Analog

Communication6/13/2015 1Analog Communication - NOISE

Thi book is compiled by abhishek kumar

This book is compiled by abhishek kumar

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NOISE

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Topics to be covered

Noise

Sources of noise

Thermal Noise, Shot Noise, Flicker noise and White noise

Noise Parameters

Signal to noise ratio

Noise factor

Effective noise temperature

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Topics to be covered

Narrow band noise

Representation of narrowband noise in terms of In phase and Quadrature Components

Noise in CW modulation Systems

Noise in linear Receivers using Coherent detection

Noise in AM Receivers using Envelope detection

Noise in FM Receivers

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Noise Sources Introduction to Noise

Shot Noise

Thermal Noise

Flicker Noise

White Noise

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Noise - Introduction

Noise Unwanted Signals that tend to disturb theTransmission and Processing of Signals inCommunication System and over which we haveincomplete control.

Noise is a general term which is used to describean unwanted signal which affects a wanted signal.

These unwanted signals arise from a variety ofsources.

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Sources of Noise

Sources of noise may be:

External

Internal

Naturally occurring external noise sources include:

Atmosphere disturbance (e.g. electric storms, lighting,ionospheric effect etc), so called Sky Noise

Cosmic noise which includes noise from galaxy, solarnoise

Hot spot due to oxygen and water vapour resonancein the earths atmosphere.

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Sources of Noise

Noise performance by external sources is shown below.

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Sources of Noise

Internal Noise is an important type of noise thatarises from the SPONTANEOUS FLUCTUATIONS ofCurrent or Voltage in Electrical Circuits.

This type of noise is the basic limiting factor ofemploying more complex Electrical Circuits inCommunication System.

Most Common Internal Noises are:

Shot Noise Thermal Noise

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Shot Noise

Shot Noise arises in Electronic Components likeDiodes and Transistors.

Due to the discrete nature of Current flow Inthese components.

Take an example of Photodiode circuit.

Photodiode emits electrons from the cathodewhen light falls on it.

The circuit generates a current pulse when anelectron is emitted.

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Shot Noise

The electrons are emitted at Random times, kwhere - < k < and assume this randomemission have been gone for a long time.

Thus the Total Current flowing through thePhotodiode may be modeled as the sum of theseCurrent Pulses.

This process X(t) is Stationary and is called SHOTNOISE

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Thermal Noise

Thermal Noise is the name given to the Electrical Noise arisingfrom the Random motion of electrons n a conductor.

It is also called Jonson Noise or Nyquist Noise.

Let VTN is the Thermal Noise Voltage appearing across the twoterminals of a resistor.

Let the applied voltage have a bandwidth or frequency), f.

Then the Mean Square value of VTN is given by:

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Thermal Noise

Where

k = Boltzmanns constant = 1.38 x 10-23 Joules per oK

T = absolute temperature in oK

R = resistance in ohms

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Jonson Noise or Nyquist Noise

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Thermal Noise

We can model a noisy resistor using the Thevenin andNorton Equivalent Circuit as shown below:

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Thermal Noise

The number of electrons inside a resistor is verylarge and their random motions inside theresistors are statistically independent.

The Central Limiting Theorem indicates thatthermal Noise is a Gaussian Distribution with Zeromean.

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Low Frequency or Flicker Noise

Active devices, integrated circuit, diodes, transistors etc also

exhibits a low frequency noise, which is frequency

dependent (i.e. non uniform) known as flicker noise .

It is also called one over f noise or 1/f noise because of

its low-frequency variation.

Its origin is believed to be attributable to contaminants and

defects in the crystal structure in semiconductors, and in

the oxide coating on the cathode of vacuum tube devices

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Low Frequency or Flicker Noise

Flicker Noise is found in many natural phenomena such asnuclear radiation, electron flow through a conductor, oreven in the environment.

The noise power is proportional to the bias current, and,unlike Thermal and Shot Noise, Flicker Noise decreases withfrequency.

An exact mathematical model does not exist for flickernoise because it is so device-specific.

However, the inverse proportionality with frequency isalmost exactly 1/f for low frequencies, whereas forfrequencies above a few kilohertz, the noise power is weakbut essentially flat.

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Low Frequency or Flicker Noise

Flicker Noise is essentially random, but because its frequencyspectrum is not flat, it is not a white noise.

It is often referred to as pink noise because most of the power isconcentrated at the lower end of the frequency spectrum.

Flicker Noise is more prominent in FETs (smaller the channel length,greater the Flicker Noise), and in bulky carbon resistors.

The objection to carbon resistors mentioned earlier for critical lownoise applications is due to their tendency to produce flicker noisewhen carrying a direct current.

In this connection, metal film resistors are a better choice for lowfrequency, low noise applications.

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White Noise

The Noise Analysis of Communication System isdone on the basis of an idealized form of noisecalled WHITE NOISE.

Its power spectral density is independent onoperating frequency.

White White light contain equal amount of allfrequencies in visible spectrum.

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White Noise

Power spectral density is given by:

The 1/2 here emphasizes that the spectrum extends to both positive and negative frequencies.

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Power Spectral Density of White Noise

A random process W(t) is called white noise if it has a flat power spectral density, i.e., SW(f) is a constant c for all f.

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Ideal Low Pass Filtered White Noise

Let

w(t) = White Gaussian Noise applied to the LPF

B = Bandwidth of LPF

n(t) = noise appearing at the output of LPF

SN(f) = Power Spectral Density of n(t)

RN() = Auto Correlation function of n(t)

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Ideal Low Pass filtered White Noise

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