6. cs lec6-30april noise (2)

8/12/2019 6. CS Lec6-30april Noise (2)

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ELECTRONIC COMMUNICATION SYSTEMS

(BY KENNEDY AND DAVIS)

CHAPTER :2

NOISE


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Outline


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WHAT IS NOISE

With reference to an electrical

system, NOISEmay be defined as anyunwanted form of energy which tends tointerfere with proper reception and reproduction

.ORNoise is random, undesirable electrical energy

that enters the communications system via thecommunicating medium and interferes with thetransmitted message. However, some noise is

also produced in the receiver.


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CLASSIFICATION OF NOISE

Noise may be put into following two categories.

External noise

Noise whose sources are external. External noise may be classified into thefollowing three types: Atmospheric noises

Extraterrestrial noises

Man-made noises or industrial noises.

Internal noiseNoises which get, generated within the receiver or communication system. Internalnoise may be put into the following four categories.

Thermal noise or white noise or Johnson noise Shot noise.

Transit time noise

Miscellaneous internal noise.


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NOTE External noise cannot be reduced except by

changing the location of the receiver or the entiresystem.

Internal noise on the other hand can be easily

a great extent by proper design.


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.


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(i) ATMOSPHERIC NOISE Atmospheric noise or static is caused by lighting discharges

in thunderstorms and other natural electrical disturbancesoccurring in the atmosphere. These electrical impulses arerandom in nature.

Atmospheric noise accordingly consists of spurious radiosignals with components spread over a wide frequencyrange. These spurious radio waves constituting the noiseget propagated over the earth in the same fashion as the

desired radio waves of the same frequency.

Accordingly at a given receiving point, the receivingantenna picks up not only the signal but also the static from

all the thunderstorms, local or remote.


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The field strength of atmospheric noise varies

approximately inversely with the frequency.

Thus large atmospheric noise is generated in low and

medium frequency (broadcast) bands while very littlenoise is generated in the VHF and UHF bands.

Further VHF and UHF components of noise are limitedto the line-of-sight (less than about 80 Km) propagation.

For these two-reasons, the atmospheric noise becomesless severe at Frequencies exceeding about 30 MHz.


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(ii) EXTRATERRESTRIAL NOISE

There are numerous types of extraterrestrial noiseor space noises depending on their sources.However, these may be put into following two

.

Solar noise

Cosmic noise


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EXTRATERRESTRIAL NOISE :SOLAR NOISE

This is the electrical noise emanating from the sun. Underquite conditions, there is a steady radiation of noise from thesun.

This results because sun is a large body at a very hightemperature (exceeding 6000C on the surface), andradiates electrical energy in the form of noise over a verywi r n r m in l in h r m r

radio communication.

The intensity produced by the Sun varies with time. In fact,the sun has a repeating 11-Year noise cycle. During the

peak of the cycle, the sun produces some amount of noisethat causes tremendous radio signal interference, makingmany frequencies unusable for communications. Duringother years. the noise is at a minimum level.


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EXTRATERRESTRIAL NOISE :COSMIC NOISE

Distant stars are also suns and have high temperatures.These stars, therefore, radiate noise in the same way asour sun.

(or black body noise) and is distributing almost uniformlyover the entire sky.

We also receive noise from the center of our own galaxy(The Milky Way) from other distant galaxies and from other

virtual point sources such as quasars and pulsars.


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(iii) MAN MADE NOISE (INDUSTRIAL NOISE)

By man-made noise or industrial- noise is meant the

electrical noise produced by such sources as automobilesand aircraft ignition, electrical motors and switch gears,leakage from high voltage lines, fluorescent lights, andnumerous other heav electrical machines.

Such noises are produced by the arc discharge taking placeduring operation of these machines.

Such man-made noise is most intensive in industrial anddensely populated areas. Man-made noise in such areas far

exceeds all other sources of noise in the frequency rangeextending from about 1 MHz to 600 MHz


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Random Noiseo The noise created by any of the passive or active

devices found in receivers is called random noise.

o It is impossible to treat it on an individual voltagebasis.

o

radio spectrum (on average).

o It is as much present on one frequency as other.

o Random noise power is proportional to thebandwidth over which it is measured.


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Types Of Internal NoiseThermal Agitation Noise. (2-2.1)

Shot Noise. (2-2.2)Transit-time Noise. (2-2.3)

Miscellaneous Noise. (2-2.4)

Flicker Or Modulation Noise. Resistance Noise.

Noise In Mixers.


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Thermal Agitation Noise The noise generated due to RESISTIVE

Component is referred to as thermal agitation/white or Johnson noise.

It is due to rapid and random motion of the

component which is also called internal kineticenergy.

In Thermodynamics, kinetic theory states that the

temperature of a particle is a way of expressingits internal kinetic energy.


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Kinetic theory Relation

So according to kinetic theory, the kinetic energy ofparticles becomes zero at absolute zero (zero Kelvin)of temperature(motion ceases).

Therefore the noise generated by a resistor is

proportional to its absolute temperature in addition tobeing proportional to the bandwidth over which thenoise is to be measured.

Pn Tf ------(i)

P=kTf --------(ii)

K=Boltzman constant=1.38x10^-23.

P=maximum noise output power.

f=bandwidth of interest.


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Random Voltage If a resistor (at standard temperature) is not

connected to any voltage source, there will be no

dc voltage across it. But a value of ac voltagemight occur which can be sensed by sensitivevoltmeter.

s e res s or s no se genera or , so ere maybe large value of voltage across it due to randomnoise.

This random noise creates random voltage

whose only RMS value can be measured, therewill be no dc component.


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Thermal Agitation Noise Equation

Assume RL is noiseless and receiver the maximum noisepower generated by R.

In general, both Rand Tdepends on frequency. In order tonow e o a no se s enoug o n egra e over a e

bandwidth. Since the signal is real, it is possible to integrateover only the positive frequencies, then multiply by 2.Assuming that Rand Tare constants over all the bandwidth ,then the root mean square (RMS) value of the voltage across

a resistor due to thermal noise is given by:


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Shot Noise

It is caused by random variations in the

arrival of electrons (or holes) at the output

electrode of an amplifying device andappears as randomly varying noise current

superimposed on the output.

When amplified it is supposed to sound thougha shower of lead shot were falling on a metal

sheet. Hence it is named shot noise.


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Shot Noise Current Shot noise current with exception of diodes is

simplified .

But for diodes, the formula is exactly the same. The RMS shot current is given as

For all other devices ,it is calculated as random noiseassumptions method.

C106.1where

2

19=

=

q

fqIi Dn


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Transit Time Noise

If the time taken by an electron to travel fromthe emitter to the collector of a transistor

ecomes s gn can o e per o o e s gnabeing amplified, transit-time effect takes place,and the noise input admittance of the transistorincreases.

The minute currents generated at input and theoperating frequency affects transit-time noise.


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Flicker / Modulation Noise At low audio frequencies, flicker or modulation

noise occurs in transistors.

It is proportional to emitter current and junctiontemperature

,may be ignored above about 500Hz.


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Resistance Noise Thermal noise is also called Resistance noise

It is also present in transistors.

It is due to the base ,emitter and collector internalresistances.

n mos c rcums ances, e ase res s ancemakes largest contribution.

From above 500Hz up to about f/5, transistornoise remains relatively constant , so that anequivalent input resistance for shot and thermalnoise may be freely used.


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Noise In Mixers

Mixers(non-linear amplifying circuits) are muchnoisier than amplifier using identical devices.

s g va ue o no se n m xers s cause y woseparate effects:

1. Conversion trans-conductance of mixers is muchlower than the trans-conductance of amplifiers.

2. If image frequency rejection is inadequate, noiseassociated with the image frequency will also beaccepted.


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NOISE FIGURE2-4.1: Signal to Noise Ratio

Signal-to-noise ratio (abbreviated SNR or S/N) is a

measure that compares the level of a desired signal to

e eve o ac groun no se. s e ne as e ra o

of signal power to noise power at the same point. A

ratio higher than 1:1 indicates more signal than noise.


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Therefore:


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2-4.2: Noise Figure

Noise Figure is also called Noise Factor. It isdenoted by F and is defined as:

The ratio of the si nal-to-noise ower

supplied to the input terminals of a receiver oramplifier to the signal-to-noise power supplied

to the output or load resistor.


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Thus,


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2-4.3: Calculation of Noise Figure

Noise Figure can be calculated from a four-terminal

network as shown in figure below:


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The figure has an input impedance Rt , an

output impedance RL and an overallvoltage gain A. It is fed from a source

(antenna) of internal impedance R , which

may or may not be equal to Rt as thecircumstances warrant.


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The calculation procedure is as follows:

1. First we will determine the signal input power Psi .

The signal input voltage is given by:

Then the signal input power is given by Psi :


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2. Now determine the noise input power Pni .

The noise input voltage is given by:

a tThus:

Hence:


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The noise input power will be:


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3- Calculate the input signal-to-noise ratio S/ Ni

from the ratio of Psi and Pni .


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4- Determine the signal output power

The output signal power will be:

The noise output power will be:


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5- Calculate the output signal-to-noise ratio S/

N0

from the ratio of Pso

and Pno

.


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6- Calculate the noise figure F from formula:


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2-4.4: NOISE FIGURE FROMEQUIVALENT NOISE RESISTANCE

Equivalent noise resistance:Equivalent noise resistance:

Is the sum of the input terminating resistance and the

equivalent noise resistance of the first stage, together with

the noise resistances of the previous stages.


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IN THIS CASE: All these resistances are added to Rt.

This resistance is now the noise makingresistance of the receiver.

More accurately it is the parallel combination ofRa and Rt.


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RESISTANCE EXCEPT Rt For convenience we have to define a noise

resistance, Req, which does not incorporate Rt.

It is given as:

Req = Req - Rt


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TOTAL EQUIVALENT NOISERESISTANCE

Now the total equivalent noise resistance for thereceiver or amplifier will be:

= eq + t From the figure:

Rt = Ra || Rt So ,

R = Req + (Rt*Ra/(Rt+Ra))


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EQUIVALENT NOISE VOLTAGE(Vni)


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THE NOISE OUTPUT (Pno)


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NOISE FIGURE IN TERMS OFEQUIVALENT NOISE RESISTANCE

(F)


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If the noise is to be minimum the ratio (Rt+Ra)/Rtmust also be minimum.

The value of Rt must be much larger than Ra.

So that the ratio approches unity.

Under these conditions the noise figure will be:

F = 1 +( Req/Ra)

Above equation is only for MISMATCHEDcondition.


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2-4.5: NOISE FIGURE FROMMEASUREMENT


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Voltage supplied to the input of the receiver:


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Putting values of constants:

v= 1.6 * 10^-19

k = 1.38*10^-23

T = 290K


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Noise calculations.

Noise due to several sources.

N i v r l m li i r .

Noise in reactive circuits.


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Noise due to several sources.

Noise due to several amplifiers in


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Noise due to several amplifiers in

cascaded:Cascaded Amplifiers are used in receivers for weak signals, which canbe seen in figure below:

Consider two stage amplifiers cascaded. The gain of 1st amplifier is A1an gain of 2nd amplifier is A2.the first stage has total input noiseresistance R11 , the second R2 and third R3.Vns are respective noisegenerating voltages.


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Cont

The expression for equivalent resistances in case

of cascaded amplifiers will be


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Noise in Reactive circuits:

Tuned circuits are mostly used for tuning and they

include reactive components. Reactive componentsproduces internal noises in the circuit .Tuned circuitslimit the band width of noise sources b not assin

noise outside its own band.


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Cont.. Consider figure which shows a parallel tuned circuit.

The series resistance of coil which is a noise source

here is shown as a resister in series with the noisegenerator and with the coil. It is required to determine

the noise voltage across the capacitor i.e. at the inputo e amp er.


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Cont..Noise current in circuit will be

Voltage across capacitor will be

Where Rp is total equivalent resistance


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2.5 NOISE TEMPERATURE DEFINITION:

A parameter use to measure noise.

APPLICATION:

Is employed extensively for antennas and low noisemicrowave amplifiers in radio astronomy.


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NOISE TEMPERATURE Noise temperature is an additive like noise power.

Tt=the total noise temperature


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EQUATING NOISE TEMP.& FIG.

By cross multiplication


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EXAMPLE


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H.W


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6. cs lec6-30april noise (2)

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