Communication

Communication

• A communication system acts as a messenger. It consists of a transmitter, communication channel and a receiver. The message signal or information is mounted on the carrier wave suitably. This is called modulation.
• Modulation is the phenomenon of superimposing the audio frequency signals (called the modulating signals) on a radio frequency carrier wave. A carrier wave is represented by the equation
  
  v_e = v_c sin (sin w_ct + f)
  Here v_e is instantaneous voltage at time t. v_c is maximum voltage, w_c is angular frequency and f is phase angle with respect to some reference.
• Amplitude modulation is the process of changing the amplitude of a carrier wave in accordance with the amplitude of the audio frequency (AF) signal.
  Voltage equation is
  sin _ct
  here v_m = Instaneous voltage of modulating & w_m = angular frequency of modulating frequency f_m.
• Frequency Modulation (FM) is the process of changing the frequency of a carrier wave in accordance with the audio frequency signal.
  Deviation in frequency
  =
  Here f = Instantaneous frequency of the FM wave at time t.
  f_c = constant frequency of the carrier wave.
• Pulse Modulation
  
  Here the carrier wave is in the form of pulses.
  (i) Pulse amplitude modulation (PAM)
  (ii) Pulse width modulation (PWM)
  (iii) Pulse position modulation (PPM)
• Demodulation is the process of extracting the audio signal from the modulated wave.
  Note: 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.
• Space Communication is the communication process utilizing the physical space around the earth.
  
• Electromagnetic waves which are used in Radio, Television and other communication system are radio waves and microwaves. The radio waves emitted from a transmitter antenna can reach the receiver antenna by the following mode of operation.
  (i) Ground wave propagation.
  (ii) Sky wave propagation.
  (iii) Space wave propagation
  
  
  
• In ground wave propagation radio wave travel along the surface of the earth (following the curvature of earth).
  
  
  
• In sky wave propagation, the waves which are reflected back to the earth by ionosphere.
  
  
  
• The space waves are the radio waves of very high frequency (30 MHz to 300 MHz) ultra high frequency (300 MHz to 3000 MHz) and microwave. (more than 3000 MHz). At such high frequencies, the sky wave as well as ground wave propagation both foils.
  
  
  
• Space wave propagation can be utilized for transmitting high frequency TV and FM signals.
  
  Television signal propagation : Frequency range for propagation is 80 MHz to 200 MHz
  
  
  Height of transmitting antenna :
  H = d = distance covered by the signal, R = Radius of the earth,
  Area covered A = d² = 2Rh
  Population cover : population density × Area covered.
  
• Satellite Communication is a mode of communication of signal between transmitter and receiver through satellite. It is like the line of sight microwave transmission. In this case, a beam of modulated microwave is projected towards the satellite.
  
• Remote sensing is a technique which is used to observe and measure the characteristic of the object at a distance.
• Optical Communication is a system by which we transfer the information from one place to another.
  
  The Optical fibres are used to transmit light signals from one place to another without any practical loss in the intensity of light signal.
• Optical fibre is mode of thin glass core (diameter 10 to 100 m) surrounded by a glass coating called cladding are protected by a jacket of plastic.
  It works on the principle of total internal reflection; There are three types of optical fibre.
  
  
  (i) Single mode optical fibre has a very narrow core of diameter about 5 mm. It is very weak in working and hence has limited practical use.
  
  (ii) Multimode optical fibre : Constructed similar to that of single mode step index fibre with the only difference that the central core is much bigger
  
  (iii) Graded index multimode fibre : Refractive index decreases smoothly from its centre to the outer surface of the fibre.
  
  There is no noticeable boundary between core and cladding.
  
  Note: Glass-core and glass cladding often called SCS fibre i.e., silica and silica fibre have the best propagation characteristics.