information, DSB signal is redundant That is in DSB the basic information is transmitted twice once in each sideband Therefore there is absolutely no reason to transmit both sidebands in order to convey the information One sideband may be suppressed The resulting signal is a single sideband commonly referred to as single sideband suppressed carrier signal

64 SINGLE SIDE BAND SPECTRUM OF BASEBAND SIGNAL SPECTRUM OF DSBSC WAVE

65 SINGLE SIDE BAND SPECTRUM OF SSB WAVE WITH THE UPPER SIDEBAND TRANSMITTED SPECTRUM OF SSB WAVE WITH THE LOWER SIDEBAND TRANSMITTED

66 EQUATION OF SSB SIGNAL The time domain description of an SSB wave s(t) in the canonical form The fourier transform of SC(t) and SS(t) are related to that of the SSB wave s(t) as On the basis of the below figure we can write Accordingly the in phase component sc(t) is defined by

67 EQUATION OF SSB SIGNAL Cont On the basis of the fig we can write Where sgn(f) is the signum function equal to +1 for positive frequencies, zero for f=0 and -1 for ve frequencies. However we note that The hilbert transform of m(t) substituting 2 nd eqn in the 1 st eqn Which shows that the quadrature component ss(t) is defined by

68 SPECTRUM OF SSB WAVE SPECTRUM OF SSB WAVE SPECTRUM OF SSB WAVE SHIFTED TO THE RIGHT BY fc SPECTRUM OF IN PHASE COMPONENT

69 SPECTRUM Cont SPECTRUM OF QUADRATURE COMPONENT The cananonical representation of an SSB wave s(t) obtained bt transmitting only the upper sideband is as follows

70 GENERATION OF SSB WAVES FREQUENCY DISCRIMINATION METHOD Block Diagram of the Frequency Discrimination Method for Generating SSB Waves Block diagram of a two stage SSB modulator

71 GENERATION OF SSB WAVES PHASE DISCRIMINATION METHOD BLOCK DIAGRAM OF THE PHASE DESCRIPTION METHOD FOR GENERATING SSB WAVES

72 PHASE DISCRIMINATION METHOD Block diagram of the phase discrimination method for generating SSB waves by using a pair of phase shifting network to realize a constant 90 degree phase difference

73 SUPPRESSION OF UNWANTED SIDEBAND TO GENERATE SSB-SC Weavers method

74 COMPARISON BETWEEN SSB SUPRESSION METHODS

75 DEMODULATION OF SSB WAVE COHERENT DETECTION OF SSB WAVE COHERENT DETECTION OF AN SSB WAVE SPECTRUM OF THE PRODUCT MODULATOR OUTPUT V(t)

76 EQUATION OF COHERENT DETECTION The product modulator output is given by The resulting demodulated signal is given by The fourier transform of the above eqn is From the definition of hilbert transform its given as Substituting the eqn in the above eqn we get

77 EFFECT OF THE FREQUENCY ERROR f ON THE O/P OF THE COHERENT DETECTOR WITH SSB WAVE s(t) AS I/P Spectrum of Baseband Signal with Energy Gap in the Interval fa<f<fa Spectrum of coherent detector o/p with s(t) containing lower side band and f >0 or with s(t) containing upper sideband and f<0

78 EFFECT OF THE FREQUENCY ERROR f ON THE O/P OF THE COHERENT DETECTOR WITH SSB WAVE s(t) AS I/P cont Spectrum of coherent detector output with s(t) containing upper sideband and f>0 or with s(t) containing lower sideband and f<0

79 VSB

80 VESTIGIAL SIDE BAND What are the limitations of SSB? What is Vestigial sideband modulation & How is VSB wave generated? SPECTRUM OF BASEBAND SIGNAL SPECTRUM OF VSB WAVE

81 SCHEME FOR THE GENERATION AND DEMODULATION OF A VSB WAVE BLOCK DIAGRAM OF VSB MODULATOR

82 EQUATION OF VSB WAVE The fourier transform of sc(t) is given by This eqn can be simplified as This relation shows that the in-phase component of the VSB wave s(t) is defined by To determine the quadrature component ss(t) of the VSB wave s(t) we first obtained the fourier transform of sc(t) as This eqn suggests that we may generate ss(t) except for a scaling factor by passing the message signal m(t) through a filter whose transfer function is defined by Thus the quadrature component of VSB wave is ss(t)=1/2 Acms(t)

83 RESPONSE OF A VSB FILTER Amplitude Response of VSB Filter (Only Positive Frequency Portion ) Frequency response of filter for producing the quadrature component of the VSB wave

84 GENERATION OF VSB WAVE Block diagram of phase discrimination method for generating a VSB wave When the minus sign is selected a vestige of the lower sideband is transmitted When the plus sign is selected a vestige of the upper sideband is transmitted

85 RESPONSE OF VSB FILTER Frequency response of a VSB filter used in TV receivers

86 ENVELOPE DETECTION OF A VSB WAVE PLUS CARRIER The factor ka modifies the modulated wave applied to the envelope detector input as The envelope detector output denoted by a(t) is therefore

87 COMPARISON OF SSB,DSB, VSB.

88 HILBERT TRANSFORM

89 HILBERT TRANSFORM What is Hilbert transform? Equation of Hilbert transform The hilbert transform of g(t) is given by The inverse hilbert transform by which the original signal can be obtained is The fourier transform of 1/ πt is equal to jsgn(f) where sgn(f) is the signum function defined by It follows therefore the fourier transform is given by

90 HILBERT TRANSFORM Phase characteristic of linear two port device for obtaining the Hilbert transform of a signal

91 PROPERTIES OF HILBERT TRANSFORM Property 1: A signal g(t) and its hilbert transform have the same spectral density Property 2: A signal g(t) and its hilbert transform have the same autocorrelation function Property 3: A signal g(t) and its hilbert transform are orthoganal Property 4: If is the hilbert transform of g(t) then the hilbert transform of is g(t)

92 What is pre envelope? Equation of pre envelope PRE ENVELOPE Consider a real valued signal g(t). We define the pre-envelope of the signal g(t) as a complex valued function Let G(f) be the fourier transform of g(t) then we may write Using the definition of the signum function sgn(f) we therefore have

93 SPECTRUM OF PRE ENVELOPE

94 What is complex envelope? Spectrum complex envelope COMPLEX ENVELOPE Amplitude spectrum of band pass signal g(t) Amplitude spectrum of pre envelope Amplitude spectrum of complex envelope

95 COMPLEX ENVELOPE SCHEME FOR DERIVING THE IN PHASE AND QUADRATURE COMPONENTS OF A BAND PASS SIGNAL

96 COMPLEX ENVELOPE SCHEME FOR RECONSTRUCTING THE BAND PASS SIGNAL FROM ITS IN-PHASE AND QUADRATUREW COMPONENTS

97 SUPERHETRODYNE RECEIVER

98 SUPER HETRODYNE RECEIVER What is superhetrodyne receiver? Operation of superhetrodyne receiver Block diagram of superhetrodyne receiver

99 SUPER HETRODYNE RECEIVER SPECTRUM OF SUPERHETRODYNE RECEIVER

100 END OF 1 ST UNIT

Amplitude Modulation II

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