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CBSE NOTES CLASS 12 PHYSICS

CHAPTER 15 COMMUNICATION SYSTEMS

Communication systems

Block diagram of communication system

Modes of communication

Point-to-point communication

Point to multipoint communication-broadcast

Terms related to communication

Transmitter

Receiver

Communication channel

Transducer

Attenuation

Amplification

Repeater

Range of communication

Bandwidth

Bandwidth of some media types

Some wireless frequency bands

Different types of signals

Electrical signal

Noise signal

Analog signal

Digital signal

Coding systems

Rectangular waves

Role of atmosphere in propagation of electromagnetic waves

Different types of waves used in communication

Ground waves

Sky waves

Space waves

Modulation

Demodulation

Baseband signal

Need for modulation

How does size of the antenna related to the frequency of transmitted wave?

How is the effective power radiated by an antenna related to the frequency of transmitted wave?

How does mixing up of signals from different transmitters make modulation necessary?

Mechanism of modulation

Sinusoidal carrier

Pulsed shaped carrier

Amplitude modulation

Modulation index

Side bands

Production and transmission of amplitude modulated wave

Band pass filter

Detection of amplitude modulated wave

Internet

Main applications of Internet

Facsimile (FAX)

Mobile telephony

CBSE NOTES CLASS 12 PHYSICS

CHAPTER 15 COMMUNICATION SYSTEMS

Terms related to communication

Transmitter

A transmitter processes the original message signal so as to make it suitable for transmission through a channel and subsequent reception.

Receiver

When a transmitted signal propagates along the channel it may get distorted due to channel imperfection and noise.

The receiver reconstructs a recognisable form of the original message signal at the channel output for delivering it to the user of information.

Communication channel

The transmitter is located at one place, the receiver is located at some other place (far or near) separate from the transmitter.

The channel is the physical medium that connects the transmitter and receiver.

A channel may be

Transducer

If the output of the information source is a non-electrical signal like a voice signal, a transducer converts it to electrical form before giving it as an input to the transmitter.

Any device that converts one form of energy into another can be termed as a transducer.

An electrical transducer may be defined as a device that converts some physical variable (pressure, displacement, force, temperature, sound etc) into corresponding variations in the electrical signal at its output.

Attenuation

The loss of strength of a signal while propagating through a medium is known as attenuation.

Amplification

It is the process of increasing the amplitude (and consequently the strength) of a signal using an electronic circuit called the amplifier. Amplification is necessary to compensate for the attenuation of the signal in communication systems. The energy needed for additional signal strength is obtained from a DC power source. Amplification is done at a place between the source and the destination wherever signal strength becomes weaker than the required strength.

Repeater

A repeater is a combination of a receiver and a transmitter. 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. A communication satellite is a repeater station in space.

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Range of communication

It is the largest distance between a source and a destination up to which the signal is received with sufficient strength.

Bandwidth

Bandwidth refers to the frequency range over which, an equipment operates or the portion of the spectrum occupied by the signal.

Bandwidth of some media types

Media type

Bandwidth

Twisted-pair for analog voice applications

1MHz

Coaxial cable

1GHz

Microwave

100GHz

Satellite

100GHz

Fiber

75THz

Some wireless frequency bands

Service

Frequency bands

Comments

Standard AM broadcast

540-1600 kHz

FM broadcast

88-108 MHz

Television

54-72 MHz

76-88 MHz

174-216 MHz

420-890 MHz

VHF (very high frequencies)

TV

UHF (ultra high frequencies)

TV

Cellular Mobile Radio

896-901 MHz

840-935 MHz

Mobile to base station

Base station to mobile

Satellite Communication

5.925-6.425 GHz

3.7-4.2 GHz

Uplink

Downlink

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