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      Class 12 PHYSICS – JEE

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      • Class 12
      • Class 12 PHYSICS – JEE
      CoursesClass 12PhysicsClass 12 PHYSICS – JEE
      • 1.Electrostatics (1)
        8
        • Lecture1.1
          Charge, Coulombs Law and Coulombs law in Vector form 41 min
        • Lecture1.2
          Electric Field; Electric Field Lines; Field lines due to multiple charges 42 min
        • Lecture1.3
          Charge Distribution; Finding Electric Field due to Different Object 01 hour
        • Lecture1.4
          Solid angle; Area Vector; Electric Flux; Flux of closed surface; Gauss Law 47 min
        • Lecture1.5
          Finding E Using Concept of Gauss law and Flux 01 hour
        • Lecture1.6
          Chapter Notes – Electrostatics (1)
        • Lecture1.7
          NCERT Solutions – Electrostatics
        • Lecture1.8
          Revision Notes Electrostatics
      • 2.Electrostatics (2)
        7
        • Lecture2.1
          Work done by Electrostatic Force; Work done by man in E-Field; Electrostatic Potential Energy 49 min
        • Lecture2.2
          Finding Electric Potential, Equipotential Surface and Motion in Electric Field 01 hour
        • Lecture2.3
          Electric Dipole and Dipole in Uniform and Non-uniform Electric field 01 hour
        • Lecture2.4
          Analysis of charge on conductors; Potential due to induced charge 58 min
        • Lecture2.5
          Conductors with cavity- Case 1: Empty cavity, Case 2: Charge Inside Cavity 41 min
        • Lecture2.6
          Connecting Two Conductors; Grounding of conductor; Electric field just outside conductor; Electrostatic pressure; Self potential Energy 54 min
        • Lecture2.7
          Chapter Notes – Electrostatics (2)
      • 3.Current Electricity (1)
        9
        • Lecture3.1
          Current, Motion of Electrons in Conductor; Temp. Dependence of Resistor 26 min
        • Lecture3.2
          Circuit Theory and Kirchoffs Laws 31 min
        • Lecture3.3
          Some Special Circuits- Series & Parallel Circuits, Open Circuit, Short Circuit 26 min
        • Lecture3.4
          Wheatstone Bridge, Current Antisymmetric 21 min
        • Lecture3.5
          Equivalent Resistance- Series and parallel, Equipotential Points, Wheatstone Bridge 25 min
        • Lecture3.6
          Current Antisymmetric, Infinite Ladder, Circuit Solving, 3D circuits 20 min
        • Lecture3.7
          Chapter Notes – Current Electricity
        • Lecture3.8
          NCERT Solutions – Current Electricity
        • Lecture3.9
          Revision Notes Current Electricity
      • 4.Current Electricity (2)
        4
        • Lecture4.1
          Heating Effect of Current; Rating of Bulb; Fuse 19 min
        • Lecture4.2
          Battery, Maximum power theorem; Ohmic and Non Ohmic Resistance; Superconductor 31 min
        • Lecture4.3
          Galvanometer; Ammeter & Voltmeter and Their Making 44 min
        • Lecture4.4
          Potentiometer and its applications ; Meter Bridge; Post Office Box; Colour Code of Resistors 32 min
      • 5.Capacitor
        6
        • Lecture5.1
          Capacitor and Capacitance; Energy in Capacitor 38 min
        • Lecture5.2
          Capacitive Circuits- Kirchoff’s Laws; Heat Production 01 hour
        • Lecture5.3
          Equivalent Capacitance; Charge on both sides of cap. Plate 52 min
        • Lecture5.4
          Dielectric Strength; Polar and Non-Polar Dielectric; Equivalent Cap. with Dielectric 01 hour
        • Lecture5.5
          Inserting and Removing Dielectric- Work (Fringing Effect), Force; Force between plates of capacitor 38 min
        • Lecture5.6
          Revision Notes Capacitor
      • 6.RC Circuits
        3
        • Lecture6.1
          Maths Needed for RC Circuits, RC circuits-Charging Circuit 19 min
        • Lecture6.2
          RC circuits-Discharging Circuit, Initial & Steady State, Final (Steady) State, Internal Resistance of Capacitor 44 min
        • Lecture6.3
          Revision Notes RC Circuits
      • 7.Magnetism and Moving Charge
        16
        • Lecture7.1
          Introduction, Vector Product, Force Applied by Magnetic Field, Lorentz Force, Velocity Selector 40 min
        • Lecture7.2
          Motion of Charged Particles in Uniform Magnetic Field 40 min
        • Lecture7.3
          Cases of Motion of Charged Particles in Uniform Magnetic Field 56 min
        • Lecture7.4
          Force on a Current Carrying Wire on Uniform B and its Cases, Questions and Solutions 59 min
        • Lecture7.5
          Magnetic Field on Axis of Circular Loop, Magnetic field due to Moving Charge, Magnetic Field due to Current 52 min
        • Lecture7.6
          Magnetic Field due to Straight Wire, Different Methods 40 min
        • Lecture7.7
          Magnetic Field due to Rotating Ring and Spiral 41 min
        • Lecture7.8
          Force between Two Current Carrying Wires 36 min
        • Lecture7.9
          Force between Two Current Carrying Wires 58 min
        • Lecture7.10
          Miscellaneous Questions 55 min
        • Lecture7.11
          Solenoid, Toroid, Magnetic Dipole, Magnetic Dipole Momentum, Magnetic Field of Dipole 54 min
        • Lecture7.12
          Magnetic Dipole in Uniform Magnetic Field, Moving Coil Galvanometer, Torsional Pendulum 01 hour
        • Lecture7.13
          Advanced Questions, Magnetic Dipole and Angular Momentum 56 min
        • Lecture7.14
          Chapter Notes – Magnetism and Moving Charge
        • Lecture7.15
          NCERT Solutions – Magnetism and Moving Charge
        • Lecture7.16
          Revision Notes Magnetism and Moving Charge
      • 8.Magnetism and Matter
        10
        • Lecture8.1
          Magnetic Dipole, Magnetic Properties of Matter, Diamagnetism; Domain Theory of Ferro 47 min
        • Lecture8.2
          Magnetic Properties of Matter in Detail 39 min
        • Lecture8.3
          Magnetization and Magnetic Intensity, Meissner Effect, Variation of Magnetization with Temperature 55 min
        • Lecture8.4
          Hysteresis, Permanent Magnet, Properties of Ferro for Permanent Magnet, Electromagnet 31 min
        • Lecture8.5
          Magnetic Compass, Earth’s Magnetic Field 20 min
        • Lecture8.6
          Bar Magnet, Bar Magnet in Uniform Field 49 min
        • Lecture8.7
          Magnetic Poles, Magnetic Field Lines, Magnetism and Gauss’s Law 32 min
        • Lecture8.8
          Chapter Notes – Magnetism and Matter
        • Lecture8.9
          NCERT Solutions – Magnetism and Matter
        • Lecture8.10
          Revision Notes Magnetism and Matter
      • 9.Electromagnetic Induction
        14
        • Lecture9.1
          Introduction, Magnetic Flux, Motional EMF 01 min
        • Lecture9.2
          Induced Electric Field, Faraday’s Law, Comparison between Electrostatic Electric Field and Induced Electric Field 43 min
        • Lecture9.3
          Induced Current; Faraday’s Law ; Lenz’s Law 56 min
        • Lecture9.4
          Faraday’s Law and its Cases 50 min
        • Lecture9.5
          Advanced Questions on Faraday’s Law 37 min
        • Lecture9.6
          Cases of Current Electricity 59 min
        • Lecture9.7
          Lenz’s Law and Conservation of Energy, Eddy Current, AC Generator, Motor 01 hour
        • Lecture9.8
          Mutual Induction 53 min
        • Lecture9.9
          Self Inductance, Energy in an Inductor 34 min
        • Lecture9.10
          LR Circuit, Decay Circuit 01 hour
        • Lecture9.11
          Initial and Final Analysis of LR Circuit 38 min
        • Lecture9.12
          Chapter Notes – Electromagnetic Induction
        • Lecture9.13
          NCERT Solutions – Electromagnetic Induction
        • Lecture9.14
          Revision Notes Electromagnetic Induction
      • 10.Alternating Current Circuit
        8
        • Lecture10.1
          Introduction, AC/DC Sources, Basic AC Circuits, Average & RMS Value 46 min
        • Lecture10.2
          Phasor Method, Rotating Vector, Adding Phasors, RC Circuit 35 min
        • Lecture10.3
          Examples and Solutions 21 min
        • Lecture10.4
          Power in AC Circuit, Resonance Frequency, Bandwidth and Quality Factor, Transformer 51 min
        • Lecture10.5
          LC Oscillator, Question and Solutions of LC Oscillator, Damped LC Oscillator 53 min
        • Lecture10.6
          Chapter Notes – Alternating Current Circuit
        • Lecture10.7
          NCERT Solutions – Alternating Current Circuit
        • Lecture10.8
          Revision Notes Alternating Current Circuit
      • 11.Electromagnetic Waves
        4
        • Lecture11.1
          Displacement Current; Ampere Maxwell Law 14 min
        • Lecture11.2
          EM Waves; EM Spectrum; Green House Effect; Ozone Layer 36 min
        • Lecture11.3
          Chapter Notes – Electromagnetic Waves
        • Lecture11.4
          Revision Notes Electromagnetic Waves
      • 12.Photoelectric Effect
        5
        • Lecture12.1
          Recalling Basics; Photoelectric Effect 50 min
        • Lecture12.2
          Photo-electric Cell 35 min
        • Lecture12.3
          Photon Flux; Photon Density; Momentum of Photon; Radiation Pressure- Full Absorption, Full Reflection; Dual nature 52 min
        • Lecture12.4
          Chapter Notes – Photoelectric Effect
        • Lecture12.5
          Revision Notes Photoelectric Effect
      • 13.Ray Optics (Part 1)
        12
        • Lecture13.1
          Rays and Beam of Light, Reflection of Light, Angle of Deviation, Image Formation by Plane Mirror 01 hour
        • Lecture13.2
          Field of View, Numerical on Field of Line, Size of Mirror 42 min
        • Lecture13.3
          Curved Mirrors, Terms Related to Curved Mirror, Reflection of Light by Curved Mirror 40 min
        • Lecture13.4
          Image Formation by Concave Mirror, Magnification or Lateral or Transverse Magnification 01 hour
        • Lecture13.5
          Ray Diagrams for Concave Mirror 45 min
        • Lecture13.6
          Image Formation by Convex Mirror; Derivations of Various Formulae used in Concave Mirror and Convex Mirror 01 hour
        • Lecture13.7
          Advanced Optical Systems, Formation of Images with more than one Mirror 24 min
        • Lecture13.8
          Concept of Virtual Object, Formation of Image when Incident ray are Converging, Image Characteristics for Virtual Object, 55 min
        • Lecture13.9
          Newton’s Formula, Longitudinal Magnification 23 min
        • Lecture13.10
          Formation of Image when Two Plane Mirrors kept at an angle and parallel; Formation of Image by two Parallel Mirrors. 43 min
        • Lecture13.11
          Chapter Notes – Ray Optics
        • Lecture13.12
          NCERT Solutions – Ray Optics
      • 14.Ray Optics (Part 2)
        13
        • Lecture14.1
          Refractive Index, Opaque, Transparent, Speed of Light, Relative Refractive Index, Refraction and Snell’s Law, Refraction in Denser and Rarer Medium 42 min
        • Lecture14.2
          Image Formation due to Refraction; Derivation; Refraction and Image formation in Glass Slab 57 min
        • Lecture14.3
          Total Internal Reflection, Critical Angle, Principle of Reversibility 01 hour
        • Lecture14.4
          Application of Total Internal Reflection 45 min
        • Lecture14.5
          Refraction at Curved Surface, Image Formation by Curved Surface, Derivation 56 min
        • Lecture14.6
          Image Formation by Curved Surface, Snell’s Law in Vector Form 01 hour
        • Lecture14.7
          Lens, Various types of Lens, Differentiating between various Lenses; Optical Centre, Derivation of Lens Maker Formula 01 hour
        • Lecture14.8
          Lens Formula, Questions and Answers 39 min
        • Lecture14.9
          Property of Image by Convex and Concave Lens; Lens Location, Minimum Distance Between Real Image and Object 01 hour
        • Lecture14.10
          Power of Lens, Combination of Lens, Autocollimation 35 min
        • Lecture14.11
          Silvering of Lens 44 min
        • Lecture14.12
          Cutting of Lens and Mirror, Vertical Cutting, Horizontal Cutting 49 min
        • Lecture14.13
          Newton’s Law for Lens and Virtual Object 01 hour
      • 15.Ray Optics (Part 3)
        6
        • Lecture15.1
          Prism, Angle of Prism, Reversibility in Prism 51 min
        • Lecture15.2
          Deviation in Prism, Minimum and Maximum Deviation, Asymmetric, Thin Prism, Proof for formula of Thin Prism 59 min
        • Lecture15.3
          Dispersion of Light, Refractive Index, Composition of Light, Dispersion through Prism 01 hour
        • Lecture15.4
          Rainbow Formation, Scattering of Light, Tyndall Effect, Defects of Image, Spherical Defect, Chromatic Defect, Achromatism. 57 min
        • Lecture15.5
          Optical Instruments, The Human Eye, Defects of Eye and its Corrections 01 hour
        • Lecture15.6
          Microscope & Telescope 02 hour
      • 16.Wave Optics
        21
        • Lecture16.1
          Introduction to Wave Optics 11 min
        • Lecture16.2
          Huygens Wave Theory 14 min
        • Lecture16.3
          Huygens Theory of Secondary Wavelets 10 min
        • Lecture16.4
          Law of Reflection by Huygens Theory 10 min
        • Lecture16.5
          Deriving Laws of Refraction by Huygens Wave Theory 10 min
        • Lecture16.6
          Multiple Answer type question on Huygens Theory 41 min
        • Lecture16.7
          Conditions of Constructive and Destructive Interference 22 min
        • Lecture16.8
          Conditions of Constructive and Destructive Interference 06 min
        • Lecture16.9
          Conditions of Constructive and Destructive Interference 23 min
        • Lecture16.10
          Incoherent Sources of Light 38 min
        • Lecture16.11
          Youngs Double Slit Experiment 12 min
        • Lecture16.12
          Fringe Width Positions of Bright and Dark Fringes 15 min
        • Lecture16.13
          Numerical problems on Youngs Double Slit Experiment 11 min
        • Lecture16.14
          Numerical problems on Youngs Double Slit Experiment 19 min
        • Lecture16.15
          Displacement of Interference Pattern 19 min
        • Lecture16.16
          Numerical problems on Displacement of Interference Pattern 28 min
        • Lecture16.17
          Shapes of Fringes 37 min
        • Lecture16.18
          Colour of Thin Films 59 min
        • Lecture16.19
          Interference with White Light 32 min
        • Lecture16.20
          Chapter Notes – Wave Optics
        • Lecture16.21
          NCERT Solutions – Wave Optics
      • 17.Atomic Structure
        6
        • Lecture17.1
          Thomson and Rutherford Model of Atom; Trajectory of Alpha particle; Bohr’s Model ; Hydrogen Like Atom; Energy Levels 58 min
        • Lecture17.2
          Emission Spectra, Absorption Spectra; De Broglie Explanation of Bohr’s 2nd Postulate; Limitations of Bohr’s Model 37 min
        • Lecture17.3
          Momentum Conservation in Photon Emission, Motion of Nucleus, Atomic Collision 58 min
        • Lecture17.4
          Chapter Notes – Atomic Structure
        • Lecture17.5
          NCERT Solutions – Atomic Structure
        • Lecture17.6
          Revision Notes Atomic Structure
      • 18.Nucleus
        6
        • Lecture18.1
          Basics- Size of Nucleus, Nuclear Force, Binding Energy, Mass Defect; Radioactive Decay 01 hour
        • Lecture18.2
          Laws of Radioactive Decay 36 min
        • Lecture18.3
          Nuclear Fission; Nuclear Reactor; Nuclear Fusion- Reaction Inside Sun 30 min
        • Lecture18.4
          Chapter Notes – Nucleus
        • Lecture18.5
          NCERT Solutions – Nucleus
        • Lecture18.6
          Revision Notes Nucleus
      • 19.X-Ray
        4
        • Lecture19.1
          Electromagnetic Spectrum, Thermionic Emission; Coolidge Tube – Process 1 22 min
        • Lecture19.2
          Coolidge Tube – Process 2; Moseley’s Law; Absorption of X-rays in Heavy Metal 39 min
        • Lecture19.3
          Chapter Notes – X-Ray
        • Lecture19.4
          Revision Notes X-Ray
      • 20.Error and Measurement
        2
        • Lecture20.1
          Least Count of Instruments; Mathematical Operation on Data with Random Error 18 min
        • Lecture20.2
          Significant Digits; Significant Digits and Mathematical Operations 30 min
      • 21.Semiconductors
        9
        • Lecture21.1
          Conductor, Semiconductors and Insulators Basics Difference, Energy Band Theory, Si element 21 min
        • Lecture21.2
          Doping and PN Junction 01 hour
        • Lecture21.3
          Diode and Diode as Rectifier 01 hour
        • Lecture21.4
          Voltage Regulator and Zener Diode and Optoelectronic Jn. Devices 01 hour
        • Lecture21.5
          Transistor, pnp, npn, Modes of operation, Input and Output Characteristics, , Current Amplification Factor 01 hour
        • Lecture21.6
          Transistor as Amplifier, Transistor as Switch, Transistor as Oscillator, Digital Gates 01 hour
        • Lecture21.7
          Chapter Notes – Semiconductors
        • Lecture21.8
          NCERT Solutions – Semiconductors
        • Lecture21.9
          Revision Notes Semiconductors
      • 22.Communication Systems
        5
        • Lecture22.1
          Basic working and terms; Antenna; Modulation and Types of Modulation 32 min
        • Lecture22.2
          Amplification Modulation, Transmitter, Receiver, Modulation index 40 min
        • Lecture22.3
          Chapter Notes – Communication Systems
        • Lecture22.4
          NCERT Solutions – Communication Systems
        • Lecture22.5
          Revision Notes Communication Systems

        Chapter Notes – Communication Systems

        Communication System

        A communication system is the set-up used in the transmission of information from one place to another. The present day communication system are electrical, electronic or optical in nature.

        In principle, a communication system consists of the following three parts :

        (i) Transmitter

        (ii) Communication Channel

        (iii) Receiver

        A schematic model of an electrical communication system is shown in Figure 1.

        (i) A transmitter :transmits the information after modifying it to a form suitable for transmission. The key to communication system is to obtain an electrical signal (voltage or current), which contains the information. For example, a microphone converts speech signals into electrical signals. Similarly, piezoelectric sensorsconvert pressure variations into electrical signals. Light signals are converted into electrical signals by photo detectors. The devices like microphone, piezoelectric sensors and photo detectors, which convert a physical quantity (called information, here) into electrical signal are known as Transducers. Such an electrical signal contains the information to be transmitted.

        We define a signal as a single valued function of time (that conveys the information). This function has a unique value at every instant of time.

        Most of the speech or information signals cannot be transmitted directly over long distances.  These signals have to be loaded or superimposed on a high frequency wave, which acts as the carrier wave. This process is known as modulation. The signal so obtained is called modulated signal/wave. The power of the signal is boosted signal using a suitable amplifire. The modulated signal is then radiated into space with the help of an antenna called transmitting antenna. The arrangement is shown in Fig.2

        (ii) Communication Channel : The communication channel carries the modulated wave from the transmitter to the receiver. In ordinary conversation, the air through which sound travels from the speaker to the listener serves as the communication channel. In case of telephony and telegraphy, communication channel is the transmission lines, which connect the transmitter and the receiver. In radio communication (or wireless communication), the free space through which the modulated signal travels serves as the communication channel.

        (iii)The receiver : In the radio communication or wireless communication, the receiver consists of :

        (a) a pick up antenna to pick the signal,

        (b)a demodulator, to separate the low frequency audio signal from the modulated signal,

        (c) an amplifier, to boost up suitably the audio signal, and

        (d) the transducer, like loud speaker to convert the audio signal (in the form of electrical pulses) into sound waves.

        The receiver part of the communication system is shown schematically in Fig.3

        Antenna 

        An antenna plays a vital role in a communication system. It is used in both, the transmission and reception of radio frequency signals.

        Infact, an antenna is a structure that is capable of radiating electromagnetic waves or receiving them, as the case may be. Basically, an antenna is generally a metallic object, often a wire or collection of wires, used to convert high frequency current into electromagnetic waves and vice-versa. Thus, a transmitting antenna converts electrical energy into electromagnetic waves, whereas a receiving antenna converts electromagnetic waves into electrical energy. Apart from their different function, transmitting and receiving antennas behave idenically i.e. their behaviour is reciprocal.

        When a transmitting antenna is held vertically, the electromagnetic waves produced are polarized vertically.

        A Hertz antenna is a straight conductor of length equal to half the wavelength of radio signals to be transmitted or received i.e. l = /2.

        A  Marconi antenna is a straight conductor of length equal to a quarter of the wavelength of radio signals to be transmitted or received i.e. l = /4. It is held vertically with its lower end touching the ground. The ground provides a reflection of the voltage and current distributions set up in the antenna. The electromagnetic waves emitted from (Marconi) antenna ground system are the same as those emitted from Hertz antenna, which is not grounded.

        The design of an antenna depends on frequency of carrier wave and directivity of the beam etc. Two common types of antenna are :

        (i) Dipole antenna, shown in Fig.4 is used in transmission of radio waves. It is omni directional.

        (ii) Dish type antenna, shown in Fig.5 is a directional antenna.

        Such an antenna has a parabolic reflector with an active element, called the dipole or horn feed at focus of the reflector. The dish type antenna can transmit waves  in a particular direction. Also, it can receive only those waves which are directed towards it. For transmission, the signal is fed to the active element, which directs it on to the reflector. The signal is then transmitted in the form of a parallel beam as shown in Fig.5. For reception, the waves directed towards the dish are reflected on to the active element, which converts them into electrical signals. The dish type antennas are commonly used in radar and satellite communication.

        Message Signals

        Message signals are electrical signals generated from the original information to be transmitted, using an appropriate transducer. A message signal is a single valued function of time that conveys the information. This function has a unique value at every instant of time. These signals are of two type :

        (i) Analog signals                (ii) Digital signals

        (i) Analog signals. An analog signal is that in which current or voltage value varies continuously with time.

        In the simplest form of an analog signal, amplitude of the signal varies sinusoidally with time. It is represented by the equation

        E = E0 sin (t + f)

        where E0 is max. value of voltage, called the amplitude, T is time period and  =  is angular frequency of the signal. In fig.6, f represents the phase angle. Such signals can have all sorts of values at different instants, but these values shall remain within the range of a maximum value (+ E0) and a minimum value ( – E0).

        Examples of analog signals are speech, music, sound produced by a vibrating tuning fork, variations in light intensity etc. These are converted into current/voltage variations using suitable transducers. The information bearing signals are called base band signals.

        (ii) Digital signals. A digital signal is a discontinuous function of time, in contrast to an analog signal, wherein current or voltage value varies continuously with time.

        Such a signal is usually in the form of pulses. Each pulse has two levels of current or voltage, represented by 0 and 1. Zero (0) of a digital signal refers to open circuit and (1) of a digital signal refers to closed circuit. Zero (0) is also referred to as ‘No’ or space and (1) is referred to as ‘Yes’ or mark. Both 0 and 1 are called bits.

        A typical digital signal is shown in Fig.7.

        The significant characteristics of a digital signal are : Pulse amplitude ; Pulse Duration or Pulse Width and Pulse Position, representing the time of rise and time of fall of the pulse amplitude, as shown in Fig.7.

        Examples of digital messages are :

        (i) letters printed in this book

        (ii) listing of any data,

        (iii) output of a digital computer,

        (iv) Electronic transmission of a document at a distant place via telephone line i.e. FAX etc.

        An analog signal  can be converted suitably into a digital signal and vice-versa.

        Note. As stated above, a digital signal is represented by binary digits 0 and 1 called bits.

        A group of bits is called a binary word or a byte. A byte made of 2 bits can give four code combinations : 00, 01, 10 ; 11.

        Types Of Communications Systems

        There is no unique way of classifying communication systems. However, for the sake of convenience, we can classify them broadly on the basis of –

        (i) nature of information source,

        (ii) mode of transmission,

        (iii) type of transmission channel used,

        (iv) type of modulation employed, as detailed below :

        (a)Based on nature of information source

        (i) Speech transmission as in radio

        (ii) Picture as well as speech transmission as in television

        (iii) Facsimile transmission, as in FAX

        (iv) Data transmission as in computers

        (b) Based on mode of transmission

        (i) Analog communication, where the modulating signal is analog. The carrier wave may be sinusoidal or in the form of pulses. For example, in telegraphy, telephony, radio network, radar, television network, teleprinting, telex etc.

        (ii) Digital communication, where the modulating signal is digital in nature. For example, Fax, mobile phone network, e-mail, teleconferencing, telemetry, communication satellites and global positioning system are all digital communication systems.

        (c)  Based on transmission channel

        (i) Line communication

        1. Two wire transmission line

        2.Co-axial cable transmission

        3. Optical fibre cable communication

        (ii) Space communication

        (d) Based on the type of modulation

        (i) For sinusoidal continuous carrier waves, the types of modulation are :

        1. Amplitude Modulation (AM)

        2. Frequency Modulation (FM)

        3. Phase Modulation

        (ii) For pulsed carrier waves, the modes of modulation are

        1. Pulse Amplitude Modulation (PAM)

        2. Pulse Time Modulation (PTM). It includes

        – Pulse Position Modulation (PPM),

        – Pulse Width modulation (PWM),

        – Pulse Duration Modulation (PDM)

        3. Pulse Code Modulation (PCM)

        5. An important step in communications modulation and it’s need

        Suppose we wish to transmit an electrical signal in the audio frequency (AF) range (20 Hz to 20 kHz) over a long distance. We cannot do it, as such because of the following reasons :

        Size of the antenna or aerial. An antenna or aerial is needed both for transmission and reception. Each antenna should have a size comparable to the wavelength of the signal, (atleastl/4 in size), so that time variation of the signal is properly sensed by the antenna.

        For an audio frequency signal of frequency n = 15 kHz, the wavelength,  = cv = 3×10815×103 = 20000 m. The length of the antenna = λ4 = 200004 = 5000 metre. To set up an antenna of vertical height 5000 metre is practically impossible. Therefore, we need to use high frequencies for transmission.

        Effective Power radiated by antenna. Theoretical studies reveal that power P radiated from a linear antenna of length l is

        P ∝1l2

        As high powers are needed for good transmission, l should be small i.e. antenna length should be small, for which wavelength l should be small or frequency n should be high.

        Mixing up of signals from different transmitters. Suppose many people are talking at the same time. We just cannot make out who is talking what. Similarly, when many transmitters are transmitting baseband information signals simultaneously, they get mixed up and there is no way to distinguish between them. The possible solution is, communication at high frequencies and allotting a band of frequencies to each user. This is what is being done for different radio and T.V. broadcast stations.

        All the three reasons explained above suggest that there is a need for  transmissions at high frequencies. This is achieved by a process, called modulation, where in we superimpose the audio frequency baseband message or information signals (called the modulating signals) on a high frequency wave (called, the carrier wave). The resultant wave is called the modulated wave, which is transmitted.

        In the process of modulation, some specific characteristic of the carrier wave is varied in accordance with the information or message signal. The carrier wave may be

        (i) Continuous (sinusoidal) wave, or

        (ii) Pulse, which is discontinuous

        A continuous sinusoidal carrier wave can be expressed as E = E0 sin (t + ).

        Three distinct characteristics of such a wave are : amplitude (E0), angular frequency (w) and phase angle (f). Any one of these three characteristics can be varied in accordance with the modulating baseband (AF) signal, giving rise to the respective Amplitude Modulation ; Frequency Modulation and Phase Modulation.

        Notes. Phase modulation is not of much practical importance. We shall, therefore, confine ourselves  to the study of amplitude and frequency modulations only.

        Again, the significant characteristics of a pulse are : Pulse Amplitude, Pulse Duration or Pulse Width and Pulse Position (representing the time of rise or fall of the pulse amplitude). Any one of these characteristics can be varied in accordance with the modulating baseband (AF) signal, giving rise to the respective, Pulse Amplitude Modulation (PAM), Pulse Duration Modulation (PDM) or Pulse Widhth Modulation (PWM) and Pulse Position Modulation (PPM).

        Application 1

        Show that the minimum length of antenna required to transmit a radio signal of frequency 10 MHz is 7.5 m.

        Solution                  

        Here, f = 10 MHz = 107 Hz

        λ=cf=3×108107 = 30 m,    Minimum length of antenna = λ4=304 = 7.5 m

        Amplitude Modulation

        When a modulating AF wave is superimposed on a high frequency carrier wave in a manner that the frequency of modulated wave is same as that of the carrier wave, but its amplitude is made proportional to the instantaneous amplitude of the  audio frequency modulating voltage, the process is called amplitude modulation (AM).

        Let the instantaneous carrier voltage (ec) and modulating voltage (em) be       represented by

        ec = Ec sin ct               ….(1)

        em = Em sin mt          ….(2)

        Thus, in amplitude modulation, amplitude A of modulated wave is made proportional to the instantaneous modulating voltage em

        i.e. A = Ec + k em                ….(3)          where k is a constant of proportionality.

        In amplitude modulation, the proportionality canstant k is made equal to unity. Therefore, max. positive amplitude of AM wave is given by

        A = Ec + em = Ec + Em sin mt         ….(4)

        It is called top envelope

        The maximum negative amplitude of AM wave is given by

        – A = – Ec – em

        = – (Ec + Em sin mt) ….(5)          This is called bottom envelope

        The modulated wave extends between these two limiting envelopes, and its frequency is equal to the unmodulated carrier frequency. Fig.8(a) shows the variation of voltage of carrier wave with time. Fig.8(b) shows one cycle of modulating sine wave and Fig.8(c) shows amplitude modulated wave for this cycle.

        As is clear from Fig.8(c)

        Em = Emax−Emin2

        and   Ec = Emax – Em

        = Emax – Emax−Emin2

        Ec = Emax+Emin2    ….(6)

        In amplitude modulation, the degree of modulation is defined by a term, called modulation index or modulation factor or depth of modulation represented by ma. It is equal to the ratio of amplitude of modulating signal to the amplitude of carrier wave i.e.

        ma = EmEc=Emax−EminEmax+Emin  ….(7)

        Obviously, modulation index (ma) is a number lying between 0 and 1. Often, ma is expressed in percentage and is called the percentage modulatoion. Importance of moduation index is that it determines the quality of the transmitted signal. When modulation index is small, veriation in carrier amplitude will be small. Therefore, audio signal being transmitted will be weak. As the modulataion index increases, the audio signal on reception becomes clearer.

        Application 2

        An audio signal of amplitude one half the carrier amplitude is used in amplitude modulation. Calculate the modulation index ?

        Solution 

        Here, Em = 0.5 Ec

        Emax = Ec + Em = Ec + 0.5 Ec = 1.5 Ec

        Emin = Ec – Em = Ec – 0.5 Ec = 0.5 Ec

        Ma = Emax−EminEmax+Emin=1.5Ec−0.5Ec1.5Ec+0.5Ec=Ec2Ec = 0.5

        6.1  Frequency Spectrum of AM wave

        A detailed study of amplitude modulation reveals that the amplitude modulated wave consists of three discrete frequencies, as shown in Fig.9. Of these, the central frequency is the the carrier frequency (fc), which has the highest amplitude. The other two frequencies are placed symmetrically about it. Both these frequencies have equal amplitudes-which never exceeds half the carrier amplitude. These frequencies are called side band frequencies i.e. fSB = fc ± fm

        Frequency of lower side band is      fLSB = fc – fm              ….(8)

        and frequency of upper side band is      fUSB= fc + fm       ….(9)

        Band width of amplitude modulated wave is = fUSB – fLSB           

        = (fc + fm) – (fc – fm) = 2fm             ….(10)

        Band width = twice the frequency of the modulating signal

        6.2  Power and Current Relations in AM wave

        Average power/cycle in the unmodulated carrier wave is  Pc = Ec22R   ….(11)

        where R is resistance (of antenna) in which power is dissipated.

        It can be shown that total power/cycle in the modulated wave is Pt = Pc (1+ma22)       ….(12)

        PtPc=1+ma22              But Pt=It2R  and Pc=Ic2R

        It2Ic2=1+ma22   or  ItIc=1+ma22−−−−−−√   ….(13)

        Amplitude Modulated Wave Production

        The following is the block diagram of a modulator for obtaining an AM signal:

        The message signal or modulating signal Am sin ωmt is added to the carrier signal Ac sin ωct to obtain the signal x(t). This signal is passed through square law device which is a non – linear device which produces output of the from y(t) = B x(t) + C x(t)2 where B and C are constants.

        The signal y(t) is then passed through bandpass filter centred at ωc to remove other unwanted signals if present. Hence, we get AM signal.

        Amplitude Modulated Wave Detection

        The block diagram of AM wave detection is shown below:

        The received AM signal is firstly passed through an amplifier as the transmitted signal gets attenuated while propagating through channel. After that, the carrier frequency is changed to a lower frequency known as an intermediate frequency before passing it to the detector.

        Following is the detailed block diagram of detector:

        The AM Wave after being converted to IF frequency is passed through a rectifier to convert it into DC signal as shown in (b). Now, in order to retrieve message signal m(t), the signal is passed through an envelope detector which can be a simple RC circuit. Finally, the message signal is passed through an amplifier to obtain the clean transmitted message signal.

        Frequency Modulation

        When a modulating AF wave is superimposed on a high frequency carrier wave in such a way that the amplitude of modulated wave is same as that of the carrier wave, but its frequency is varied in accordance with the instantaneous value of the modulating voltage, the process is called frequency modulation (FM).

        Let the instantaneous carrier voltage (ec) and modulating voltage (em) be represented by

        ec = Ec sin ct        ….(15)                     em = Em sin mt             ….(16)

        Fig.10(a) represents the variation of carrier voltage with time, and Fig.10(b) represents the variation of modulating AF voltage with time.

        In frequency modulation, the amount by which carrier frequency is varied from its unmodulated value (fc = c/2) is called the deviation. This deviation is made proportional to the instantaneous value of the modulating voltage. The rate at which the frequency variation takes place is equal to the modulating frequency. Fig.10(c) represents an exaggerated view of frequency modulated wave. Fig.10(d) shows the frequency variation with time in the FM wave. This is identical to the variation of the modulating voltage with time, Fig.10(b). Note that

        (i) All signals having same amplitude will change the carrier frequency by the same amount, whatever be their frequencies

        (ii) All modulating signals of same frequency, say 1 kHz, will change the carrier frequency at the same rate of 1000 times, per second-whatever be their individual amplitudes

        (iii) The amplitude of the frequency modulated wave remains constant at all times, being equal to the amplitude of the carrier wave

        If ƒ is frequency of FM wave at any instant t and ƒc is constant frequency of the carrier wave, then

        deviation (in frequency),  = (ƒ – ƒc)       ….(17)

        By definition of frequency modulation,

          em

        or     Em sin mt

         = k Em sin mt                              ….(18)

        where k is a constant of proportionality, Using (17), we get

         = ƒ –ƒc = k Em sin mt

        or   ƒ = ƒc + k Em sin mt     ….(19)

        The deviation will be maximum, when

        (sinmt)max = ± 1

        From (19), ƒmax = ƒc ± k Em                   ….(20)

        or   dmax = ƒmax – ƒc = ± k Em                ….(21)

        The modulation index (mƒ) of a frequency modulated wave is defined as the ratio of maximum frequency deviation to the modulating frequency i.e.

        mƒ = δmaxfm=±kEmfm   ….(22)

        Clearly, modulating index increases, as modulating frequency (ƒm) decreases. mƒ has no units, it being the ratio of two frequencies.

        The instantaneous amplitude of frequency modulated wave is given by

        A = A0 sin 

        where  is the function of carrier angular frequency (c) and modulating angular frequency (m). Infact,

         = (ωct+δfmsinωmt) ….(23)

        The frequency spectrum of FM wave is far more complex than the frequency spectrum of AM wave. Infact,

        (i) The output of an FM wave consists of carrier frequency (ƒc) and almost an infinite number of side bands, whose frequencies are (ƒc ± ƒm), (ƒc ± 2ƒm), (ƒc ± 3ƒm),… and so on. The sidebands  are thus separated from the carrier by ƒm, 2ƒm, 3ƒm ….etc i.e. they have a recurrence frequency of ƒm.

        (ii) The number of sidebands depends on the modulation index (mƒ). The number of sidebands increases, when frequency deviation () is increased, keeping (ƒm) constant. Similarly, number of sidebands decreases, when frequency of modulating signal (ƒm) is increased keeping frequency deviation constant

        (iii) The sidebands are disposed symmetrically about the carrier. Further, sidebands at equal distances from the carrier have equal amplitudes.

        (iv) As the distance of sidebands from carrier frequency increases, their amplitude decreases. Therefore, number of significant sideband pairs is limited

        (v) In frequency modulated wave, the information (audio signal) is contained in the sidebands only. Since the sidebands are separated from each other by the frequency of the modulating signal (ƒm), therefore,

        Band width = 2n × (ƒm)  ….(24)

        where n is the number of the particular sideband pair.

        Application 3

        As audio signal of 2.8 kHz modulates a carrier of frequency 84 MHz and produces a frequency deviation of 56 kHz. Calculate

        (i) frequency modulation index

        (ii) frequency range of FM wave

        Solution

        Here, ƒm = 2.8 kHz, ƒc = 84 MHz ;  = 56 kHz

        (i) Frequency modulation index = mƒ = δfm=562.8=20

        (ii) Frequency range of FM wave  = ƒc ± ƒm = (84 ± 2.8 × 10–3) MHz = 84.0028 MHz and 83.9972 MHz

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