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# CBSE Class 12th Physics Notes for Chapter 15 Communication Systems

CHAPTER 15 COMMUNICATION SYSTEMS

• Communication is the act of transmission of information. Electronic communication refers to the faithful transfer of information in the form of electrical voltage and current from one point to another point.

• Transmitter, transmission channel and receiver are three basic units of a communication system.

• Two forms of communication system:
• Analog: Information transmitted in continuous waveform.
• Digital: Information transmitted has only discrete or quantized levels.

• Every message signal occupies a range of frequencies. The bandwidth of a message signal refers to the band of frequencies which are necessary for satisfactory transmission of the information contained in the signal.

• Low frequencies cannot be transmitted to long distances. Therefore, they are superimposed on a high frequency carrier signal by a process known as modulation.

• In modulation, some characteristics of the carrier signal like amplitude, frequency or phase varies in accordance with the modulating or message signal. Correspondingly, they are called Amplitude Modulated (AM), Frequency Modulated (FM) or Phase Modulated (PM).

• For transmission over long distances, signals are radiated into space using devices called antennas. The radiated signals propagate as electromagnetic waves and the mode of propagation is influenced by the presence of the earth and its atmosphere. Near the surface of the earth, electromagnetic waves propagate as surface waves. Surface wave propagation is useful up to a few MHz frequencies.

• Long distance communication between two points on the earth is achieved through reflection of electromagnetic waves by the ionosphere. Such waves are called sky waves. Sky wave propagation takes place up to frequency of about 30 MHz.

• If an antenna radiates electromagnetic waves from a height hT , then the range dT is given by√(2Rh_T ) where R is the radius of the earth.