Friday, 18 October 2013

Noise in communication systems


In every communication system, unwanted random signals or noise is present. Noise is defined as an unwanted form of electrical signal which tends to interfere with the desired signals. Many disturbances of an electrical nature produce noise in receivers, modifying the signal in an unwanted manner.
  • In radio receiver noise may produce hiss in loud speaker output.
  • In television receivers it may appear as snow or confetti(colored snow) superimposed on the picture.
  • In pulse communication systems, noise may produce unwanted pulses which may cancel the wanted ones and lead to serious error in the detected signal.
Noise is thus seen as limiting the range of systems for a given transmitted power. It also affects the sensitivity(ability to amplify weak signals) of receivers.


External noise:-   originating from sources outside the receiver.

Man-made noise: 
               This noise is strongest in industrial area. The frequency of man-made noise ranges between 1 to 600 MHz. The intensity of noise made by human easily outstrips than that created by any other source, internal or external to the receiver.

Atmospheric noise:
              This noise arises from lightning during thunderstorms and other electrical disturbances occurring in the atmosphere. This noise becomes less at frequencies above 30 MHz because the effect of noise components is very less in the VHF(Very High Frequency) range and above.

Extra-terrestrial noise:
              It arises from sun and other distant stars.

Solar noise:
              It arises from the sun. Under normal conditions, there is a constant noise radiated from the sun simply because it is a large body heated at a very high temperature(6000 C on the surface).

Cosmic noise:
              This refers to noise coming from distant stars other than sun. These stars are also heated bodies with radiated signals spreading a wide range of frequency. This is the strongest component of noise in the range 20-120 MHz.

Internal noise:-   arising from the system itself.

Shot noise:
              Shot noise arises in electronic devices because of discrete nature of current flow in the device. When this noise is amplified and fed to loudspeaker it sounds like a shower of lead shots striking a metal plate and hence the name 'shot noise'.

Thermal noise:   
              This noise arises due to the random motion of free electrons in the conducting medium such as resistors. Each free electron inside a resistor is in motion due to its thermal energy. 


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