Other Important Chapter/Topics for Competitive Examinations Preparation.

UNIT – X Communication Systems

CHAPTER COMMUNICATION SYSTEMS

Chapter Analysis

List of Topics 2016 2017 2018

D D OD OD D/OD

Elements of 1 Q Communication System 1 Q (2 marks) 1 Q

(2 marks) 1 Q (3 marks)

(3 marks)

Modulation 1 Q 1 Q

(1 mark) 1 Q 1 Q 1 Q (2 marks)

1 Q (3 marks) (1 mark) (3 marks) 1 Q

(3 marks) (3 marks)

TOPIC-1

Elements of Communication System

Revision Notes

TOPIC - 1

Elements of Communication System

... P. 391

• Communication is the method of transmitting and receiving information of data.

> There may be two types of communication :

TOPIC - 2

Modulation .... P. 399

• Point to point communication : In this type of communication transmitter and receiver are single point. Example: Telephonic communication.
• Broadcasting communication: In this type of communication, there is one transmitter and many receivers. Example: transmission of radio or television.

> When input message (any form of signal variation) is combined with some intelligence input (encoding and compatibility in machine language), it becomes information.

• Generalised communication system :

Communication System

Information Message Source Signal Transmitter Signal Signal Received ►Receiver Transmitted Channel Message Information Jser of

Signal

Noise

Block diagram of a generalised Communication System

392 ]

Oswaal CBSE Chapterwise & Topicwise Question Bank, PHYSICS, Class – XII

Important terminology and their application in communication system

> Information source : The information source is the generator of information which we want to communicate. It may be audio, video or data.

Electric Transducer: Electric transducer convert physical variable into electrical signal variable.

Signal: Information converted in electrical form and suitable for transmission is called signal.

> Transmitter: Process of the incoming message signal and making it suitable for transmission through particular channel.

Noise: Unwanted signal which interferes with the information signal and disturbs the information.

> Channel: It is the medium through which signal from transmitter propagates to the receiver. For example: optical fibre, coaxial cables etc.

Receiver: Collects the message from the channel and extracts signal.

• Attenuation: Signal loses energy during propagation through channel. This is called attenuation.

➣ Amplifier: It is the device which increases the strength by increasing its amplitude.

> Range: Maximum distance between transmitter and receiver at which signal can be recovered is called the range of communicating system.

Bandwidth: It is the frequency range over which an equipment operates or range of frequencies a signal has.

➣ Modulation: Mixing of signal with carrier frequency is known as modulation.

Demodulation: Extracting of signal from carrier frequency is known as demodulation.

Repeater: It receives the signal, reconditions it and then retransmits it.

Signal may be classified in two categories :

> Analog Signal : Continuous variation of signal with respect to time is known as analog signal. For example : telephonic signal, video signal etc.

In modern technology we can convert analog signal to digital signal for communicating and convert back to analog signal at receiver.

Digital Signal: Discrete value of signal variation with respect to time is known as digital signal. For example: computer etc.

• Coding helps in sending digital signal with much more accuracy. There are several coding techniques. For example in computer data, we employ suitable combinations of number systems such as the binary coded decimal (BCD), American Standard Code for Information Interchange (ASCII)

Operational advantages of digital communication system over analog communication systems are.

An improved form of sending messages securely.

• Increased immunity to noise and external interference.
• A common format for encoding different kinds of message signals for the purpose of transmission.
• Flexibility in configuration of digital communication system.
• Hence in modern technology, analog signals are transmitted through digital communication. In the final stage they are converted back to analog signals.
• Different types of message signals have different range of frequencies.
• Audio signal – 20 Hz to 20kHz
• Video signal – 4.2 MHz
• TV signal – 6 MHz (audio + video)

➣ Large bandwidth is required to accommodate complete information of wave.

> Frequency bands of some important wireless communications :

Service Frequency bands Comments

Standard AM broadcast 540-1600 kHz FM broadcast 88-108 MHz Television 54-72 MHz VHF (very high frequencies)

76-88 MHz TV

174-216 MHz UHF (ultra high frequencies)

420-890 MHz TV

Cellular Mobile Radio 896-901 MHz Mobile to base station

840-935 MHz Base station to mobile

Satellite Communication 5.925-6.425 GHz Uplink

3.7-4.2 GHz Downlink