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Note for Analog and Digital Communication - ADC By ANNA SUPERKINGS

  • Analog and Digital Communication - ADC
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Course Material (Lecture Notes) UNIT I FUNDAMENTALS OF ANALOG COMMUNICATION Introduction In the Microbroadcasting services, a reliable radio communication system is of vital importance. The swiftly moving operations of modern communities require a degree of coordination made possible only by radio. Today, the radio is standard equipment in almost all vehicles, and the handie-talkie is a common sight in the populace. Until recently, a-m (amplitude modulation) communication was used universally. This system, however, has one great disadvantage: Random noise and other interference can cripple communication beyond the control of the operator. In the a-m receiver, interference has the same effect on the r-f signal as the intelligence being transmitted because they are of the same nature and inseperable. The engines, generators, and other electrical and mechanical systems of modern vehicles generate noise that can disable the a-m receiver. To avoid this a different type of modualation, such as p-m (phase modulation) or f-m (frequency modulation) is used. When the amplitude of the r-f (radio-frequency) signal is held constant and the intelligence transmitted by varying some other characteristic of the r-f signal, some of the disruptive effects of noise can be eliminated. In the last few years, f-m transmitters and receivers have become standard equipment in America, and their use in mobile equipments exceeds that of a-m transmitters and receivers. The widespread use of frequency modulation means that the technician must be prepared to repair a defective f-m unit, aline its tuned circuits, or correct an abnormal condition. To perform these duties, a thorough understanding of frequency modulation is necessary. Carrier Characteristics The r-f signal used to transmit intelligence from one point to another is called the carrier. It consists of an electromagnetic wave having amplitude, frequency, and phase. If the voltage variations of an r-f signal are graphed in respect to time, the result is a waveform such as that in figure 2. This curve of an unmodulated carrier is the same as those plotted for current or power variatons, and it can be used to investigate the general properties of carriers. The unmodulated carrier is a sine wave that repeats itself in definite intervals of time. It swings first in the positive and then in the negative direction about the time axis and represents changes in the amplitude of the wave. This action is similar to that of alternating current in a wire, where these swings represent reversals in the direction of current flow. It must be remembered that the plus and minus signs used in the figure represent direction only. The starting point of the curve in the figure 2 is chosen arbitrarily. It could have been taken at any other point just as well. Once a starting point is chosen, however, it represents the point from which time is measured. The starting point finds the curve at the top of its positive swing. The curve then swings through 0 to some maximum amplitude in the negative direction, returning through 0 to its original position. CS6304 – Analog And Digital Communication UNIT 1 Page 1

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Course Material (Lecture Notes) The changes in amplitude that take place in the interval of time then are repeated exactly so long as the carrier remains unmodulated. A full set of values occurring in any equal period of time, regardless of the starting point, constitutes one cycle of the carrier. This can be seen in the figure, where two cycles with different starting points are marked off. The number of these cycles that occur in 1 second is called the frequency of the wave. Noise Models CS6304 – Analog And Digital Communication UNIT 1 Page 2

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Lecture Notes