com-sys-2 - icbsexiiphysics
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1 You Can’t Miss It Previous Years Questions with Expected Answer/value Points Syllabus:- Communication System 5Marks Elements of a communication system (block diagram only); bandwidth of signals (speech, TV and digital data); bandwidth of transmission medium. Propagation of electromagnetic waves in the atmosphere, sky and space wave propagation. Need for modulation. Production and detection of an amplitude-modulated wave. --------------------------------------------------------------------------------------------------------------------------Q1. Explain the function of a repeater in a communication system. 2 A1. A repeater, picks up the signal from the transmitter, amplifies and retransmits it to the receiver sometimes with a change in carrier frequency. Repeaters are used to extend the range of a communication system. Q2. What is space wave propagation ? Give two examples of communication system which use space wave mode. A TV tower is 80 m tall. Calculate the maximum distance upto which the signal transmitted from the tower can be received. A2. When waves travel in space in a straight line from the transmitting antenna to the receiving antenna, this mode of propagation is called the space wave propagation. 1 Examples: Television broadcast, microwave links, satellite communication (Any two) ½+½ ½+½ Q3. Write two factors justifying the need of modulating a signal. A carrier wave of peak voltage 12 V is used to transmit a message signal. What should be the peak voltage of the modulating signal in order to have a modulation index of 75% ? A3. (i) Appropriate size of the antenna or aerial (ii) Effective power radiated by an antenna (iii) To avoid Mixing up of signals from different transmitters (Any Two) ½+½ Modulation Index ½ ½ Q4. Which mode of propagation is used by short wave broadcast services having frequency range from a few MHz upto 30 MHz ? Explain diagrammatically how long distance communication can be achieved by this mode. Why is there an upper limit to frequency of waves used in this mode ? A4. Sky wave propagation/ Ionospheric reflection ½ 2 Electromagnetic waves of these frequencies are reflected, by the ionosphere towards the earth. 1 Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. 1 Q5. (i) Define modulation index. 2 (ii) Why is the amplitude of modulating signal kept less than the amplitude of carrier wave ? A5. Modulation index is defined as the ratio of amplitude of modulating signal and amplitude of carrier wave Alternatively, 1 The amplitude of modulating signal is kept less than amplitude of carrier wave to avoid/minimize distortion / noise. 1 Q6. Draw a schematic diagram showing the (i) ground wave (ii) sky wave and (iii) space wave propagation modes for em waves. 3 Write the frequency range for each of the following: (i) Standard AM broadcast (ii) Television (iii) Satellite communication A6. 3 Q7. What is sky wave communication ? Why is this mode of propagation restricted to the frequencies only upto few MHz ? 2 A7. Ionospheric reflection of radiowaves, back towards the earth, is known as sky wave communication. 1 The ionospheric layers act as a reflector for a certain range of frequencies (3-30MHz.) Electromagnetic waves of frequencies higher than30MHz (or few MHz) penetrate the ionosphere and escape. 1 Alternatively Diagrammatic explanation 2 Q8. Write briefly any two factors which demonstrate the need for modulating a signal. Draw a suitable diagram to show amplitude modulation using a sinusoidal signal as the modulating signal. 3 A8. Need for modulation: Size of antenna would be very large in the absence of modulation. Effective power radiated by the antenna would be small in the absence of modulation. Mixing up of the signals from different transmitters. 1+1 (Any Two) 4 Q9. Q10. A10. 5 Q11. 2 Q12 Q13 6 Q
