CBSE Class 12 Physics Chapter Notes on Communication System

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CBSE Class 12 Physics Chapter Notes on Communication System

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CBSE Class 12th Physics Notes: Notes: Communication Communication Systems (Part – II) Feb 3, 2017 12:18 IST ि वापन

MAY AYAN ANK K UTTAM UTTAM

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CBSE Class 12th 12th Physics Physics Chapter Chapter wise wise Notes base based d on the the chapter chapter Commun Communicatio ication n Systems (Chapter (Chapter 15 of Class 12 Physics NCERT N CERT textbook) are available here.

These key notes are

important important for coming CBSE coming CBSE Class 12 Physics Physic s board exam 2017. These These notes are are continuation continuation of CBSE Class Class 12th Physics Notes: Notes: Communication Systems Systems (Part ‒ I).

In part I, we I, we have studied about communication system, elements of communication system and basic terminology used in electronic communication systems etc. Now in part II II we will study about the topics given below

Bandwidth of Signals

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Bandwidth of Signals Bandwidth refers to the frequency range over which an equipment operates or the portion of the http://www.jagranjosh.com/articles/cbse-class-12th-physics-notes-communication-systems-part-ii-1486108555-1

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spectrum occupied by the signal. Dierent types of signals (music, picture or computer data)

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require dierent bandwidth. Speech signal requires a bandwidth of 2800 Hz (3100 Hz – 300 Hz) for telephonic conversation.

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To transmit music signal an approximate bandwidth of 20 kHz is required because of the high frequencies produced by the musical instruments. The audible range of frequencies extends from 20 Hz to 20 kHz. Video signals for transmission of pictures require about 4.2 MHz of bandwidth. A TV signal contains both voice and picture and is usually allocated 6 MHz of bandwidth for transmission. CBSE Class 12th Physics Notes: Dual Nature of Radiation and Matter

Bandwidth of Transmission Medium Dierent types of transmission media oer dierent bandwidths. The commonly used transmission media are wire, free space and ber optic cable. Coaxial cable oers a bandwidth of approximately 750 MHz. Such cables are normally operated below 18 GHz.

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Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz.

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Range of frequencies used for dierent services are given below Sub-Category Some Important Wireless Communication Frequency Bands

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Propagation of Electromagnetic Waves Various propagation modes for em waves are shown in the gure given below

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Image Source: NCERT textbooks

Ground wave propagation Radiowaves traveling through atmosphere and moving along the surface of the earth are termed as ground waves. Ground wave propagation is suitable for low frequencies (500 kHz to 1500kHz) or for radio

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broadcast at long wavelength i.e., upto 1 MHz.

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To radiate signals with high eciency, the antennas should have a size comparable to the wavelength λ of the signal (at least ~ λ/4). At

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The frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by ionospheric reection of radio waves back towards the earth. This mode of propagation is called sky wave propagation and is used by short wave broadcast services. The ionosphere is further subdivided into several layers as given below in table:


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The ionospheric layer acts as a reector for a certain range of frequencies (3 to 30 MHz). Electromagnetic waves of frequencies higher than 30 MHz penetrate the ionosphere and escape. The phenomenon of bending of em waves so that they are diverted towards the earth is similar to total internal reection in optics. CBSE Class 12 Video Tutorials

Space wave propagation Television broadcast, microwave links and satellite communication are some examples of communication systems that use space wave mode of propagation. A space wave travels in a straight line from transmitting antenna to the receiving antenna. Space waves are used for line-of-sight (LOS) communication as well as satellite communication. At frequencies above 40 MHz, communication is essentially limited to line-of-sight paths.


If the transmitting antenna is at a height h T , then you can show that the distance to the horizon d T is given as d T is given as, d T = (2Rh T )1/2, where R is the radius of the earth (approx. 6400 km). dT is also know as the radio horizon of the transmitting antenna.

From the gure given above, maximum line-of-sight distance d M between the two antennas having heights h T and h R above the earth is given by d M = (2Rh T )1/2 + (2Rh R )1/2

Here, h R is the height of receiving antenna. Space waves are of very high frequency (30 MHz to 300 MHz). Space waves can travel through atmosphere directly from one point to another. Height of the transmission antenna can be calculated from the relation, d = (2hr )1/2 CBSE Class 12 Chapter Notes: All Subjects

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CBSE Class 12 Physics Chapter Notes on Communication System

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