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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

Production and transmission of amplitude modulated wave

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The modulating signal Am sin ωmt is added to the carrier signal Ac sin ωct to produce the signal x(t). This signal, x(t) = Am sin ωmt + Ac sin ωct is passed through a square law device which is a non-linear device which produces an output y(t) = Bx(t ) + Cx2(t) , where B and C are constants. Thus,

yt= BAmsinωmt+BAcsinωct

+ C Am2sin2ωmt+Ac2sin2ωct+2 AmAcsinωmtsinωct

Using the trigonometric relation,

sin2A =1 cos2A2

and

sin A sin B =12 [cos(A  B)  cos (A + B)]

we get,

yt= BAmsinωmt+BAcsinωct+ C2 Am2+Ac2- CAm22 cos2ωmt

-CAc22cos2ωct+CAmAccos(ωc-ωm)t-CAmAccos(ωc+ωm)t 

There is a DC term C/2(Am2+Ac2) and sinusoids of frequencies ωm, 2ωm, ωc, 2ωc, ωc – ωm and ωc + ωm. This signal is passed through a band pass filter which rejects DC and the sinusoids of frequencies ωm, 2ωm and 2ωc and retains the frequencies ωc, ωc – ωm and ωc + ωm. The device which only passes frequencies within a certain range and rejects frequencies outside that range is called band pass filter. The output of the band pass filter is of the same form as an AM wave.

The modulator is to be followed by a power amplifier which provides the necessary power and then the modulated signal is fed to an antenna of appropriate size for radiation.

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