- 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