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CBSE 12th Board Class 12 2018 : Physics

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SUMMARY Main elements of the communication system are : (1) Transmitter (2) Transmission channel and (3) Receiver. Transducer : The device which transforms one form of energy into another form of energy is called a transducer. Transmission Channel : Transmission channel is a link or medium between transmitter and receiver. The signals of information are propagating through this medium and received at receiver. Noise : Noise is an unwanted signal. They get admixed with the information signal in the transmission channel and distort the information signal. Signal : For a transmission, the information is transformed into electric signal is called a signal. There are two types of signals. (1) Analog signal and (2) Digital signal. Communication Systems 263 Bandwidth : In communication system the difference of maximum frequency and minimum frequency efficiently transmitted by the system is called the bandwidth of the system. The bandwidth of audio signal is 20 kHz and bandwidth of video signal is 4.2 MHz. Modulation : The process of superposing low frequency audio signals on high frequency is called modulation. Low frequency is called the modulating signal and high frequency wave is called the carrier waves. The resultant wave is known as modulated wave. There are three types of the modulation. (1) Amplitude modulation (AM), (2) Frequency modulation (FM) and (3) Phase modulation (PM). Amplitude Modulation : A modulation in which the amplitude of the carrier wave is changed according to instantaneous value of the modulating wave is called amplitude modulation. Here, phase and frequency of carrier wave remain unchanged. Modulation Index : The ratio of the amplitude of modulating wave and amplitude of carrier wave is called a modulation index. ma = Em Value of ma is always less than 1. Ec Demodulation : The process of separating information signal from carrier wave is called demodulation. This process is performed in the receiver. The circuit which performs the process of demodulation is called a detector circuit. Propagation of an Electromagnetic Waves : (1) The wave propagating along the surface of the earth are called ground wave or surface wave. The frequencies up to 2 MHz is propagating through ground waves. (2) The waves returned to the earth after getting reflected by the ionosphere at a height of about 60-400 km is known as sky waves. 2 MHz to 30 MHz frequencies are propagating through the sky waves. (3) The waves from the transmitting antenna, reaching a receiver in a straight line after the reflection from the ground are called space waves. 348

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