• Noise means any unwanted sound.
  • It can also be defined as unwanted form of electrical energy which tend to interfere with wanted signals.

Classification of noise :

According to the source of noise it can be classified as below.

  1. External Noise
  2. Internal Noise

External Noise :

  • It is external to the communication equipment like radio receiver,television,telephone,etc…
  • External nose are uncontrollable at a given geographical location.
  • External noise can be further classified into three types as follows.
  1. Atmospheric Noise
  2. Extraterrestrial Noise
  3. Solar Noise

a.Atmospheric Noise :

  • Atmospheric noise is radio noise caused by natural atmospheric processes, primarily lightning discharges in thunderstorm.
  • Atmospheric noise is commonly called static electricity noise.
  • Atmospheric noise has wide range of frequency hence it has large bandwidth.
  • Amplitude of this noise is inversely proportional to the frequency.

b.Extra-terrestrial Noise :

  • Noise consists of electrical signals that originate from outside Earth’s atmosphere.
  • It is sometimes called as deep space noise.
  • It has frequency range from 8MHz to 1.43GHz.
  • Extraterrestrial nose can be divided into two categories.
  • Solar Noise
  • Cosmic Noise
  • Solar Noise :
  • Source of this noise is sun.
  • This noise is generated due to constant radiation from sun at high temperature.
  • Spectrum of this radiation is broad and interferes with communication channels.
  • Magnitude of this noise changes with time.
  • Cosmic Noise :
  • Source of this noise is stars.
  • These stars are at large distance and they have also high temperature due to which they are radiating cosmic noise.
  • Cosmic noise uniformly distributed over entire sky and uniformly distributed over frequency spectrum
  • If we receive it from galaxy, it is known as galactic noise.

c.Industrial Noise :

  • Source of industrial noise are automobile-air craft ignition,electric motor-switch gears,fluorescent light,etc.. a
  • It is man-made noise.

Internal Noise :

  • Electrical interference generated within a device or circuit itself.
  • This is due to the functioning of active and passive component inside the device itself.
  • Internal noise can be reduced by proper receiver design.
  • Internal noise can be further classified into three types as follows.
  1. Shot Noise
  2. Transit Time Noise
  3. Thermal Noise

a.Shot Noise :

  • The other source of internal noise in communication receiver is shot noise.
  • Active devices like diode,transistor,etc… has shot noise due to electron and holes coming randomly at the output electrodes.
  • It is occur in bipolar transistor.
  • The root-mean-square value of the shot noise current iis given by the Schottky formula


where , I – DC current.

q – charge of electron.

∆B – bandwidth in hertz.

b.Transit Time Noise :

  • This noise occurs in the semiconductor devices, where the transit time of the carrier crossing the junction is comparable with periodic time of the signal.
  • In this case some of the carriers may defuse back to the source and thus transit time noise occurs.
  • Transit time noise occurs at high frequency only.
  • Based on which equipment is taking more time to transmit carrier signal time delay can be divided into three types.
  1. Base time delay
  2. Emitter time delay
  3. Collector time delay

c.Thermal Noise :

  • It is sometimes called as Johnson or Nyquist noise.
  • It is generated by the random thermal motion of charge carriers, inside an electrical conductor, which happens regardless of any applied voltage.
  • Thermal noise is approximately white, meaning that its power spectral density is nearly equal throughout the frequency spectrum.
  • The root mean square(RMS) voltage due to thermal noise vn, generated in a resistance R(ohms) over bandwidth ∆f(hertz), gven by

Vn = √4kBTR∆f

where,                         k is Boltzmann’s constant (J/K).

T is the resistor’s absolute temperature (kelvin).

  • An example of such a noise source may be a cable or transmission line.


  • The noise power is given by:



Where,             k = Boltzman’s constant (1.38 x 10-23 J/K)

T = temperature in degrees Kelvin

B = bandwidth in Hz