Class 12 Physics Communication System Notes | Detailed Explanation with Questions

Communication is one of the most important aspects of modern science and technology. From sending a text message to live video streaming across the world, everything depends on a communication system.

In Class 12 Physics, the topic Communication System helps students understand the principles behind information transmission using electrical and electromagnetic signals. This chapter introduces key concepts like transmitters, receivers, modulation, propagation of waves, and modern communication technologies like satellite and optical fiber communication.

By the end of this chapter, students will clearly understand how signals are transmitted over long distances without significant loss or distortion.

Class 12 Physics Communication System Notes | Detailed Explanation with Questions

Basic Elements of a Communication System

A communication system mainly consists of three basic elements:

1. Transmitter

• The transmitter converts the original message or information into a signal suitable for transmission.

• It includes devices like microphones, amplifiers, and oscillators.

• The transmitter also performs modulation and amplification before sending the signal to the communication channel.

2. Communication Channel

• The channel is the physical medium through which the signal travels.

• It can be wired (optical fiber, coaxial cable) or wireless (radio waves, microwaves).

• The quality of transmission depends on the channel and the level of noise present.

3. Receiver

• The receiver captures the transmitted signal from the channel.

• It performs demodulation and amplification to recover the original message.

• Examples include television sets, radios, and mobile phones.

Types of Signals in Communication

1. Analog Signals

• These signals vary continuously with time.

• Examples: Human voice, electrical voltage from a microphone.

• Represented as a continuous waveform.

2. Digital Signals

• Digital signals have discrete values, often represented as 0s and 1s (binary code).

• They are less affected by noise and are used in modern communication systems.

• Examples: Computer data, mobile communication.

Bandwidth of Signals

• Bandwidth is the range of frequencies required to transmit a signal without distortion.

• Measured in Hertz (Hz).

• Examples:

  • Telephone Audio Signals → 300 Hz to 3.4 kHz

  • Television Video Signals → Up to 6 MHz

  • FM Radio → About 88 MHz to 108 MHz

Higher bandwidth means better data transmission and sound quality.

Noise in Communication System

Noise refers to any unwanted signal that mixes with the original message signal during transmission.

Types of Noise

1. Internal Noise:

   • Produced by the components of the communication system itself, e.g., thermal noise.

2. External Noise:

   • Comes from natural sources (like lightning) or man-made sources (like engines, machinery).

Noise reduces the signal-to-noise ratio and lowers the clarity of communication.

Modulation in Communication System

Modulation is the process of superimposing the message signal onto a high-frequency carrier wave for efficient transmission.

Need for Modulation

• To reduce antenna height: For a signal of frequency $f$, antenna height h=λ4h = \frac{λ}{4}h=4λ​. Low-frequency signals require very tall antennas.

• To increase range: High-frequency signals travel longer distances.

• For multiplexing: Multiple signals can be transmitted on different carrier frequencies.

• To reduce noise: High-frequency waves are less affected by external interference.

Types of Modulation

1. Amplitude Modulation (AM)

• The amplitude of the carrier wave varies as per the message signal.

• Frequency and phase remain constant.

• Used in AM radio broadcasting.

2. Frequency Modulation (FM)

• The frequency of the carrier wave changes as per the message signal.

• Amplitude and phase remain constant.

• Provides better sound quality and is used in FM radio and television sound transmission.

3. Phase Modulation (PM)

• The phase of the carrier wave changes as per the message signal.

• Used in digital communication systems.

Demodulation

Demodulation is the reverse process of modulation.

• At the receiver end, the original message signal is extracted from the modulated carrier wave.

• The process restores the information for audio, video, or data output.

Propagation of Electromagnetic Waves

Electromagnetic waves travel through space in different ways depending on frequency and distance.

1. Ground Wave Propagation

• Radio waves travel along the surface of the Earth.

• Suitable for frequencies up to 2 MHz.

• Used for AM broadcasting.

2. Sky Wave Propagation

• Waves are reflected back to Earth by the ionosphere.

• Suitable for 2 MHz to 30 MHz.

• Used for long-distance radio transmission.

3. Space Wave Propagation

• Direct transmission from transmitter to receiver.

• Suitable for frequencies above 30 MHz.

• Used in television, FM radio, and satellite communication.

Satellite Communication

Satellites are widely used for modern communication systems.

• A geostationary satellite remains fixed above a point on the equator at a height of 36,000 km.

• Communication occurs in two steps:

  • Uplink: Earth to satellite.

  • Downlink: Satellite to Earth.

Applications:

• Television broadcasting

• Weather forecasting

• GPS and telecommunication services

Optical Fiber Communication

• Optical fibers transmit data using light signals through thin glass fibers.

• Works on the principle of total internal reflection (TIR).

Advantages of Optical Fibers:

• High-speed transmission

• Low signal loss

• Immune to electromagnetic interference

• Compact and lightweight

Applications: Internet, medical endoscopy, cable television.

Internet and Mobile Communication

Internet Communication

• Uses optical fibers, satellites, and wireless networks to connect millions of computers worldwide.

• Services: Email, VoIP, Video conferencing, Cloud storage.

Mobile Communication

• Mobile networks divide areas into small regions called cells.

• Technologies: 2G, 3G, 4G, and 5G for faster speed and better quality.

Block Diagram of a Communication System

Information Source → Transmitter → Channel → Receiver → Destination

Each block plays a specific role:

• Source: Generates message.

• Transmitter: Processes and sends the signal.

• Channel: Medium for signal transmission.

• Receiver: Recovers the message.

Applications of Communication Systems

• Radio and Television Broadcasting

• Mobile Telephony

• Internet Services

• Satellite Navigation (GPS)

• Remote Sensing and Weather Forecasting

Objective Questions (MCQs)

Q1. Which of the following is not an element of the communication system?

a) Transmitter
b) Channel
c) Antenna
d) Receiver

Answer: Antenna

Q2. The range of frequencies required to transmit a signal without distortion is called:

a) Modulation
b) Bandwidth
c) Demodulation
d) Amplification

Answer: Bandwidth

Q3. The process of extracting the original message signal from the carrier wave is called:

a) Modulation
b) Amplification
c) Demodulation
d) Transmission

Answer: Demodulation

Q4. Which modulation is used in AM radio broadcasting?

a) Amplitude Modulation
b) Frequency Modulation
c) Phase Modulation
d) None of these

Answer: Amplitude Modulation

Q5. Sky wave propagation is effective for frequencies:

a) Below 2 MHz
b) Between 2 MHz and 30 MHz
c) Above 30 MHz
d) None of these

Answer: Between 2 MHz and 30 MHz

Q6. Which communication uses total internal reflection?

a) Satellite Communication
b) Optical Fiber Communication
c) Sky Wave Propagation
d) Ground Wave Propagation

Answer: Optical Fiber Communication

Q7. In frequency modulation, which parameter of the carrier wave remains constant?

a) Frequency
b) Amplitude
c) Phase
d) Both b and c

Answer: Amplitude and Phase

Q8. The antenna height for a frequency f is given by:

a) h = λ/2
b) h = λ/3
c) h = λ/4
d) h = λ

Answer: h = λ/4

Q9. Which type of modulation provides the best noise immunity?

a) AM
b) FM
c) PM
d) None

Answer: FM

Q10. The uplink in satellite communication refers to:

a) Earth to Satellite
b) Satellite to Earth
c) Satellite to Satellite
d) Earth to Earth

Answer: Earth to Satellite

Q11. What is the frequency range for FM radio transmission?

a) 2–20 MHz
b) 20–40 MHz
c) 88–108 MHz
d) 100–150 MHz

Answer: 88–108 MHz

Q12. Noise in communication refers to:

a) Desired signal
b) Unwanted signal
c) Carrier wave
d) Modulation

Answer: Unwanted signal

Q13. Which of the following is a digital signal?

a) Sine wave
b) Cosine wave
c) Square wave
d) Triangular wave

Answer: Square wave

Q14. In optical fiber, signal loss is mainly due to:

a) Scattering
b) Absorption
c) Dispersion
d) All of these

Answer: All of these

Q15. The ionosphere reflects which type of waves?

a) Microwaves
b) Radio waves
c) Infrared waves
d) X-rays

Answer: Radio waves

Q16. Which device converts sound into electrical signals?

a) Amplifier
b) Microphone
c) Loudspeaker
d) Oscillator

Answer: Microphone

Q17. Which mode of wave propagation is used in TV broadcasting?

a) Ground wave
b) Sky wave
c) Space wave
d) All of these

Answer: Space wave

Q18. What is the full form of GPS?

a) Global Position System
b) General Positioning System
c) Global Positioning System
d) Geographic Positioning System

Answer: Global Positioning System

Q19. Which modulation is used in digital communication?

a) FM
b) PM
c) AM
d) None

Answer: PM

Q20. The speed of light in vacuum is:

a) 3×106 m/s3 \times 10^6 \, m/s3×106m/s
b) 3×108 m/s3 \times 10^8 \, m/s3×108m/s
c) 3×104 m/s3 \times 10^4 \, m/s3×104m/s
d) 3×1010 m/s3 \times 10^{10} \, m/s3×1010m/s

Answer: 3×108 m/s3 \times 10^8 \, m/s3×108m/s

Short Answer Questions

Q1. What is modulation and why is it necessary?

Answer: Modulation is the process of superimposing a low-frequency message signal on a high-frequency carrier wave. It is necessary to increase the transmission range, reduce antenna size, improve signal quality, and enable multiplexing.

Q2. Define bandwidth. Give its importance.

Answer: Bandwidth is the frequency range required to transmit a signal without distortion. It determines the data-carrying capacity of a communication channel. Higher bandwidth means better signal quality and faster data transmission.

Q3. What is demodulation?

Answer: Demodulation is the process of extracting the original message signal from the modulated carrier wave at the receiver end.

Q4. List the applications of optical fiber communication.

Answer: Optical fiber communication is used in internet services, cable TV, medical imaging (endoscopy), and telecommunication.

Q5. What are the advantages of digital signals over analog signals?

Answer: Digital signals are less affected by noise, provide better quality, allow data compression, and are suitable for computer and mobile communication.

Long Answer Questions

Q1. Explain the elements of a communication system with a block diagram.

Answer: A communication system consists of a transmitter, channel, and receiver. The transmitter converts the information into signals, the channel transmits it, and the receiver reconstructs the original message. The block diagram is:
Information Source → Transmitter → Channel → Receiver → Destination.

Q2. Describe the types of wave propagation used in communication.

Answer: The three main types are:

• Ground Wave: Used for AM broadcasting below 2 MHz.

• Sky Wave: Used for long-distance transmission via the ionosphere between 2–30 MHz.

• Space Wave: Used for TV, FM, and satellite communication above 30 MHz.

Q3. What is satellite communication? Explain its working.

Answer: Satellite communication uses artificial satellites for data transmission. Uplink sends signals from Earth to satellite, and downlink sends amplified signals back to Earth. It is used in TV broadcasting, GPS, and weather forecasting.

FAQs

Q1. What is the need for modulation in communication systems?

Answer: To transmit signals over long distances with less noise and smaller antenna sizes, modulation is essential.

Q2. Which modulation is better – AM or FM?

Answer: FM is better than AM because it offers higher noise immunity and better sound quality.

Q3. Why is optical fiber preferred for internet communication?

Answer: Optical fiber offers high speed, low loss, and immunity to electromagnetic interference, making it ideal for high-speed data transfer.

Q4. What is the typical height of a geostationary satellite?

Answer: About 36,000 km above the Earth’s surface.

Q5. What is the main disadvantage of analog signals?

Answer: They are highly affected by noise and distortion, leading to poor quality over long distances.

Conclusion

The Communication System in Class 12 Physics is a fundamental topic that explains how information is transmitted efficiently over long distances. By learning about transmitters, receivers, modulation techniques, propagation methods, optical fibers, and satellites, students gain a clear understanding of modern telecommunication technologies that connect our world today.

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