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Mar 11

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COMMUNICATION SYSTEMS

IMPORTANT POINTS TO REMEMBER

  • Communication system is a system that transmits information from one point to another. The three basic components of a communication system are :
  1. Transmitter
  2. Communication channel
  3. Receiver
  • Transmitter is a device that transmits the receiving end through a communication channel.It basically transforms a message signal into a suitable form for transmission to the receiver. A transmitter consists of :
  1. A signal generator
  2. A modulator
  3. An antenna
  • Message signal is a time varying electrical signal produced by a transducer,a device that converts energy in one form to another. This signal carries information. There are two types of message signals:
  1. Analog, and
  2. Digital
  • A signal has two important characteristics-(a) amplitude ,and (b) frequency or phase.Analog signal is a continuous function of time.It may take any value in a given range. Digital signal is a discrete signal discontinuous in time.They are sampled with time.
  • Bandwidth is the range over which frequencies in a signal vary
  • Base band is the range of frequency representing the original signal;the one delivered by source.
  • Modulation is the modification of a message signal by varying some of the characteristics of a wave to transmit information. The radio frequency carrier wave is superimposed by an audio frequency signal.
  • There are three types of modulation:  
  •     (a)Amplitude modulation
  •     (b)Frequency modulation
  •     (c) Pulse modulation,

(a) Amplitude modulation: The amplitude of the carrier wave is varied in proportion to amplitude of modulating signal.

(b) Frequency modulation: The frequency of a carrier wave is varied in accordance with the modulating signal.

(c) Pulse modulation: It is the process of sampling signal at periodic  intervals. The above modulation techniques are used for transforming digital signal to analog signal.

(1) The analog pulse modulation is of two types:

(a) Pulse Amplitude Modulation (PAM)

(b) Pulse Time Modulation (PTM)

(2)  Pulse digital modulation is also of two types.

(a)Pulse Code Modulation (PCM)

(b) Pulse Delta Modulation (PDM)

Antenna is a conductor which radiates or intercepts  waves in space carrying information.The design of antenna depends upon (a)frequency of carrier wave (b) Directivity of the beam.There are two       types of antenna-(a) Dipole type (b) Dish type.

The medium  that links a transmitter and a receiver is called a communication channel.They are of two types-(a) Guided media and (b) Unguided media .In a guided media, communication is from one point to the other. Some of them are (1) Twisted Pair (2) Coaxial cable (3) Optical fibre.

In unguided media, waves travel through space from transmitter to receiver. On the basis of communication channel used , communication is classified into (a) line communication (b)optical communication (c) space communication.

Receiver’s function is to reconstruct the original message after it is propagated through the communication channel.The process of separating  carrier wave and the modulating signal is called demodulation.

Modem (Modulator +Demodulator ) is a device connecting one computer to another using telephone lines. It converts a digital signal to analog and vice versa.

Fax is a device to reproduce an exact document at the receiving end. The process involves the following steps:

(1) Conversion of the document into digital form

(2)Transmission of digital data through communication channel

(3) Reproduction of document at the receiving end.

Space communication is the processing ,sending and receiving information using space as a communication medium.The signal to be transmitted is emitted from the antenna at the transmitting end and received at the receiving end.Depending on the frequency of two waves to be transmitted, space communication have different types:

(1)surface wave or ground wave propagation  (upto 2 MHz)

(2) Sky wave propagation –2 MHz to 30 MHz

(3) Space wave propagation –30 MHz to 300 MHz.

Skip distance is the distance of the transmitter along the surface of the earth to the point where sky wave returns to earth  after reflection from ionosphere.

Critical frequency (CF) is the highest frequency of radiowaves incident normally to the ionosphere when the wave is reflected and returns to earth.If frequency is more than critical it will not return.

Maximum usable frequency (MUF) is the highest frequency of radio wave which when incident at some angle gets reflected back to earth.

MUF=CF/cos? where ?  is the angle between normal and incident ray.

Fading is the variation in the strength of the signal received due to the interference of waves.

Satellite communication is the mode of communication where satellite acts as a link between transmitter and receiver.The satellite receives signal from the earth station and returns it back to the earth after amplification at some different frequency to avoid  interference.The transmission of data is at higher rate.The orbits of a communication satellite are of three types:

           (1)Inclined-elliptical

           (2)Polar-circular

           (3)Geostationary

Remote sensing is a science of obtaining information about an area by a sensor which has no direct contact with the target.The satellite equipped with the above sensors takes photograph and collects other datas and transmits them to the earth station.The above technique is used to prepare wasteland maps,ground water survey , weather forecasting resource exploration etc.

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