topic 3-decibels and noise

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Telecommunication engineering Principles

Chapter 3

Decibels and Noise

Ade Syaheda Wani Binti Marzuki

Department of Electronic Engineering

Semester 1, Session 2009/2010


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Outlines

3.1 Introduction

3.2 Decibels for Power and Voltage

3.3 Decibels Calculation: Examples3.4 Decibel Reference Values

3.5 System Measurements with dB

3.6 Decibels and Bandwidth3.7 Noise and Its Effects

3.8 Sources and Types of Noise

3.9 Noise MeasurementsKNL2833

Telecommunication Engineering Principles


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PART 3.1

Introduction

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What is decibels

and noise?

3.1 Introduction


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3.1 Introduction


DECIBEL (dB)

Signal in communication system span a range of

extremely wide decibel scale.

Decibel scale

Uses ratios and logarithms

Compress wide span of magnitude into

smaller, easier-to-manipulate range ofnumbers

Can be used to compare any 2 signals


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3.1 Introduction


Decibel scale (cont.)

Can be used to measure one signal against

another signal of defined value

Allow relative signal gain and loss to bemeasured easily

Allow total gain through multiple stage to be

calculated by simple addition


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PART 3.2Decibels for power and voltage

KNL1233Electrical Engineering Technology


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3.1 Introduction


NOISE

Unwanted, interfering signal

Often measured in decibels in relation to the

desired signal

Any circuit may produce its own noise

Amount of noise from the circuit itself adds to the

noise of the received signal must also bemeasured as part of the overall system

performance


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3.2 Decibels for power and

voltage


Power and dB

To compare two power signal

Use 2 power value (instead using single

absolute value such as Watt)

Use base 10 logarithm of this ratio (To compresswide range of signal values into a much smaller

range)

oP

PdB 1log10

3 2 f


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3.2 Decibels for power and

voltage


Voltage and dB

To compare two voltage signal

Using the factor of 20 as the multiplier

Note of caution:

When using the dB scale with voltage and power, the

voltage or current must be measured at points at the

same point in a circuit

oV

VdB 1log20

3 3 D ib l C l l ti


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3.3 Decibel Calculation:

Example


Example 3.1The dB value for a signal at 10 W compared to one at

0.5 W.

Example 3.2

A signal enters the circuit with a value of 0.1 V and is

amplified to 5V. The input and output resistances are the

same. The dB ratio that shows the gain in magnitude is

______Example 3.3

A signal is amplified 100 times in power. The dB gain is

______.

3 3 D ib l C l l ti


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Example


dB scale can also be NEGATIVE value (when asignal is reduced in value because of loss)

Example 3.4

Consider a signal whose power is reduced by a factor of100

Example 3.5

A signal enters the circuit with a value of 5 V and is

attenuated to 0.1V. The input and output resistances are

the same. The dB ratio that shows the gain in magnitude

is ______


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PART 3.3

Decibel Calculation: Example


3 3 D ib l C l l ti


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dB scale can also be ZERO value (when the twosignals have equal power (or voltage) values.

Another important case is when the power isdoubled (P1 = 2 x P0)


Example


voltagefordB

V

V

powerfordBP

P

dB

o

o

0log20

0log10

1

1

powerforP

PdB 010.32log102log10

0

1

3 3 D ib l C l l ti


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3dB corresponds to a power factor of 2(approximately)

It means a power factor of one-half

Another one-half is from second signal

For voltage, voltage ratio of 2 is very close to 6

dB


Example


voltageforV

VdB 021.62log202log20

0

1

3 3 D ib l C l l ti


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Example 3.6A new communications cable is installed and the signal

level, in volts, increases by one-half, or 50%. What is the

increase in dB?


Example


3 3 D ib l C l l ti


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In many instances, the system is specified usingdB values whenever possible, but at some point

we must calculate the exact value of power or

voltage as an absolute value.

For power:

For voltage:


Example


1001 10

valuedB

PP 10

10

10valuedB

PP

2001 10

valuedB

VV 20

10

10

valuedB

VV

3 3 D ib l C l l ti


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Example (for power):For a signal that has been a,plified by 13 dB and

is measured at 1.2 W, the original value was

Another example (for power):

If a 0.2W signal comes into a circuit having a

specified gain of 22 dB, its final value will be


Example


WWP 060.010

2.1

10

2.1 3.110

130

WP 7.31102.0 1022

1

3 3 D ib l C l l ti


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Example (for voltage):

A signal of 1.6 V that is amplified by 2.5 dB will

become

Another example (for voltage):

A signal that comes out the final stage of a 9dB

gain amplifier with a measured value of 15Vmust had an original input value of


Example


VV 13.2106.1 20

5.2

1

VV 3.5

10

15

20

90


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PART 3.4

Decibel Reference Value



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dB scale has resulted in the use of severalcommon reference values in electronics and

communications industries

One common reference is 1mW (equal to

0.001W) for lower power circuit

This is where the dB scale is called dBm

3.4 Decibel Reference Value


Means that the 0 dB

point reference is 1mW


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Example:A signal of 200 mW expressed in dBm is

Another example:

One ten-thousandth of a watt would then be



dBm

mW

mWdB 23

1

200log10

dBmW

WdB 10

001.0

0001.0log10


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For higher-power circuit, 1W is commonlyused as the reference value, dBW

Example:

An audio amplifier might have an output of7dBW. This correspond to:

Please note that:



WP 01.5101 107

1

dBWdBm

dBWdBm

030

300


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Similarly, the voltage ratio dB scale hassome standard values.

The most common is 1V, denoted by dBV.

Example:

A signal of 8.2 V corresponds to



dBV3.181

2.8log20


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Exercise:

What is the dBV value for 25.7 V?

Another exercise:

Express 0.05V as dBV.


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PART 3.5

System Measurements with dB


3 5 System Measurements


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3.5 System Measurements

with dB


Most practical communication systems consistsof many stages

As a signal passes from one stages from one

stage to another, it undergoes some changes in

signal level as well as other characteristics

Example:A signal from tape cassette as it

travels from the playback head of the cassette

unit to the antenna of the broadcast station



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with dB


The output power at each stage can also be

obtained

+20 dB

+10 dB

to

+20 dB

+15 dB

Cassette

playback

head

Preamplifier Main

amplifier

Power

amplifier

overall gain: +45 dB to +55 dB

antenna

WP 30



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with dB


The value at the output of the preamplifier

The value at the output of the main amplifier

If gain setting is set to 10 dB

If gain setting is set to 20 dB

WP erpreamplifi 30010310/20

mWWP amplifiermain 3300010310/30

mWWP amplifiermain 303000010310/40



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with dB

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Finally at the antenna (according to middlestage)

For +45 dB gain

For +55dB gain

The dB scale and addition/ substraction of dB values

at each stage make it very simple to incorporate

changes in the overall result

gaindBformWWPantenna 458.949486810310/45

gaindBformWWPantenna 5594994868310310/55


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PART 3.6

Decibels and Bandwidth

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Electrical Engineering Technology


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3.6 Decibels and bandwidth

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Concept of bandwidth Bandwidth needed to convey information or data

Determined by

Shannon formula

Looking at the amount of data to be transmitted in

fixed time period

Seeing how much bandwidth required to convey datasignal accurately


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Concept of bandwidth Also considered to be the width of frequency

spectrum between -3dB power point of the signal

amplitude versus frequency graph

-3dB is shortened to 3dB (-ve sign is understood)

0

-3

-6

-9

-12

Bandwidth

dB

f


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-3dB is the frequency value where signal powerhas fallen to one-half its maximum value, the

same as declined by 3 dB (since 3dB represents

a factor of 2 in power)


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0

-3

-6

-9

-12

Bandwidth

dB

f


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PART 3.7

Noise and Its Effect

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Noise Unwanted signal that corrupts and distorts

desired signals in any way

Original signal can be distorted in shape,increased or decreased in amplitude, delayed

slightly in time, or otherwise corrupted or

modified

Noise is random, but it has some kind of

characteristics.

3.7 Noise and Its Effects

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Effects of Noise Can cause listener to misunderstand the original

signal or be unable to understand it at all

Can cause the receiving system to malfunction

Result in less efficient system

Noise is an unavoidable fact, and the effect of

noise must be taken into account in designing

and operating any communications system.

3.7 Noise and Its Effects

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PART 3.8

Sources and Types of Noise

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3 8 Sources and Types of


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Noise can be divided into two main sources:external and internal

External noise can be hand made or from

natural origins

Internal noise generated by the circuit

components themselves

3.8 Sources and Types ofNoise

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External noise Atmospheric noise

produced mostly by lightning discharge in

thunderstorms As a source in relatively quiet locations at frequencies

below about 20MHz or so

The level of atmospheric noise also decreases with

increasing latitude on the surface of the globe


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External noise Galactic noise

Caused by disturbances originating outside the

earths atmosphere The primary frequency range us from about 15MHz to

perhaps 500MHz

Its power spectrum decreases with increasing

frequency


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External noise Impulse noise

Sudden change in voltage and current

Primarily from sudden on/off event

Interference

Interference from nearby communication system

produce some interfering effects


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Internal noise Flicker noise

Also called 1/fnoise

Most significant near dc and at a few Hertz, and isusually negligible above about 1kHz or so

Shot noise Arises from discrete nature of current flow in

electronic device

Effect usually similar to thermal noise


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Internal noise Thermal noise (Johnson Noise)

Result from random motion of charged particles in a

conducting medium such as resistor Power spectrum of thermal noise is quiet wide

Amount noise generated increases with the

temperature


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kTBP

Where P = power

k= Boltzmanns constant, 1.38 x 10-23 J/K

T = absolute temperature in KelvinsB= bandwidth in Hz



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The noise voltage

Equivalent noise resistance the value of a resistor that

would produce the noise value (rms voltage) that was

measured


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kTBRVN 4

Where VN = noise voltagek= Boltzmanns constant, 1.38 x 10-23 J/K

T = absolute temperature in Kelvins

R = equivalent noise resistance

B = bandwidth in Hz



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Try this!What is the noise power at room temperature,

25oC, when the bandwidth is 1kHz?

Another exercise!

What is the equivalent noise resistance when

the measured noise is 300V, the temperature is300 K and the bandwidth is 3kHz?


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PART 3.9

Noise Measurements

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3 9 N i M t


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Noise will definitely change the original signal It must be measured in a meaningful way.

Noise measurements:

Rms value

SNR (signal-to-noise ratio)

Noise Figure

3.9 Noise Measurements

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3 9 N i M t


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rms value Formed by taking the square root of the average

of individual noise voltages, which have been

squared

For example:

Consider a series of 10 noise values

measured with a voltmeter as -0.3, 1.0, 0.2,

0.5, -0.6, 0.3, 0.1, -0.15, and 0.9V.

The rms value is 0.55V

3.9 Noise Measurements

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3.8 Sources and Types of


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Try this!Noise values in milivolts as follows are

measured at various times: 10, - 100, 35, -57,

90, 26, 26, -10, -15, -20. What is the rms noise

value?


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Signal-to-noise ratio Shows how much stronger the desired signal is,

compared with the unwanted noise

Often expressed in dB

where S is the signal power and N is the noise power


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N

SdBinSNR log10)(



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Noise Figure, NF Defined as the dB ratio of the input signal and

noise ratio to the output signal and noise ra

A good low-noise transistor for low-level

amplification may have NF as low as 1dB

NF of 2 5dB are common


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noiseoutputsignaloutputnoiseinputsignalinputNF

//log10


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topic 3-decibels and noise

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