بسم ا الرمحن الرحيم

LecturerAhmed H. Hadi

Training package in

NoiseFor students of second class

Ministry of Higher Education and Scientific ResearchFoundation of Technical Education

Technical College / Al-Najaf

1 / B –Rationale :-

After studying this package, you should be familiar thetypes and sources of noise. The methods of calculatingthe noise produced by various sources will be learned

/ Over view1/ Over view1

1 / A –Target population :-For students of second class inCommunications Techniques Engineering Department

1 / C –Central Idea :-• Define noise• Random noise• Thermal Noise

• Noise Figure

1 / C –Central Idea :-• Define noise• Random noise• Thermal Noise

• Noise Figure

:Objectives–/ D1• Define the word noise as it applies

to this material• Name at least six different types of

noise.• Demonstrate an understanding of

signal-to-noise (S/N)• Calculate noise figure

:Objectives–/ D1• Define the word noise as it applies

to this material• Name at least six different types of

noise.• Demonstrate an understanding of

signal-to-noise (S/N)• Calculate noise figure

2/ Pre test :-2/ Pre test :-

2. The ideal value of noise figure is

(a) 1 dB (b) 0 dB (c) dB

2. The ideal value of noise figure is

(a) 1 dB (b) 0 dB (c) dB

Multiple Choice Questions With Answer

١١1. The noise in a temperature limited diode is

(a)Filcher noise (b) Thermal noise (c) Shot noise

1. The noise in a temperature limited diode is

(a)Filcher noise (b) Thermal noise (c) Shot noise

٢٢

٣٣3. The RMS value of thermal noise power is proportional to

(a) square root of bandwidth (b) bandwidth

(c) reciprocal of bandwidth (d) square of bandwidth

3. The RMS value of thermal noise power is proportional to

(a) square root of bandwidth (b) bandwidth

(c) reciprocal of bandwidth (d) square of bandwidth

٤٤

5. In communication system noise is most likely to affect the signal

(a) at the transistor (b) in the channel

(c) in the information source (d) at the destination

5. In communication system noise is most likely to affect the signal

(a) at the transistor (b) in the channel

(c) in the information source (d) at the destination

3/ Performance Objectives :-3/ Performance Objectives :-

NoiseNoise: Undesired electrical signal which are introduced witha message signal during the transmission or processing ofthe latter are called noise. Noise is thus an unwanted signalthat corrupts a desired message signal.

NoiseNoise: Undesired electrical signal which are introduced witha message signal during the transmission or processing ofthe latter are called noise. Noise is thus an unwanted signalthat corrupts a desired message signal.

Random noise:Random noise varies randomly with time. Identification ofmessage signal at the receiver depends upon the amountof noise accompanied by the message during the processof communication. The presence of noise complicates thesystem of communication. The amount of noise powerpresent in the received signal decides the minimum powerlevel of the desired message signal at the transmitter.

Random noise:Random noise varies randomly with time. Identification ofmessage signal at the receiver depends upon the amountof noise accompanied by the message during the processof communication. The presence of noise complicates thesystem of communication. The amount of noise powerpresent in the received signal decides the minimum powerlevel of the desired message signal at the transmitter.

The general classification of random noise Thereare

various sources of random noise they are broadlyclassified as

-External noise: sources are external i.e., outsidethe circuit

-Internal noise: noise is created by the active andpassive components present within the circuit

itself.

The general classification of random noise Thereare

various sources of random noise they are broadlyclassified as

-External noise: sources are external i.e., outsidethe circuit

-Internal noise: noise is created by the active andpassive components present within the circuit

itself.

Thermal NoiseThermal Noise

Thermal noise is produced as a result ofthe thermally excited random motion offree electrons in a conducting medium,such as a resistor. The path of eachelectron in motion is randomly orienteddue to collisions. The net effect of themotion of all electrons is an electriccurrent in the resistor which is randomwith a mean vale of zero.

Thermal noise is produced as a result ofthe thermally excited random motion offree electrons in a conducting medium,such as a resistor. The path of eachelectron in motion is randomly orienteddue to collisions. The net effect of themotion of all electrons is an electriccurrent in the resistor which is randomwith a mean vale of zero.

From thermodynamic and quantum mechanical consideration, thepower spectral density of thermal noise is given byFrom thermodynamic and quantum mechanical consideration, thepower spectral density of thermal noise is given by

]1)2[exp(

kTh

hS n

kTS n 2)( ,/2 hkT watts per Hz for

T= temperature of the conducting medium in Kelvin,

K= Boltzmann's constant=

h= Plank's constant=

,/1038.1 23 KJoule

.10625.6 34 SecJoule

For frequencies above thermal

noise is no longer white. However, thesefrequencies are so high for electrical signals thatwe can safely assume that thermal noise is whitefor our purposes here

( for example, KT/h= 6000 GHz for T=290 K).

For frequencies above thermal

noise is no longer white. However, thesefrequencies are so high for electrical signals thatwe can safely assume that thermal noise is whitefor our purposes here

( for example, KT/h= 6000 GHz for T=290 K).

,/ hkT

the mean-square (open-circuit ) voltage generatedby a resistor R in a bandwidth B isthe mean-square (open-circuit ) voltage generatedby a resistor R in a bandwidth B is

.4)(,22

1)( 222

2

2 voltsKTRBtdKTRtB

B

the mean-square (short-circuit) current generatedthe mean-square (short-circuit) current generated

.4)(,22

1)( 222

2

2 amperesKTGBtidKTGtiB

B

T=273 + 27= 300 ºKRs= R1 + R2

=300 + 400Rs= 700Bandwidth B=7 MHz

T=273 + 27= 300 ºKRs= R1 + R2

=300 + 400Rs= 700Bandwidth B=7 MHz

Example:Example:Calculate the rms noise voltage at the input of a videoamplifier using a device having 300 equivalent noiseresistance and 400 Input resistor. It is given that thebandwidth of the amplifier is 7 MHz and the ambienttemperature is 27 ºC

Calculate the rms noise voltage at the input of a videoamplifier using a device having 300 equivalent noiseresistance and 400 Input resistor. It is given that thebandwidth of the amplifier is 7 MHz and the ambienttemperature is 27 ºC

Solution: Given thatSolution: Given that

VV

kTBRV

nr

snr

9

1073001038.14

4

623

Example:Example:

Find the noise figure of an amplifier for which noiseequivalent resistance Req = 2518 and Rt = 600 . It isdriven by a generator whose output impedance is 50 . Itis given that this constitutes a enough mismatch.

Find the noise figure of an amplifier for which noiseequivalent resistance Req = 2518 and Rt = 600 . It isdriven by a generator whose output impedance is 50 . Itis given that this constitutes a enough mismatch.

Solution: Given that Req = 2518, Rt = 600, Ra = 50Solution: Given that Req = 2518, Rt = 600, Ra = 50

4.394.38150

19181

1,

F

F

R

RF

a

eq

dB

dBF n

84.15

)4.39(log10 101

Noise FigureNoise Figure

Let the input and output signal voltages (or currents) ina given system be Si(t), So(t), respectively, and let theinput and output noise voltages (or currents) be ni(t),no(t). The input-to-noise ratio, (S/N)i, is

Let the input and output signal voltages (or currents) ina given system be Si(t), So(t), respectively, and let theinput and output noise voltages (or currents) be ni(t),no(t). The input-to-noise ratio, (S/N)i, is

,)()((S/N) 22i tnts ii

the output signal-to-noise ratio isthe output signal-to-noise ratio is )()((S/N) 22o tnts oo

The noise figure, F, to be the ratio of the input signal-to-noise ratio divided by the output signal-to-noise ratio:The noise figure, F, to be the ratio of the input signal-to-noise ratio divided by the output signal-to-noise ratio:

The noise figure in decibels:The noise figure in decibels:

o

i

(S/N)

(S/N)F

o

e

T

TF 1

)(log10 10 FFdB

Note: The standard value for To is 290 K.Note: The standard value for To is 290 K.

The noise figure, F, to be the ratio of the input signal-to-noise ratio divided by the output signal-to-noise ratio:The noise figure, F, to be the ratio of the input signal-to-noise ratio divided by the output signal-to-noise ratio:

The noise figure in decibels:The noise figure in decibels:

o

i

(S/N)

(S/N)F

o

e

T

TF 1

)(log10 10 FFdB

Note: The standard value for To is 290 K.Note: The standard value for To is 290 K.

or

Example:Example:

first stage has a noise figure of 2 dB and a powergain of 12 dB. The second stage has a noise figureof 6 dB and a power gain of 10 dB. Find the overallNoise Figure in dB.

first stage has a noise figure of 2 dB and a powergain of 12 dB. The second stage has a noise figureof 6 dB and a power gain of 10 dB. Find the overallNoise Figure in dB.

First stage Second stage

Input output

First stage Second stage

Input outputF1=2 dBA1=12 dB

F2=2 dBA2=10 dB

Fig. Overall noise figure F determination of a two-stage amplifierFig. Overall noise figure F determination of a two-stage amplifier

Solution:Solution:

)(log102

)(log10

110

10

ratio

ratiodB

F

FF

59.1)(

2.0log)(

10

2log)(

,1

10,1

10,1

ratio

ratio

ratio

F

AntiF

AntiF

In the same wayIn the same way10)(

9.15)(,4)(

,2

,1,2

ratio

ratioratio

A

AF

779.19.15

359.1

9.15

1459.1

F

F

dBF

F

FF

dB

dB

ratiodB

5.2

)779.1(log10

)(log10

10

10

١١

٢٢

Define the following terms

Shot noise

Quiz /Quiz /

Noise is thus an unwanted signal that corrupts adesired message signal.

or schottly noise rises due to the fact that in a tubeor a semiconductor device the current flow due to themovement of discrete electric charges carried by electrons orholes and this current flows not a perfect continuous flow.

-/ Post test :5

1. One of the following type of noise of great importance athigh frequencies.

(a) Shot noise (b) Random noise

(c) Impulse noise (d) transit-time noise

1. One of the following type of noise of great importance athigh frequencies.

(a) Shot noise (b) Random noise

(c) Impulse noise (d) transit-time noise

6. Thermal noise is also known as

(a) Johnson noise (b) modulation noise

(c) Flicher noise (d) shot noise

6. Thermal noise is also known as

(a) Johnson noise (b) modulation noise

(c) Flicher noise (d) shot noise

References:References:

11

2

T. R. Ganesh Babu, and G. Srinivasan:“ Communication Theory and systems”, 2006.

Sanjay Sharma: “Communication Systems(Analog and Digital) ”Sanjay Sharma: “Communication Systems(Analog and Digital) ”

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