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Communication Systems

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Communication Systems PDF Notes, Important Questions and Synopsis


SYNOPSIS

  • Basic elements of a communication system
    Transmitter. The function of the transmitter is to transform the message or information (e.g. music, speech, picture, written message etc.) into a suitable form and transmit it over the communication channel.
  • Communication channel. It is the medium by which the modulated waves are transmitted from the transmitter to the receiver.
  • Receiver. The function of the receiver is to receive the modulated waves transmitted by the transmitter and to do demodulation or detection or decoding.
  • Analog and digital modes of transmission: An analog message is a physical quantity which varies with time usually in a smooth and continuous fashion.
  • A digital message is an ordered sequence of symbols selected from a finite set of discrete elements.
  • Bandwidth of signals
  • Speech signals require bandwidth=3100 Hz-300 Hz=2800 Hz.
    Bandwidth of video signals. For transmission of pictures, the video signals require a bandwidth of     about 4.2 MHz.
    Bandwidth of transmission medium
    Coaxial cables. These are used for signals below 18 GHz. The commonly used coaxial cables have a bandwidth of 750 MHz
  • Optical fibers. Optical fibers are suitable for microwaves and UV waves. The bandwidth of anoptical fiber is about  Hz.
  • Operational advantages of a digital communication system over an analog communication system are
  1.   An improved security message
  2.   Increased immunity to noise and external interference
  3.   A common format for encoding different kinds of message signals for the purpose of transmission
  4.   Flexibility in configuration digital communication system
  • Propagation of electromagnetic waves
    Ground wave propagation: In ground wave propagation, the radio waves travel along the surface of earth. This mode of propagation is possible only when the transmitting and receiving antennas are closeto the surface of the Earth.
    Sky wave propagation: When the radio waves from the transmitting antenna reach the receiving antenna after reflection from the ionosphere, it is called sky wave propagation or ionosphere propagation.
    Space wave propagation: At frequencies above 30 MHz ground waves are quickly attenuated and at frequencies above 30 MHz, the ionosphere is unable to reflect the sky waves back to earth. Therefore, we use direct waves which travel directly in straight lines from the transmitting antenna to the receiving antenna which is known as space wave propagation.
  • Attenuation, distortion, interference and noise are the undesirable effects in the source of signal transmission.
  • Modulation:The process of changing some characteristic, e.g. amplitude, frequency or phase, of a carrier wave in accordance with the intensity of the signal is known as modulation.

  • Types of modulation
  1.   Amplitude modulation
  2.   Frequency modulation
  3.   Phase modulation
  • Amplitude modulation: The amplitude of the carrier wave changes according to the intensity of the signal. The amplitude variation of the carrier wave is at the signal frequency ¦s.
  • Amplitude modulated signals contains frequencies . Amplitude modulated wave can be produced by application of message signal and carrier wave to a non-linear device, followed by a band pass filter.
  • Frequency modulation
    When the frequency of carrier wave is changed in accordance with the intensity of the signal, it is
    called frequency modulation.
  • Modulation factor: The ratio of the change of amplitude of the carrier wave to the amplitude of the normal carrier wave is called the modulation factor (m).
  • Pulse modulation can be classified as pulse amplitude modulation (PAM), pulse duration modulation (PDM) or pulse width modulation (PWM) and pulse position modulation (PPM).
  • Demodulation: Demodulation is the process of recovering the signal intelligence from a modulated carrier wave.
  • Fax: Facsimile or fax is the exact reproduction of a document, picture, letter, map etc. at the receiver’s end.
  • Modem: The term modem means modulator–demodulator. It converts a series of binary pulses of digital information into an analog signal and transmits across the phone lines.
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