SOURCE OF NOISE
- 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.
- External Noise
- 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.
- Atmospheric Noise
- Extraterrestrial Noise
- 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.
- Shot Noise
- Transit Time Noise
- 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 in is 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.
- Base time delay
- Emitter time delay
- 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, kB 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:
Pn α TB
Where, k = Boltzman’s constant (1.38 x 10-23 J/K)
T = temperature in degrees Kelvin
B = bandwidth in Hz