shealding

ABSTRACT

With the advent of technology a lot of equipment uses electromagnetic signals

for their operation. The devices intentionally or unintentionally interfere with the

operation of other circuits and effect their performance. For the proper functioning of

the devices this interference needs to be avoided at least suppressed. The various

electronic/electrical systems are forced to work in the close proximity. So the effects

of EMI are highly influencing the circuit operations.

Our project deals with the electromagnetic interference and its nature. Also

the methods of suppressing EMI. We laid emphasis on shielding method of

suppressing EMI. At high frequency shielding is effective method. The shield must

completely enclose the electronics and must have no penetrations such as holes,

seams, slots or cables.

The shielding effectiveness of different metals like Copper, Aluminum for

various thicknesses and results are dealt with in the project. The shielding

effectiveness is estimated up to 10MHz.

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LIST OF FIGURES

Figure No. Name of the figure Page No.

1.1 Noise path 9

3.1 Total loss is the sum of

absorption loss and

reflection loss

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3.2 Absorption Loss of

different materials

20

4.1 Shielding Effectiveness

of Aluminium for

variable thickness

26

4.2 Shielding Effectiveness

of Copper for variable

thickness

27

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CHAPTER- 1

INTRODUCTION

In the environment the electromagnetic interference (EMI) in the equipment is originated by

physical phenomenon. EMI is somewhat arbitrarily defined to cover the frequency spectrum from

10Hz to 100GHz. For radiated emission a lower frequent limit of 10 KHz is often used, although

EMI can exist in many equipment and systems below this frequency. The electromagnetic

environment will be variable from place to place. Depending on the environment a wide variety of

interferences can be encountered.

All electric and electronic devices or installations influence each other when connected or

close to each other. For example any time varying voltage source or current source generates

electromagnetic waves which propagate in space with time. This is how useful signals are received

by receiving antenna. On the other hand these signals will be received by any other circuit or system.

This will intercept it and causes interference.EMI can be generated from power transients, radio

frequency interference, electro static discharge and power line electric and magnetic fields.

Electromagnetic interference can cause disturbance and distortion in signals namely in

distribution of line carrier high speed VLSI circuits. Most of the systems require at least some

shielding for proper operation. Hence it is required to isolate or shield high powered sources and also

equipment of high sensitivity. To eliminate EMI we go for Electromagnetic compatibility (EMC)

which includes several methods like shielding, grounding, filtering.

1.1 Electromagnetic Interference (EMI)

Electromagnetic interference is a disturbance that affects an electrical circuit due to either

electromagnetic conduction or electromagnetic radiation emitted from an external source. The

disturbance may be any object, artificial or natural, that carries rapidly changing electrical currents

such as an electrical circuit.EMI is caused by undesirable radiated electromagnetic fields or

conducted voltages and currents. The interference is produced by a source emitter and is detected by

the susceptible victim via a coupling path. The coupling path may involve one or more of the

following coupling mechanisms like conduction, radiation, capacitive, inductor.

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Table 1.1 Examples of source, coupling path, and victim

Source Coupling path Victim

Lightening cables IC

RF transmitter Antennas RF receiver

High speed data trace Power lines High speed data trace

EMI can be transmitted by the following path

Fig.1.1 Noise path

1.1.1 Types of EMI

Electromagnetic interference is divides into several categories according to the source and signal

characteristics.

1.1.1. a) Continuous interference

Continuous, or Continuous Wave (CW), interference arises where the source regularly emits a given

range of frequencies. This type is naturally divided into sub-categories according to frequency range,

and as a whole is sometimes referred to as "DC to daylight".

Audio Frequency, from very low frequencies up to around 20 KHz. Frequencies up to

100 KHz may sometimes be classified as Audio.

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Sources include:

o Mains hum from power supply units, nearby power supply wiring, transmission lines

and substations.

Radio Frequency Interference, RFI, from 20 KHz to a limit which constantly increases as

technology pushes it higher.

Sources include:

o Wireless and Radio Frequency Transmissions

o Television and Radio Receiver

Broadband noise may be spread across parts of either or both frequency ranges, with no

particular frequency attentuated.

Sources include:

o Solar Activity

o Continuously operating spark gaps such as arc welders

1.1.1. b) Pulse or transient interference

Electromagnetic Pulse(EMP) also sometimes called Transient disturbance, arises where the source

emits a short-duration pulse of energy. The energy is usually broadband by nature, although it often

excites a relatively narrow-band damped sine wave response in the victim.

Sources divide broadly into isolated and repetitive events.

Sources of isolated EMP events include:

o Switching action of electrical circuitry, including inductive loads such as relays,

solenoids, or electric motors.

o Electrostatic Discharge (ESD), as a result of two charged objects coming into close

proximity or even contact.

Sources of repetitive EMP events, sometimes as regular pulse trains, include:

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o Electric Motors.

o Gasoline engine ignition systems.

1.1.2 TYPES OF EMI based on the spectrum division

EMI can be classified by its spectrum distribution. EMI can be either narrowband or broadband

interference.

1.1.2. a) Narrowband emissions

A narrow band signal occupies a very small portion of the radio spectrum. The magnitude of narrow

band radiated emissions is usually expressed in terms of volts per meter (v/m).Such signals are

usually continuous sine wave and may be continuous or intermittent in occurrence, communication

transmitters such as single channel AM, FM & SSB fall into this category. Spurious emissions such

as harmonic outputs of narrow band communication transmitters, power line hum, local oscillators,

signal generators, test equipment and many other man made sources are narrow band emissions.

1.1.2. b) Broadband emissions

A broadband signal may spread its energy across hundred of megahertz or more. The magnitude of

broadband radiated emissions is meter per MHz (V/m/MHz). This type of signal is composed of

narrow pulses having relatively short rise and fall times. Broadband signals are further divided into

random and impulse sources. These may be transient, continuous or intermittent in occurrence.

Examples include unintentional emissions from communication and radar transmitters electric

switch contacts, computers, thermostats, motor speed controls, thyratron circuits, voltage regulators,

pulse generators and intermittent ground connections. They may also result from galactic and solar

noise, lightening electromagnetic pulses and by radio frequency pulses associated with electrostatic

discharge.

We can prevent the EMI by three ways

Suppress the emission at its source.

Make the coupling path as in efficient as possible.

Make the receptor less susceptible to the emission.

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1.1.3 Effects of EMI

Most of the electronic instruments get affected by the electromagnetic interference. A malfunction

occurs because of this EMI.In monitors and printers some common EMI radiation susceptibility

problems are

Display memory corruption.

Display shaking.

Paper not advancing.

Computer system and monitors generally get affected by electrical fast Transients. For high

levels of electrical fast transients general mal function observed in computers and printers is “system

hanging”, where monitors manifest “display jumping”. In addition to that wire buses cables and

electronic of the I/O accessories plays an important role in finding EMI environment.

1.2 Electromagnetic Compatibility

Electromagnetic compatibility means that a device is compatible with its electromagnetic

environment and it does not emit levels of electromagnetic energy that cause EMI in other devices in

the vicinity. The different forms of EM energy that can cause EMI are conduction, radiation and

electrostatic discharge (ESD). It is a branch of electrical sciences which studies unintentional

generation, propagation and reception of electromagnetic energy with reference to the unwanted

effects that such energy may induce.

EMC can be achieved from different design levels, such as from chip level integrated design,

PCB, module or enclosure, and interconnect to software control. Different design techniques are

developed for various EMI problems, depending on the particular system, its electronic design, and

the type of interference source.

In order for correct operation of equipment EMC pursues two different kinds of issues

Emission issues are related to unwanted generation of EM energy by some source and the

counter measures to be taken to reduce such generation and to avoid the escape of any

remaining energies in to the external environment.

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Susceptibility or immunity refers to the correct operation of electrical equipment, referred to

as victim, in the presence of unplanned EM disturbances.

A system is said to be electromagnetically compatible with its environment if it satisfies three

criteria

1. It does not cause interference with other system.

2. It is not susceptible to emissions from other systems.

3. It does not cause interference with itself.

1.2.1 Need for EMC

The purpose of electromagnetic compatibility is to keep all the side effects caused by

interference under reasonable Control. EMC designates all the existing and future techniques and

technologies for.

EMC is needed for the correct operation of different electrical and electronic equipment

which involve electromagnetic phenomena in their operation in the same environment. It is also

needed for the reduction in the unintentional generation of electromagnetic energy.

The primary advantages of EMC are

1. Minimizing the additional cost required by suppression elements or redesign in order to

satisfy the regulatory requirements.

2. Maintaining the development and product announcement schedule.

3. Ensuring that the product will operate satisfactorily in the presence of inevitable extreme

noise sources at its location.

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1.3 Sources of EMI

They can be broadly classified into four categories

1. Inherent EMI

2. Man Made EMI

3. Natural EMI

4. Functional EMI

1.3.1 Inherent EMI

Inherent interference is noise within a piece of electronic equipment, caused by thermal agitation of

electrons flowing through circuit resistance. (This noise is usually noticed as the background noise

heard in a radio receiver when it is tuned to a frequency between stations.)

1.3.2 Man-Made EMI

Man-made EMI is produced by a number of different classes of electrical and electronic equipment.

They  include,  but  are  not  limited  to:  transmitters, welders,  power  lines,  motors  and

generators,  lighting, engines  and  igniters,  and  electrical  controllers.  These devices  can  cause

severe  EMI,  which  can  degrade  the operation  of  shipboard  or  shore based  data  processing

equipment. The discussion of EMI will be directed to the recognition and elimination of the man-

made EMI that you are apt to encounter ashore or afloat.

1.3.3 Natural EMI

Natural interference is caused by natural events, such as snow storms, electrical storms, rain

particles, and   solar   radiation.   This   type   of   interference   is commonly called static or

atmospheric noise.  It can cause problems with RF communications and older data links between

shore, ship, and air; however, it does not cause many problems with modern digital data

equipment.EMI can be caused by natural phenomena, such as electrical storms, rain particles and

solar and interstellar radiation.

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1.3.4 Functional EMI

EMI can originate from any source designed to generate electromagnetic energy and which may

create interference as a normal part of its operation. The functional sources of the EMI can be

originated from the sources like electric fields, magnetic fields, conducted interference

1.3.4. a) Electric fields

When a current flowing in a conductor is suddenly switched off, high voltages can occur, which if

large enough can cause sparking. Electrical arcs such as these emit electromagnetic radiation in the

form of an electric field across a wide band of the spectrum, which can interfere with the radio

control of the robot as well as inducing harmful voltages in any other electronics situated closed by.

1.3.4. b) Magnetic fields

There is another form of electromagnetic radiation, the magnetic field which is produced by high

currents flowing in conductors. If the current is charging or the conductor is moving or vibrating,

then the currents an be induced in the conductors situated near by.The large currents going into the

motors, which may be AC can therefore induce harmful currents in electronics.

1.3.4. c) Conducted interference

The other way that interference can get in to the electronics is by conduction through the wires that

the electronics uses. This is commonly along the power supply lines, but can be through any of the

electronic signal wires also.

EMI can also be generated from power transients, radio frequency interference, Electrostatic

discharge and power line electric and magnetic fields.

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CHAPTER – 2

METHODS TO SUPRESS EMI

2. Methods to suppress EMI

There are several methods to suppress electromagnetic interference. Some of them are

1. Move components on the printed circuit board (PCB).

2. Change circuit components-less noisy components.

3 Use special shielding techniques.

The most common methods to suppress EMI are

1. Filtering

2. Grounding

3. Shielding

2.1 Filtering

The noise (EMI) needs to be eliminated while not eliminating the desired signals. This is

generally solved by using a filter to select out the offending noise and allow through the good

signals. An EMI filter is a passive electronic device used to suppress conducted interference present

on any power or signal line. It may be used to suppress the interference generated by the device itself

as well as to suppress the interference generated by other equipment to improve the immunity of a

device to the EMI signals present within its electromagnetic environment. Most EMI filters include

components to suppress both common and differential mode interference. Filters can also be

designed with added devices to provide transient voltage and surge protection as well as battery

backup.

2.1.1 Working of EMI filter

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An EMI filter has a high reactive component to its impedance. That means filter looks like a

much higher resistance to higher frequency signals. This high impedance attenuates or reduced the

strength of these signals. So they will have less of an effect on other devices. Most filters are

frequency specific. Ferrites can act as filters and absorbers. Filtering usually needs longer capacitors

or inductors to be connected in the circuit. Thus the cost is higher and the connection is complicated.

2.2 Grounding

Grounding is one of the important ways of minimizing the noise and interference generated from

within the system and from outside. Proper use of the grounding and shielding, in combination, can

solve a large percentage of all noise problems. A good grounding system must be designed just like

the rest of the circuit. Grounding is one of the primary ways minimize the unwanted noise and pick

up. In most generated sense a ground can be defined as an equipment surface or point which serves

as a reference voltage for a circuital ground is connected to the earth through a low Z path, it can

then be called an “earth ground”. With reference to EMC design, three fundamental grounding

concepts are used they are:

Single point Grounding System

Multiple point Grounding System

Floating point Grounding System

These three grounding systems can be used individually or in combination in any system. In single

ground point a single physical point the circuitry is the defined ground reference. All ground

connections are connected to this point. At, high frequencies to which the equipment ground plane

dimensions of cable length approach wavelength, the single point ground system is not practical.

Therefore, the multipoint grounding is one in which a ground plane is used in one of the individual

return wires for each circuit. The ground plane must be a equipment chassis or a ground wire. i.e.,

carried throughout system. The multiple ground approach has the following advantages.

The construction is easier.

The standing wave effects in the ground system are avoided at high Frequencies.

Floating ground system is a method to electrically isolated circuits or equipments.

Floating grounds depend for their effectiveness on true floating. The accumulation of static grounds

and other access grounds such as navel deck, flight decks are the main advantages. The grounding

principles covered here are just as applicable to large complex electronic systems as they are to

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individual circuits on a single printed wiring board. There are two basic objectives involved in

designing good grounding systems. The first is to minimization of the noise voltage generated by

currents from two or more circuits flowing through common ground impedance. The second is to

avoid creating ground loops which are susceptible to magnetic fields and differences in ground

potential.

2.3 Shielding

Shielding can be described as a conductive or ferromagnetic material which either reflects, absorbs

or carries electromagnetic interference to ground. The shielding effectiveness of a material is

determined by measuring the unshielded field intensity and then, the field intensity under the same

conditions with the shield included. This ratio is then the shielding effectiveness (SE) of the

material. Schematically, the effects of a shielding material, on a plastic substrate, can be

characterized as follows. Many factors play a role in defining the (SE) for an actual electronic

product in operation. These include the distance from the energy source, the type of radiating field

involved, discontinuities in the material, operational voltage, the thickness and the conductivity of

the shielding material. External factors such as the temperature, humidity, environmental conditions

during operation, mechanical durability of the material, recyclability, etc. help to determine the

suitability of alternative shielding material choices for a given application

The term shield usually refers to a metallic enclosure that completely encloses an electronic product

or a portion of that product. The shield must completely enclose the electronics and must have no

penetrations such as holes, seams, slots or cables. Any penetration in a shield unless properly treated,

may drastically reduce the effectiveness of the shield. Shielding is more efficient and less expensive

to deal with suppression at the source. Virtually all high speed electronic devices employ shielding

in some form. Computers, cell phones, videogames etc. Shielding enclosures that are properly

designed and installed can be a very effective means of attenuating radiated emissions and protecting

products from external source of interference. Infact, a metallic enclosure with no apertures, seams

or cable penetrations can typically reduce radiated emissions and improve radiated immunity by 40

dB or more. A reliable way of earthing is needed for desirable effects in shielding. When the

earthing is not readily achieved, it is difficult to suppress the EMI effectively using the methods of

shielding and therefore the application is limited. There are two purposes of shielding

1. To prevent the emissions of the electronics of the product or a portion of those electronics from

radiating outside the boundaries of the product.

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2. To prevent radiated emissions external to the product from coupling to the electronics products,

this may cause interference in the product.

CHAPTER-3

SHIELDING EFFECTIVENESS

3.1 Shielding Effectiveness

Shielding effectiveness is typically measured as an attenuation of the electromagnetic signal

after a shield is introduced. Thus attenuation is a measure of the reduction in the intensity of the

electromagnetic field and is normally reported as decibels (dB). This value is actually the ratio of the

field strength without the shield to the field strength with the shield and is given mathematically as

follows

For electric field dB

For magnetic field dB

Where E0 (Ho) is the incident field strength, E1 (H1) is the transmitted field strength

The shielding effectiveness of a material is determined by a variety of factors in relation to the

characteristic influence of the wave itself. Metals have fundamental impedance that is inversely

proportional to their conductivity. For low impedance magnetic fields, more energy is absorbed and

less reflected because the impedances of both the metal and the field are similar. In the other case,

the wave impedance of an electric field is very high, so most of the energy is reflected. The specific

ability of a given material to control the unwanted electromagnetic energy can be defined as its

shielding effectiveness (in dB of attenuation) by the following general equation.

Total shielding Effectiveness is SE=A+R+B

Where S.E. is the shielding effectiveness expressed in dB,

R is the reflection factor expressed in dB,

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A is the absorption term expressed in dB, and

B is the correction factor due to reflections from the far boundary expressed in dB

B represents the possibility of re-reflected wave energy inside the shield. This can be a positive or a

negative value and is generally less important when the E-field or plane wave region is dominant. B

is often negligible for E-field and plane wave situations

Fig. 3.1 The total loss is the sum of the absorption and the reflection losses

3.2 Absorption Loss

Absorption loss representation for the planewaves is given as,

Notes: Usually at high-F (f > 1 MHz) absorption loss is predominate; however at low-F (f< 1KHz) ,

absorption loss is very small and reflection is dominate. When 1 KHz < f < 1MHz, both “A” and “R”

have effects

Fig 3.2 Absorption loss of different materials.

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3.3 Reflection Loss

Reflection loss is dependent on the type of field, and the wave impedance (i.e. characteristic

impedance). R is the reflected wave radiation and is related to the E-field properties of the shielding

material. Its value represents the wave intensity lost as a result of reflection of the EMI wave. The

reflection term is largely dependent upon

1. The relative mismatch between the incoming waves

2. The surface impedance of the shield.

3.3.1 Reflection loss to plane wave

Zw = 377 ohms, substituting , therefore

dB

Therefore, the lower the shield impedance, the greater the reflection loss is

At very low frequency (means in near field of H field), magnetic material like steel makes a better

magnetic shielding than does a good conductor like copper, however at high-F, it’s opposite

Table 3.1 Electric Conductivity and Relative permeability of different materials

S.No Metal Electric

conductivity

Relative permeability

μr

1 Aluminium 62 1

2 Copper 100 1

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3 Gold 71 1

3.4 Source Code

clear all

clc

clear

n=1;

for f=1:1000:10^7

A(n)=3.34*10^(-3)*0.5*(sqrt(f*62));

R(n)=168+(10*log10((62)/f));

SE(n)=A(n)+R(n);

n=n+1;

end

f=(1:1000:10^7);

figure

subplot(3,1,1);

plot(f,SE);

%axis[(0 10^7 0 250)]

title('shielding effectiveness Vs frequency for 0.5mm thickness for Aluminium');

xlabel('Frequency(HZ)');

ylabel('SE(dB)');

n=1;

for q=1:1000:10^7

A(n)=3.34*10^(-3)*1*(sqrt(q*62));

R(n)=168+(10*log10((62)/q));

SE(n)=A(n)+R(n);

n=n+1;

end

subplot(3,1,2);

plot(f,SE);

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title('shielding effectiveness Vs frequency for 1mm thickness for Aluminium');

xlabel('frequency(HZ)');

ylabel('SE(dB)');

n=1;

for s=1:1000:10^7

A(n)=3.34*10^(-3)*1.5*(sqrt(s*62));

R(n)=168+(10*log10((62)/s));

SE(n)=A(n)+R(n);

n=n+1;

end

subplot(3,1,3);

plot(f,SE);

title('shielding effectiveness Vs frequency for 1.5mm thickness for Aluminium');

xlabel('frequency(HZ)');

ylabel('SE(dB)');

4. RESULTS

The experiment made on shielding effectiveness for variable conducting metals for

variable thicknesses are estimated and are presented in the table and the graphs of shielding

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effectiveness is estimated by considering both absorption loss and reflection loss for variable

thicknesses. The shielding effectiveness is estimated up to 10MHz. For good EMC design

the shielding effectiveness must be as high as possible. The experimental values of shielding

effectiveness of Aluminium, copper of thicknesses 0.5 milli inch, 1.0 milli inch, 1.5 milli

inche are tabulated

Table 4.1 Experimental Values of Shielding Effectiveness of different metals.

S.NO METAL Shielding Effectiveness (A+R) in dB at10MHz.

0.5 milli inch 1.0 milli inch 1.5 milli inch

1. Aluminium 157.5 199.1 240.7

2. Copper 170.8 223.6 276.4

5. PLOTS:

Fig. 4.1 Shielding Effectiveness of Aluminium for variable thicknesses

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Fig. 4.2 Shielding Effectiveness of Copper of variable thicknesses

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5. CONCLUSION

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In this project, we present suppression techniques of EMI through

shielding. Shielding is one of the best techniques to suppress EMI and also cost

effective. In shielding we use different metals to enclose the system to suppress

the EMI

In this project, we studied the shielding effectiveness of Aluminium and

Copper up to 10MHz. The shielding effectiveness of above mentioned metals

are calculated and plotted by using MATLAB.

The metal which has high shielding effectiveness values, it is the better

one for suppression. So, by this we came to know that Copper is the better metal

for suppression of EMI through shielding. At high frequency shielding is

effective method.

6. REFERENCES

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1. www.ce-mag.com

2.”Matlab for Beginners” by Chapman, 3rd Edition, Pearson Education, 2000.

3. Burgoon, J.R., Jr., “Fundamentals of Electrical Shield Design”,

Insulation/Circuits, August, 1970, pp.20-40.

4. Jordan, E.C., and K.G.Balmain “Electromagnetic Waves and Radiating

Systems”, Second Edition, Prentice-Hall, Englewood Cliffs, N.J., 1968, pp.111-

123.

5. Circuits and System Magazine, Volume-9, Issue-4, IEEE.

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