Vidyarthi Academy

Home NCERT Solutions Chapter Notes Test Papers Contact Us

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

Amplitude modulation

In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal.

Let c(t) = Ac sin ωct represent carrier wave and m(t) = Am sin ωmt represent the message or the modulating signal where ωm = 2πfm is the angular frequency of the message signal.

???

???

???

???

???

The modulated signal cm (t) can be written as,

Cmt=Ac+Amsinωmtsinωct

=Ac1+AmAcsinωmtsinωct 

Cmt=Acsinωct+μ Ac sinωmt sinωct

Where, μ = AmAc is the modulation index.

In practice, μ is kept ≤ 1 to avoid distortion.

Using the trigonometric relation,

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

We can write,

Cmt=Acsinωct+μAc2 cos(ωc-ωm)t-μAc2cos(ωc+ωm)t

Here ωc – ωm and ωc + ωm are called the lower side and upper side frequencies, respectively.

The modulated signal consists of the carrier wave of frequency ωc plus two sinusoidal waves each with a frequency slightly different from ωc, known as side bands.

???

If the broadcast frequencies (carrier waves) are sufficiently spaced out so that sidebands do not overlap, different stations can operate without interfering with each other.

Previous
Next