Principles of Communication Revision Notes - IIT JEE/NEET Preparation | Nucleon

Principles of Communication

  • INTRODUCTION
  • Communication is the act of transmission of information. Every living creature in
    the world experiences the need to impart or receive information almost continuously
    with others in the surrounding world. For communication to be successful

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  • INTRODUCTION
  • Communication is the act of transmission of information. Every living creature in
    the world experiences the need to impart or receive information almost continuously
    with others in the surrounding world. For communication to be successful

    Read more
  • ELEMENTS OF A COMMUNICATION SYSTEM
  • Communication pervades all stages of life of all living creatures. Irrespective of its nature, every communication
    system has three essential elements transmitter, medium/channel and receiver. The block diagram shown in
    figure-1 depicts the general form of a communication system.

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  • BASIC TERMINOLOGY USED IN ELECTRONIC COMMUNICATION SYSTEMS
  • BASIC TERMINOLOGY USED IN ELECTRONIC COMMUNICATION SYSTEMS

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  • BANDWIDTH OF SIGNALS
  • In a communication system, the message signal can be voice, music picture or computer data. Each of these
    signals has different ranges of frequencies. The type of communication system needed for a given signal depends
    on the band of frequencies which is considered essential for the communication process.

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  • BANDWIDTH OF TRANSMISSION MEDIUM
  • Similar to message signals, different type of transmission media offer different bandwidths. The commonly
    used transmission media are wire, free space and fiber optic cable.Coaxial cable is a widely used wire medium,
    which offers a bandwidth of approximately 750 MHz. Such cables are normally operated below
    18 GHz. Communication through free space using radio waves takes place over a very wide range of frequencies:

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  • PROPAGATION OF ELECTROMAGNETIC WAVES
  • In communication using radio waves, an anetnna at the transmitter radiates the electromagnetic wave (em
    waves), which travel through the space and reach the receiving antenna at the other end. As the em wave travels
    away from the transmitter, the strength of the wave keeps on decreasing. Several factors influenece the propagation
    of em waves and the path they follow. At this point, it is also important to understand the composition of the
    earth's atmosphere as it plays a vital role in the propagation of em waves.

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  • Sky waves
  • In the frequency range from a few MHz up to 30 to 40 MHz, long distance communication can be achieved by
    ionospheric reflection 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 so called because of the presence
    of a large number of ions or charged particles. It extends from a height of ~65 Km to about 400 Km above the
    earth's surface. Ionisation occurs due to the absorption of the ultraviolet and other high-energy radiation coming
    from the sun by air molecules. The ionosphere is further subdivided into several layers.

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  • Space wave
  • Another mode of radio wave propagation is by space waves. 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. At these frequencies, the antennas are relatively smaller and can be placed at heights of many wavelengths
    above the ground. Because of line-of-sight nature of propagation, direct waves get blocked at some point
    by the curvature of the earth as illustrated in figure-5 below

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  • Modulation and its necessity
  • As already mentioned, the purpose of a communication system is to transmit information or message signals.
    Message signals are also called baseband signals, which essentially designate the band of frequencies representing
    the original signal, as delivered by the source of information. No signal, in general, is a single frequency
    sinusoid, but it spreads over a range of frequencies called the signal bandwidth

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  • Size of the antenna or aerial
  • For transmitting a signal, we need an antenna or an aerial. This antenna should have a size comparable to the
    wavelength of the signal (at least ?/4 in dimension) so that the antenna properly senses the time variation of the
    signal. For an electromagnetic wave of frequency 20 kHz. the wavelength ? is 15 km

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  • Effective power radiated by an antenna
  • A theoretical study of radiation from a linear antenna

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  • Mixing up of signals from different transmitters
  • Another important argument against transmitting basband signals directly is more practical in nature. Suppose
    many people are talking at the same time or many transmitters are transmitting baseband information signals
    simultaneously. As these signals will get mixed up and there is no simple way to distinguish between them. This
    points out towards a possible solution by using communication at high frequencies and allotting a band of
    frequencies to each message signal for its transmission.

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  • Amplitude Modulation
  • In amplitude modulation the amplitude of the carrier is varied in accordance with the information signal. Here we
    explain amplitude modulation process using a sinusoidal signal as the modulating signal

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  • Side band frequencies and band width in AM wave
  • Side band frequencies  The AM wave contains three frequency

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  • PRODUCTION OF AMPLITUDE MODULATED WAVE
  • Amplitude modulation can be produced by a variety of methods. A conceptually simple method is shown in the
    block diagram of figure–10

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  • DETECTION OF AMPLITUDE MODULATED WAVE
  • The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore
    to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is
    usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the
    detection. The detected signal may not be strong enough to be made use of and hence is required to be
    amplified. A block diagram of a typical receiver is shown in figure-12.

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  • DETECTION OF AMPLITUDE MODULATED WAVE
  • The transmitted message gets attenuated in propagating through the channel. The receiving antenna is therefore
    to be followed by an amplifier and a detector. In addition, to facilitate further processing, the carrier frequency is
    usually changed to a lower frequency by what is called an intermediate frequency (IF) stage preceding the
    detection. The detected signal may not be strong enough to be made use of and hence is required to be
    amplified. A block diagram of a typical receiver is shown in figure-12.

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  • FREQUENCY MODULATION
  • It is the second type of continuous wave or sinusoidal wave modulation.
    In this mode of modulation, the frequency of the carrier signal varies in accordance with the modulating signal.
    The amplitude of the carrier wave is fixed while its frequency is changing.
    In frequency modulation, the frequency of the carrier wave is modified in accordance with the amplitude of the
    modulating wave.

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  • NECESSITY OF FREQUENCY MODULATION
  • Various electrical machines and noises cause amplitude disturbance in the transmission
    of amplitude-modulated wave. This makes the reception noisy. So, there is
    a need for different type of modulation which

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  • FM is preferred for transmission of music
  • Frequency modulation (FM) gives better quality transmission and has a larger bandwidth. In
    FM signals, the intelligence (information or message signal) is in the form of frequency variations
    and, therefore, the atmospheric or man-made noises (which are generally amplitude

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  • Frequency Modulation (FM)
  • The process of changing the frequency of a carrier wave in accordance with the audio frequency signal is
    known as frequency modulation

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  • FREQUENCY-MODULATED COMMUNICATION (Height of Transmitting Antenna)
  • The TV signals are frequency-modulated. Their transmission cannot be obtained by ground wave propagation.
    This is because the signals get absorbed by ground due to their high frequency. The transmission via sky wave
    propagation is also not desirable. This is becasuse the ionosphere is unable to reflect radio waves of frequencies
    greater than 40 MHz

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  • Pulse code Modulation
  • The pulse amplitude, pulse width and pulse position modulations not completely digital.
    A completely digital modulation is obtained by pulse code modulation (PCM).
    An analog signal is pulse code modulated by following three operation.

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  • Demodulation
  • The process of extracting the audio signal from the modulated wave is known as
    demodulation or detection

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  • Simple demodulator circuit
  • A diode can be used to detect or demodulate an amplitude modulated (AM) wave. A diode basically acts as
    a rectifier i.e. it reduces the modulated carrier wave into positive envelope only.

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  • Data Transmission and Retrieval
  • The term data is applied to a representation of facts, concepts or instructions suitable for communication
    interpretation or processing by human beings or by automatic means. Data in most cases consists of
    pulse type of signals

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