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INTRODUCTION:

Electro magnetics (EM) is a branch of physics or electrical engineering in

which electric and magnetic phenomena are studied.

Electromagnetic interference (EMI) is electromagnetic energy that

adversely affects the performance of electrical/electronic equipment by

creating undesirable responses or complete operational failure.

Electromagnetic compatibility (EMC) is the ability of electrical or

electronic equipment/systems to function in the intended operating

environment without causing or experiencing performance degradation

due to intentional EMI.

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EM Signal:

E=Electric field

B=Magnetic field

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E

B

ELECTRO MAGNETIC

INTERFERENCE

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Example:

Electromagnetic

interference in

TV signal

BASIC ELEMENTS OF EMI

SITUATION

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Interference occurs if the received energy causes the receptor to

function in unwanted manner.

Whether the receiver is functioning in wanted or unwanted

manner, depends on the coupling path as well as the source and

victim.

The medium is to be made as inefficient as possible.

CLASSIFICATION OF EMI:

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RADIATED INTERFERENCE

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Narrow band interference usually arises from intentional transmissions such as radio

and TV stations, pager transmitters , cell phones etc. It is a high frequency operation .

Broad band interference usually comes from incidental radio frequency emitters.

These includes electric power transmission lines, electric motors etc. It is a low

frequency operation

CONDUCTED INTERFERENCE

Conducted electromagnetic interference is caused by the physical contact of the

conductors as opposed to radiated EMI, which is caused by induction (without

physical contact of the conductors).

Electromagnetic disturbances in the EM field of a conductor will no longer be

confined to the surface of the conductor and will radiate away from it.

This persists in all conductors and mutual inductance between two radiated

electromagnetic fields will result in EMI

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Differences between conducted and

radiated interference

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Intra system EMI

causes

Inter system EMI

causes

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Effects of EMI

Momentary disturbance in TV and radio reception due to

operation of mixer-grinder/electric shavers/a passing vehicles

etc

Reset of computers and loss of data.

Change of setting of status of control equipment.

Malfunctioning of flight controlling system due to use of

laptop by passenger.

Biological hazards.

Sources of EMI:The sources of EMI can be broadly classified into two groups

Natural sources of EMI

example: lightning

Manmade sources of EMI

example: commercial radio and telephone communications

In specific we can classify as

Functional: EMI can originate from any source designed to generate

electromagnetic energy and which may create interference as a normal part of its

operation

Incidental: EMI can originate from man made sources .These sources are not

designed specifically to generate electromagnetic energy but which do in fact cause

interference.

Natural: EMI can be caused by natural phenomena, such as electrical storms ,

rain particles , solar and interstellar radiation.

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Fig:1

Fig1-lightning hitting

between the

Buildings.

EMI CONTROL TECHNIQUES:

To control or suppress EMI, the

three common means employed

in the design process are

Grounding

Shielding

Filtering

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Grounding:

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Grounding is the establishment of an electrically conductive

path between two points to connect electrical and electronic

elements of a system to one another or to some reference point,

which may be designated as the ground. .

An ideal ground plane is a zero-potential ,zero-impedance body

that can be used as a reference for all signals in associated

circuitry and to which any undesired current can be transferred

for the elimination of its effects.

The purpose of the ground plane

The main purpose is to provide low grounding

impedance.(the effective resistance of an electric circuit

or component to alternating current, arising from the

combined effects of ohmic resistance and reactance.)

The secondary purpose is to act as a shield.

< 1 MHz: Single-point grounding

> 10 MHz: Multi-point grounding

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Bonding is the establishment of a low-impedance path

between two metal surfaces. Grounding is a circuit concept,

while bonding denotes the physical implementation of that

concept.

Bonds provide protection from electrical shock,

power circuit current return paths, and antenna ground

plane connections, and also minimize the potential

difference between the devices. They have the ability

to carry large fault current.

Shielding:

The purpose of shielding is to confine radiated energy to a specific region

or to prevent radiated energy from entering a specific region.

Shields may be in the form of partitions and boxes as well as in the form of

cable and connector shields.

Shield types include solid, nonsolid (e.g., screen), and braid, as is used on

cables. In all cases, a shield can be characterized by its shielding

effectiveness.

The shielding effectiveness is defined as

SE=10 log incident power density

transmitted power density

where the incident power density is the power density at a measuring point

before a shield is installed and the transmitted power is the power density

at the same point after the shield is in place.

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Shielding:

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Filtering: An electrical filter is a network of lumped or distributed constant resistors,

inductors, and capacitors that offers comparatively little opposition to

certain frequencies, while blocking the passage of other frequencies.

Filter provides the means whereby levels of conducted interference are

substantially reduced.

The most significant characteristic of a filter is the insertion loss it provides

as a function of frequency.

Insertion loss is defined as

IL=20 log V2/V1

Where V1 is the output voltage of a signal source with the filter in the

circuit,

V2 is the output voltage of the signal source without the use of the filter.

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Filtering:

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ELECTRO MAGNETIC

COMPATIBILITY Electromagnetic compatibility (EMC) is the branch of

electrical science which studies the unintentional generation,

propagation and reception of electromagnetic energy with

reference to the unwanted effects (Electromagnetic

interference, or EMI) that such energy may induce.

The goal of EMC is the correct operation, in the same

electromagnetic environment, of different equipment which

use electromagnetic phenomena, and the avoidance of any

interference effects.

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ECM

A system is said to be electro magnetically compatible if :-

• It doesn't cause interference with other system .

• It is not susceptible to emissions from other systems.

• It doesn’t cause interference with itself.

EMI is a phenomenon while EMC is an equipment characteristic or a property not to generate EMI above a certain limit and not to be affected or disturbed by EMI. The statement "Live and let live" is the best way to describe EMC.

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The methodologies used to prevent EMI are:-

Suppress the emissions at source point , best

method to control EMI.

Make the coupling path as inefficient as

possible.

Make the receiver less susceptible to emission.

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COUPLING MECHANISM:

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The basic arrangement of noise source, coupling path and victim, receptor or sink is

shown in the figure below. Source and victim are usually electronic hardware devices,

though the source may be a natural phenomenon such as a lightning strike, electrostatic

discharge(ESD)

There are four basic coupling mechanism :

1. Conductive

2. Capacitive

3. Magnetic/Inductive

4. Radiative

Conductive coupling:Conductive coupling occurs when the coupling path between the source and the

receptor is formed by direct contact with a conducting body, for example a

transmission line, wire, cable or metal enclosure.

. Capacitive coupling:

Capacitive coupling occurs when a varying electrical field exists between two

adjacent conductors typically less than a wavelength apart, inducing a change

in voltage across the gap.

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Radiative coupling:

Radiative coupling or electromagnetic coupling occurs when source and

victim are separated by a large distance, typically more than a wavelength.

Source and victim act as radio antennas: the source emits or radiates

an electromagnetic wave which propagates across the open space in

between and is picked up or received by the victim.

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Layout design

In simple non-critical devices, the layout design is often

enough.

◦ Especially, if there are no cables to/from the device.

About 90 % of post-design EMC-problems are caused

by poor layout design!

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Good layout design Split the system in parts

Choose grounding points well and minimize the

grounding impedance.

Remember that every conducting part can carry

interfering currents!

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Splitting the system in parts

Decide which parts are critical vs. non-critical.

Place the parts which are neither sensitive nor sources for interference, into separate locations even on their own circuit boards.

For example, linear power supplies, non-clocked logic circuits and power amplifiers are usually immune to interference.

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Ferrite chokes

Very common in data cables.

Adds series inductance to cable.

Effective on frequencies 1-1000 MHz (approximately).

Disadvantage: relatively low attenuation (10-20 dB).

Advantage: easy to add afterwards.

Ferrite choke attenuates also fast transients caused by

ESD.

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Electromagnetic Interference in airplane

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EMI is a major problem in modern digital avionics systems, and it is getting worse.

The most common Natural EMI source is Lightning.

Three basic EMI sources:1. Transients2. 400 Hz power3. Radio Frequency

Ten specific EMI sources:1. Electrostatic 6. Magnetic field(400Hz)

2. 1 MHz Resonant frequency 7. Computer clock/Data signals3. Voltage potential drop 8. Switching Regulators 4. Inductive switching transient 9. HF/VHF5. Electric field (400 HZ) 10. Super high frequency

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EMC CRITICALITY CATEGORIES

Category 1:Electromagnetic compatibility (EMC) problems that could result in loss of life, loss of vehicle , Mission abort, or unacceptable reduction in system effectiveness.

Category 2:EMC problems that could result in injury, damage to vehicle, or reduction in system effectiveness that could endanger mission success.

Category 3:EMC problems that could result in annoyance, minor discomfort, or no reduction in system effectiveness.

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Examples of alleged Electro-magnetic

interference (EMI) based incidents

●CRJ200 First Officer reports compass system

malfunctions during initial climb. When passengers are

asked to verify that all electronic devices are turned off the

compass system returns to normal.

●MD88 crew has problem on VHF com radio. The static

stopped when the passengers were asked to turn off their

electronic devices

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EMI management?

●Today aircraft type designs such as the Boeing 787,

Airbus A350 and A380 are designed taking EMC into

account

Today cabin systems:

●Equipment qualification for new systems is constantly

updated –where equipment qualification for installed

aircraft systems stay on the original qualified status.

●System configuration and age of the aircraft defines

qualification level of the equipment on board

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Guidance and regulations

Guidance Material:

●RTCA/DO -313 Certification Guidance for Installation of Non-

essential, Non- required Aircraft Cabin Systems & Equipment

●RTCA/DO-160 Environmental Conditions and Test Procedures for

Airborne Equipment

●EUROCAE ED14 Environmental Conditions and Test Procedures for

Airborne Equipment

●RTCA/DO-294 Guidance on Allowing Transmitting Portable

Electronic Devices (T-PEDS) on Aircraft

●EURAOCAE ED-130 Guidance for the use of Portable Electronic

Devices (PEDs) on Board Aircraft

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NEED FOR EMC STANDARDS:

The EMC standards are required for trouble free co-existence

and to ensure satisfactory operation.

They are also required to provide compatibility between

electrical, electronic, computer, control and other systems.

Standards are required as manufacturer-user interaction and

user’s knowledge on EMI are limited.

They are also required for establishing harmonized standards

to reduce international trade barriers and to improve product

reliability and life of the product.

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EMC STANDARDS:

These are of two types

Military Standards :

Military EMC standards are made in order to ensure system-to-system

compatibility in the real time military environment. Military standards

are more stringent than civilian standards. Most of the military standards

are broadly based on MIL-STD 461 and 462.

Civilian Standards:

The civilian EMC standards are applicable for equipments used for

commercial, industrial and domestic applications. The emission

standards are specified to protect the broadcast services from

interference..

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ADVANTAGES OF EMC

STANDARDS

The advantages are:

Compatibility, reliability and maintainability

are increased.

Design safety margin is provided.

The equipment operates in EMI scenario

satisfactorily.

Product life and profits are increased.

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EMC STANDARDS IN DIFFERENT

COUNTRIES:Sl

no

Standard

name

Meaning Country

1 CISPR(IEC) Committee International

Special

Perturbations Radioelectriques

– Europe

International

committee

2 FCC Federal Communications

Council

USA

3 SAE Society of Automobile

Engineers

Trade Association

Technical Committee

4 VG Military standard Germany

5 VDE Verband Deutscher

Electrotecknikev

Germany

6 ICT EMI measurements &

measuring apparatus

India

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CONCLUSION: The fact that EMI was recognized as a problem of significant practical

concern three-quarters of a century ago generally suggests that by now this

must be a well-understood field, and solutions to problems are

consequently a routine technology.

The position is that a great deal of theoretical, analytical and practical

information is today available to understand EMI, have graduated from the

traditional EMC fixes based on trail and error approach.

EMC has developed into a very interdisciplinary subject.

Many problems and topics in this field are how ever still open for further

research.

There is aerial need for further research on several aspects of EMC .These

include characterization of interferences, measurements techniques etc...

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REFERENCES:

“aircraft digital electronics and computer system ” a textbook by MIKE

TOOLY.

“DIGITAL AVIONICS SYSTEM ” a textbook by CARY.SPITZER

http://www.arrl.org/tis/info/rfigen.html RadioFrequency

Interference/ElectroMagnetic Interference, ARRL

http://www.kyes.com/antenna/interference/tvibook.html

INTERFERENCE HANDBOOK

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