Noise

Introduction:-

Noise is probably the only topic in electronics and tele communications with which everyone must be familiar, electrical disturbances that interfere with signals produces noise and this noise ever present and limits the performance of the most of the systems. Measuring noise is very controversial almost everybody has a different method of quantifying noise and its effects.

definition:- noise is unwanted energy that interfere with the required signal. In receivers :- noise is disturbance in electric nature.

  • Radio receivers—> noise appears as “hiss”.
  • TV receivers —–> it appears a snow (or) colored snow pictures.
  • In Pulse communication systems —->noise produces unwanted pulses.

In receivers noise effects sensitivity and band width and it decreases sensitivity as well as band width.

  Basically noise can be classified as Internal and External noise .

External Noise Internal Noise
when noise sources are external to the receiver . i.e, noise source is located outside of the receiver. It is difficult to treat quantitatively external noise. Noise is created with in the receiver itself.i.e, noise source is internal to the receiver. internal noise can be treated quantitatively and reduction is also possible by appropriate receiver design.

External Noise:-

Atmospheric noise:- 

If we try to listen to short waves on a receiver which is not well equipped to receive them, an astonishing variety of strange sounds will be heard, all tending to interfere with the program. most of these sounds are the result of spurious sources of disturbance, which represents atmospheric noise generally called as “static”.

  • Atmospheric noise is caused by lightning discharge in thunderstorms and other natural electric disturbances occurring in the atmosphere.
  • It originates in the form of amplitude modulated impulses , and are spread over most of the RF spectrum normally used for broadcasting.
  • i.e, It consists of spurious radio signal with components distributed over a wide range of frequencies. Atmospheric noise propagates over the earth in the same way as ordinary Radio waves of the same frequencies.
  • Static is more severe in the case of Radio than that of Tele-vision and it becomes less severe at frequencies above 30 MHz. Since higher frequencies are limited to line of sight propagation.
  • This noise is created in VHF range and above.

Extraterrestrial noise:-

This noise is generated in the earth’s outer space (atmosphere)

Extraterrestrial noise is divided into

  1. Solar noise .
  2. Cosmic noise.

Solar noise:- 

  • The sun radiates so many things our way noise is noticeable among them.
  • Under “quiet” conditions , there is a constant noise radiation from the sun simply because its a large body at high temperature ≈ 6000o C.
  • ∴ The radiation consists of the frequencies which we use for communications and interferes with them.
  • However, the disturbances in the sun is variable and undergoes cycles at the peak of which electrical disturbances erupt. These additional disturbances are several orders of magnitude greater than the noise generated during periods of the quiet sun. The solar cycle repeats these period of great electrical disturbances approximately every 11 years, further these 11 year cycle peaks reach even a higher maximum peak every 100 years.
  • Thus the noise generated by sun changes periodically with the solar disturbances.

Cosmic noise:-

  • stars are also suns and have high temperatures, they radiate RF noise in the same manner as our sun, this refers to noise coming from distant stars other than sun.
  • The noise received from such stars is also called “black-body noise” and is distributed fairly uniformly over the entire sky.
  • Space noise is observable in the range from about 8 MHz to about 1.43 GHz this is the strongest component of noise in the range(20-120) MHz.

Industrial (or) Man-made noise:-

  • This noise is strongest in Industrial areas and the frequency of Man made noise spans between 1 to 600 MHz.
  • Man made noise is found in urban, sub-urban and industrial areas. The intensity of the noise made by human easily outstrips that created by any other source,  internal or external to the receiver.
  • under this, sources such as Automobile, Aircraft ignition, electric motors and switching equipment leakage from high voltage lines and a multitude of other heavy electric machines are all included.
  • Fluorescent lights are another powerful source of such noise and therefore should not be used where sensitive receiver is installed.

Internal Noise:-

This noise is created by any of the active (or) passive devices found in receivers. It is created by various components used in processing the received signal and is completely internal to the system. The effect of this noise is significant at the front end of the receiver.This appears as  thermal and shot noise  caused by resistors, inductors and capacitors. 

Thermal Noise:-

This noise is also known as agitation noise, Jhonson noise / white noise. thermal noise is random in nature, this mainly occurs due to random(or) rapid motion of molecules, atoms and electrons of which resistor is made up of.

from the theory of dynamics the noise generated by a resistor is proportional to it’s absolute temperature and BW over which the noise is to be measured.

P_{n}\propto T\Delta f where B= BW= \Delta f

P_{n}= kT\Delta f

  • where k – boltzmann’s constant =1.38X10-23 J/K.
  • T- Absolute temperature in Kelvin,   K = 273+ oC.
  • \Delta f = BW of interest.
  • P_{n} is the maximum noise power output of a resistor.

In an ordinary resistor at the standard temp of 17oC is not connected to any voltage source and if we are measuring voltage using a DC volt meter to measure voltage across it shows a zero. Actually a resistor itself is a noise generator, if we use a very sensitive electronic volt meter it shows a very large voltage across R.

This noise voltage is caused by the random movement of electrons with in the resistor, which constitutes a current. The rate of arrival of electrons at either end of the resistor therefore varies randomly, and so does the potential difference exists between the two ends.

from the circuit diagram,

P_{n}=\frac{V_{n}^{2}}{4R_{L}}  —- equation(1).

The maximum power  is delivered to  load when R =RL.

V_{n}= i(R + R_{L})

V_{n} = i 2R

i = \frac{V_{n}}{2R}

load voltage V = i R_{L}

V = \frac{V_{n}}{2 R}

V = \frac{V_{n}}{2} volts

Vn -source noise voltage.

V- ouput voltage measured across RL.

from equation (1) V_{n}^{2}=P_{n}4R_{L}

V_{n}^{2}=4k T\Delta fR_{L}

V_{n}=\sqrt{4k T\Delta fR_{L}}

Vn is known as RMS noise voltage asross a resistor.

Shot Noise:-

This occurs due to shot effect, it occurs in all active and amplifying devices (diodes/transistors).

It is caused by random variations in the arrival of electrons (or holes) at the output electrode of an amplifying device and appears as a randomly varying noise current super imposed on the output.It sounds like a shower of a lead shot were falling on a metal plate. Hence named it as shot noise.

  • In electronic tubes shot noise is caused because of random emission of electrons from cathode.
  • In semi-conductors shot noise occurs due to random diffusion of minority carriers .
  • i(t) = I_{o} + i_{n}(t)

Total current = Mean DC constant current + shot noise current.

Shot noise is given by i_{n} = \sqrt{2qi_{p}\Delta f}

  • in -RMS shot-noise current.
  • q-charge of an electron 1.6 X10-19 C.
  • ip– direct diode current.
  • \Delta f– BW of the system.

This formula for shot noise is valid for vaccum tube diode under so called temp-limited conditions.

In all other cases we use the concept of equivalent noise resistance intead of shot noise formula.

Transit-time noise (or) High-frequency noise:-

It is generally observed in semi conductor devices when the transmit time of charge carriers crossing a junction is comparable with the time-period of the signal, some charge carriers diffuse back to the source (or) emitter.

this gives rise to input admittance Y   

conductance G = 1/Y , this G increases with frequency which causes noise . This is also called as high frequency noise.

 

 

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Author: Lakshmi Prasanna

Completed M.Tech in Digital Electronics and Communication Systems and currently working as a faculty.