CBSE Class 12 Physics Communication Systems Frequently Asked Questions

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UNIT X
COMMUNICATION SYSTEMS
2 MARKS

1. Draw a block diagram of communication system. [2 Marks]

Answer:
1. Information source → Transmitter → Communication Channel → Receiver → User information.
2. In the communication channel, noise may be introduced during transmission.
3. The block diagram clearly represents the unidirectional flow of information from the source to the destination.

[Figure: Block diagram showing Information Source, Transmitter, Communication Channel with Noise, Receiver, and User Information]

Teacher's Note:
a) The basic blocks of a communication system are transmitter, channel, and receiver.
b) Always label the noise block connected to the communication channel.
c) A neat pencil diagram fetches full credit in board exams.

2. Distinguish between point to point and broadcast communication modes. Give one example of each. [2 Marks]

Answer:
1. Point to point communication mode involves communication over a link between a single transmitter and a single receiver.
2. Broadcast communication mode involves a single transmitter and a large number of unconnected receivers.
3. Example of point to point communication is telephony.
4. Example of broadcast communication is radio or television broadcast.

Teacher's Note:
a) Focus on the number of transmitters and receivers involved in each mode.
b) Do not miss giving examples for both modes as asked in the question.
c) Telephony is strictly point-to-point, whereas mass media uses broadcasting.

3. Explain the following terms.
a) Ground waves b) Space waves and c) sky waves. [2 Marks]

Answer:
1. Ground waves: Radio waves that travel directly through the atmosphere along the surface of the earth, used for low-frequency communication.
2. Space waves: Radio waves that travel directly in a straight line from the transmitting antenna to the receiving antenna, typically used for VHF and UHF bands.
3. Sky waves: Radio waves reflected back to the earth by the ionosphere, enabling long-distance shortwave communication.

Teacher's Note:
a) Clearly mention the frequency range or propagation path for each wave type.
b) Ground wave propagation is limited to lower frequencies due to high ground attenuation.
c) Sky wave propagation relies entirely on the ionized layers of the upper atmosphere.

4. What does the term LOS communication mean? Name the types of waves that are used for this communication. Give typical examples, with the help of a suitable figure, of communication systems that use space wave mode propagation. [2 Marks]

Answer:
1. LOS (Line of Sight) communication means transmission of signals in a straight line from transmitting antenna to receiving antenna.
2. Space waves are used for LOS communication.
3. Examples include television broadcast, microwave links, and satellite communication.
4. Space wave propagation requires both antennas to be visible to each other without obstruction by the curvature of the earth.

[Figure: Line of sight propagation showing transmitting antenna, receiving antenna, and direct wave path above earth's curvature]

Teacher's Note:
a) Mention the line of sight restriction explicitly in terms of antenna height and earth curvature.
b) Space waves operate at frequencies above 40 MHz where ground and sky waves fail.
c) Diagrams must clearly show the direct wave trajectory between antenna tips.

5. Write the function of 1) Transducer and 2) repeater in the context of communication system. [2 Marks]

Answer:
1. Transducer: A device that converts one form of energy into another, typically converting physical audio/video signals into electrical signals or vice versa.
2. Repeater: A combination of a receiver and a transmitter that picks up a signal from a distant transmitter, amplifies it, and retransmits it to extend the communication range.

Teacher's Note:
a) Transducers are placed at both input and output stages of a communication system.
b) Repeaters overcome the attenuation of signals over long distances or geographical obstacles.
c) Satellites function as space-borne repeaters for global communication networks.

7. We do not choose to transmit an audio signal by just directly converting it to an E.M wave of the same frequency. Give two reasons for the same. [2 Marks]

Answer:
1. Size of the antenna: For efficient radiation and reception, antenna length must be comparable to a quarter wavelength (\(\lambda / 4\)), which is impractically large for low audio frequencies.
2. Effective power radiated: The power radiated from an antenna is inversely proportional to the square of the wavelength, making audio frequency power radiation extremely inefficient.
3. Mixing up of signals: Transmitting low frequency audio signals directly from different sources without modulation would result in overlapping signals that cannot be separated.

Teacher's Note:
a) State the antenna size requirement formula \(l = \frac{\lambda}{4}\) clearly.
b) Explain power radiation dependence on wavelength to secure full marks.
c) Modulation solves all three practical difficulties of direct audio transmission.

8. Explain briefly with the help of diagrams the terms (i) amplitude modulation and (ii) Frequency modulation. Which of these (i) gives better quality transmission? (ii) Has a larger coverage [2 Marks]

Answer:
1. Amplitude modulation (AM): The amplitude of the carrier wave varies in accordance with the information signal while frequency and phase remain constant.
2. Frequency modulation (FM): The frequency of the carrier wave varies in accordance with the information signal while amplitude and phase remain constant.
3. (i) Frequency modulation (FM) gives better quality transmission because it is less susceptible to noise.
4. (ii) Amplitude modulation (AM) has a larger coverage due to its lower frequency bands allowing ground wave propagation.

[Figure: Waveform diagrams showing carrier wave, modulating audio signal, amplitude modulated wave, and frequency modulated wave]

Teacher's Note:
a) Define AM and FM precisely based on parameter variation.
b) Address both sub-questions regarding quality and coverage separately.
c) Noise affects amplitude more than frequency, making FM superior in audio fidelity.

9. Why is short wave bands used for long distance transmission of signals? [2 Marks]

Answer:
1. Short wave bands consist of high frequency radio waves that can undergo total internal reflection from the ionosphere.
2. This sky wave propagation mechanism allows signals to bounce between the ionosphere and the earth surface, bypassing the earth curvature to reach distant locations globally.

Teacher's Note:
a) Mention the role of the ionosphere in reflecting short waves back to earth.
b) High frequency short waves penetrate lower atmospheric layers but get refracted by ionized layers.
c) This technique enables intercontinental radio broadcasting.

10. Optical and radio telescope are built on the ground but x-ray astronomy is possible only from satellite? [2 Marks]

Answer:
1. The Earth atmosphere is transparent to visible light and radio waves, allowing optical and radio telescopes to operate effectively from the ground.
2. X-rays are heavily absorbed by the Earth atmosphere (specifically ozone and other gases), so X-ray telescopes must be placed above the atmosphere on satellites in space.

Teacher's Note:
a) Explain atmospheric transparency windows for optical and radio waves.
b) State the high absorption property of the atmosphere for high-energy X-ray radiation.
c) Space-based observatories are essential for parts of the electromagnetic spectrum blocked by air.

11. Draw a block diagram for a transmitter and a receiver of AM wave. [2 Marks]

Answer:
1. Transmitter block diagram: Microphone → Audio Amplifier → Modulator ← Carrier Oscillator → Power Amplifier → Transmitting Antenna.
2. Receiver block diagram: Receiving Antenna → Tuner / RF Amplifier → Detector (Demodulator) → Audio Amplifier → Loudspeaker.

[Figure: Two block diagrams showing AM transmitter blocks on top and AM receiver blocks below with proper component labels]

Teacher's Note:
a) Clearly separate transmitter and receiver blocks in your diagram.
b) Show the modulator stage combining carrier and message signals in the transmitter.
c) Show the detector stage recovering the message signal from the modulated wave in the receiver.

3 MARKS

12. Define the term modulation index for an AM wave. What would be the modulation index for an AM wave for which the maximum amplitude is 'a' and the minimum amplitude is 'b' [3 Marks]

Answer:
1. Modulation index is defined as the ratio of the amplitude of the modulating signal to the amplitude of the carrier wave.
2. Formula: \(m = \frac{A_m}{A_c}\), where \(A_m\) is message amplitude and \(A_c\) is carrier amplitude.
3. Maximum amplitude \(A_{max} = A_c + A_m = a\) and minimum amplitude \(A_{min} = A_c - A_m = b\).
4. Solving for amplitudes: \(A_c = \frac{a + b}{2}\) and \(A_m = \frac{a - b}{2}\).
5. Therefore, modulation index \(m = \frac{a - b}{a + b}\).

Teacher's Note:
a) The modulation index determines the quality of the transmitted signal.
b) Ensure that \(m\) lies between 0 and 1 to avoid distortion.
c) This is a standard board derivation frequently asked in communication chapters.

13. A TV tower has a height 'h'. Derive an expression for maximum distance up to which the signal can be received from the earth. [3 Marks]

Answer:
1. Let R be the radius of the earth and h be the height of the TV tower.
2. Let d be the maximum line-of-sight distance up to which the signal can be received.
3. From right-angled triangle formed by earth's center, tower top, and horizon point, using Pythagoras theorem: \((R + h)^2 = R^2 + d^2\).
4. Expanding the LHS: \(R^2 + h^2 + 2Rh = R^2 + d^2\), which simplifies to \(d^2 = 2Rh + h^2\).
5. Since \(R \gt\gt h\), \(h^2\) can be neglected, giving the final expression \(d = \sqrt{2Rh}\).

Teacher's Note:
a) Always state the approximation \(R \gt\gt h\) clearly during the derivation.
b) The coverage area for broadcast can also be calculated as \(A = \pi d^2 = 2\pi R h\).
c) Students often forget to include the radius of earth \(R\) in the geometric triangle.

14. What is meant by the term modulation? Explain with the help of a block diagram, how the process of modulation is carried out in AM broadcasts? [3 Marks]

Answer:
1. Modulation is the process in which some characteristic of a carrier wave is varied in accordance with the instantaneous value of the baseband (modulating) signal.
2. It is required for long-distance transmission, smaller antenna size, and avoiding mixing of signals.
3. Block diagram involves a modulating signal source and a high-frequency carrier oscillator feeding into a modulator stage to produce the amplitude modulated wave.
[Figure: Block diagram showing a modulating signal and a carrier wave entering a modulator box to yield an AM wave output]
4. In AM broadcasts, the amplitude of the high-frequency carrier wave is changed in proportion to the amplitude of the audio modulating signal.

Teacher's Note:
a) Emphasize why low-frequency signals cannot be transmitted directly.
b) Draw clear block diagrams with labeled arrows indicating signal direction.
c) Mention the three types of modulation: amplitude, frequency, and phase.

15. What is meant by 'production' of a modulated carrier wave? Describe briefly the essential steps with block diagram production. [3 Marks]

Answer:
1. Production of a modulated carrier wave refers to the generation of a wave where the parameters of a high-frequency carrier are controlled by a low-frequency message signal.
2. Essential steps include generating a high-frequency carrier using an oscillator and producing a baseband message signal using a transducer like a microphone.
3. Both signals are fed into a non-linear device or modulator circuit.
[Figure: Block diagram showing audio signal and carrier oscillator connected to a modulator followed by a power amplifier and transmitting antenna]
4. The output of the modulator is passed through a power amplifier to obtain sufficient signal strength before feeding into the transmitting antenna.

Teacher's Note:
a) Focus on the role of the non-linear element in generating sidebands.
b) Include the power amplifier stage in the block description for completeness.
c) Link production directly to the transmitter side of communication systems.

16. What is meant by 'detection' of a modulated carrier wave? Describe briefly the essential steps with block diagram detection. [3 Marks]

Answer:
1. Detection (or demodulation) is the process of recovering the original modulating baseband signal from the received amplitude modulated wave.
2. The modulated wave received from the antenna is first passed through a rectifier to remove the negative half cycles or envelope.
3. The rectified wave is then passed through an envelope detector (typically a parallel combination of resistor and capacitor) to filter out high-frequency carrier components.
[Figure: Block diagram showing AM input, detector/rectifier, low-pass filter, and output baseband audio signal]
4. A blocking capacitor or low-pass filter removes any remaining radio frequency ripples, leaving the pure audio signal.

Teacher's Note:
a) Detection is essentially the reverse process of modulation.
b) Explain the role of the RC network as a low-pass filter clearly.
c) Mention that the diode acts as a rectifier during the demodulation process.