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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

Need for modulation

  1. Size of the antenna or aerial

    The antenna should have a size comparable to the wavelength of the signal (at least λ/4 in dimension) to sense the time variation of the signal. For an electromagnetic wave of frequency 20 kHz, the wavelength λ is 15 km. Hence direct transmission of such baseband signals is not practical due to large antenna requirement. We can transmit with reasonable antenna lengths if transmission frequency is high (for example, if ν is 1 MHz, then λ is 300 m). Therefore, there is a need of translating the information contained in the original low frequency baseband signal into high or radio frequencies before transmission.


  2. Effective power radiated by an antenna

    Power radiated by an antenna is related to length L of the antenna and wavelength λ as follows,

    P  ∝ Lλ2

    Therefore, for the same antenna length, the power radiated increases with decreasing λ, i.e., increasing frequency. Hence, the effective power radiated by a long wavelength baseband signal would be small. For a good transmission, we need high powers and hence the need of using high frequency transmission.


  3. Mixing up of signals from different transmitters

    If many people are talking at the same time or many transmitters are transmitting baseband information signals simultaneously, all these signals will get mixed up and there is no simple way to distinguish between them. Using communication at high frequencies and allotting a band of frequencies to each message signal for its transmission can solve the problem.

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