EMI /EMC TOPIC

1.Sources of EMI

2. Conducted EMI

3.Radiated EMI

4. EMI-EMC Definitations and Units of parameters

5.EMI Specifications/Standards/Limits:Units of Specifications

6.Civilian Standards

7.Military Standards

1.

Designers usually do not intend their devices to be sources of interference .However ,what is a desired signal in one path may be an undesired signal or “noise” in a path into which it is unintentionally coupled.

2.

Devices are designed for a wide variety of power levels, all the way from microwatts to megawatts. A relatively small arc associated with power switching may result in a serious disturbance to a sensitive(i.e low power level) circuit.

Electromagnetic Interference (EMI) is defined as electromagnetic energy from sources external

or internal to electrical or electronic equipment that adversely affects equipment by creating

undesirable responses (degraded performance or malfunctions). EMI can be divided into two

classes: continuous wave (CW) and transient.

Electromagnetic Compatibility (EMC) is defined as an electrical system's ability to perform its

specified functions in the presence of electrical noise generated either internally or externally by

other systems. The goal of EMC is to minimize the influence of electrical noise

1. Intrasystem EMI

2. Intersystem EMI

The cause of an EMI problem may be either within the system one is dealing with ,in which case the problem is labelled an Intrasystem

The EMI may come from the outside ,in which case the problem is given the INTERSYSTEM

In all cases EMI arieses because of a combination of three factors :Sourse, Transmission path, and response,at least one of which is unplanned.

RECEPTORSOURCE

1

2

3

3

4

Power Line

PATH 1

Direct radiation from source to receptor

PATH 2

Direct radiation from source picked up by the electrical power cables or the signal/control cables connected to the receptor,which reaches the receptor via conduction

PATH 3

Electromagnetic inteference radiated by the electrical power,signal,or control cables of the source

PATH 4

The electromagnetic interference carried by various power/signal/control cables connected to the source,which may couple into the power/signal/control cables of the receptor

TRANSMISSION PATHS

Transmission paths are both Conducted and Radiated.

“Conducted” means carried by metallic paths,including lumped components such as capacitors and transformers.

“Radiated is the term used to characterize non-metallic paths,even though the transfer mechanism be the “near field “ or induction,rather than radiation,field of an antenna.

EEMITTER

SSUSCEPTOR

CE

RE RS

CS

The charateristics of an emitter are said to be its conducted emission and its radiation emission.

The charateristics of a susceptor are said to be its conducted susceptibility and its radiated susceptibilty

Conducted Emission (CE):

The potential EMI that is directly coupled through conduction (with attenuation) from one network or device to another. It may be generated inside equipment and transferred through power lines, I/O lines, or control leads.

Conducted Susceptibility (CS):

The determination or measurement of a device's capability to function in the presence of undesirable conducted EMI.This usually involves conduction through with the I/O cables, signal leads, or power lines.

Radiated Emission (RE):

Desired or undesired electromagnetic energy that is propagated into or across space, either as a transverse electromagnetic wave or by capacitive or inductive coupling.

Radiated Susceptibility (RS):

The determination or measurement of a device's capability to function in the presence of undesirable radiated EMI from external electromagnetic sources

EMITTERS SUSCEPTORS

1. Automoblie Ignition systems 1. Display Devices

2. Power Supplies 2. Relays

3. Generators 3.Navigation Instruments

4. Computers 4. Computers

5. Radar Transmitters 5. Radar Receivers

6. Radio Transmitters 6. Radio Receivers

7. Fluorescent Lights 7. Ordance

Note: “Emitter” is used to denote a source of electromagnetic energy. “Susceptor” is used to denote a device that respondes to electromagnetic energy. Any item in an emitter column may interfere with any item in a susceptor column .

Ordnance weapon: a personal weapon issued to a member of a military unit

Aircraft ordnance, weapons carried by and used by an aircraft

EMITTERS SUSCEPTORS

1.Radio Transmitters 1.Radio Receivers

2.Microwave Relay 2.Microwave Realy

3.Broadcast 3.Broadcast

4.Aircraft 4.Aircraft

5.Shipboard 5.Shipboard

6.Land mobile 6.Land mobile

7.Radio Receivers 7.Radio Transmitters

8.Local Oscillator 8.low-level circuits

9.Radar transmitters 9.Radar Receivers

10.Power lines 10.Heat Pacers

11.Lighting Strokes 11.Computers

12.Motors 12.Navigational Instruments

13.Fluorescent Lights 13.Industrial Controls

Radar interference with aircraft navigation systems

Power line Interference with telecommunications systems

Mobile radio interference with television receivers

Powerline transient interference to computer systems

Aircraft radio interference with shipboard systems

Taxicab radio interference with police radio systems

Distant FM and TV transmitter interference with Grade B coverage of nearby FM and TV transmitters

INTERSYSTEMINTERSYSTEM EMI COMBINATIONS OF SOURCE AND RECEPTORSEMI COMBINATIONS OF SOURCE AND RECEPTORSINTERSYSTEMINTERSYSTEM EMI COMBINATIONS OF SOURCE AND RECEPTORSEMI COMBINATIONS OF SOURCE AND RECEPTORS

Interference from an automotive ignition system to a radio receiver with in the car

Leakage of radar transmitter energy into the radar receiver

Inadvertent interstage coupling via ground loop currents

Interference caused by digital circuits operating from a common power supply with low level analog circuits

Interference caused by magnetic field of a tape drive to low level digital circuits within a computer system

F

C

D E

GJ

FIG: Coupling Paths

H

Noise source ‘A’ radiates and conducts interference .

Sensitive device B accepts the direct radiation.

Sensitive C is radiation coupled via structural member H.

Sensitive devices D and E are coupled to the noise source via common cabling.

Sensitive devices F and G are inductively coupled to the noise source via adjacent wires at J.

CHARACTERISTICS OF INTERFERENCE

1.Bandwidth1.1 Narrowband Interference

1.2 Broadband Interference

2.Amplitude Behavior 2.1 Thermal Noise

2.2 Impulsive Noise

Narrowband Interference is usually thought of as occupying only tens or hundreds of kilohertz at the most. Examples are

1.Single channel radio systems using amplitude modulation ,frequency modulation phase-shift keying or ssb transmission.

2.Certain miltiplexed analog and digital systems

3.The harmonic outputs of communication transmitters

4.Prime power(60hz and 400hz) outputs and their harmonics

5.Local Oscillators

6.Radio altimeters

Broadband Interference

Broadband energy may be dispersed across tens or hundreds of megahertz or more

Broadband interference often is composed of narrow pulses having relatively short rise and fall times

Amplitude Behavior

Interference amplitudes can behave in a variety of manners,regardless of the associated bandwidths.In addition to different types of amplitude distributions

Radiated emission and radiation susceptibilty are measured in terms of field strength (Volts per meter,or tesla).

Conducted emission and conducted susceptibility are measured as voltages and currents(volts, or amperes).

Single–frequency or very narrowband measurements are expressed as amplitude, whereas broadband measurements are expressed on a per unit bandwidth(e.g.per hertz) basis.

Voltage

Volts=millivolts,microvolts.

dBv=dB above one volt reference level

dBmv=dB above one millivolt reference level

dBmicrovolt=dB above one microvolt reference level

dBV= 1020log [( )1 ]Vvolts volt

Current

Amps =milliamps, microampsdBA, dBmA, dBmicro amops

Power

watts=milliwatts,microwatts,picowatts, dBmw, dBmicrowatt

dBw= 1010log [( ) /1pwatts watt

Electric field

Volts per meterdBv/meter,dBmv/meter, etc.

Magnetic field

Tesla=webers/m

Source strength of weak celestial sources

Flux init(FU) =-260dBw/meter square/Hz

Conducted Interference

Conducted interference is interference that propagates through a metallic conductor such as wiring or any metallic structure.

It includes interference propagated through inductors,capacitors and transformers.

A corona is a type of plasma "atmosphere" of the Sun or other celestial body, extending millions of kilometers into space, most easily seen during a total solar eclipse, but also observable in a coronagraph. The Latin root of the word corona means crown.

Sources of Conducted Interference

Source Spectrum Magnitude

Heater Circuits 50khz to 25khz

Fluorescent Lamps 0.1 to 3 Mhz 20 to 300 Microvolt/khz

Mercury Arc Lamps 0.1 to 1.0 Mhz 8000

Computer logic box 50khz to 20 Mhz

Command programmerSignal LinesPower Lines

0.1 to 25 Mhz1 to 25Mhz

Multiplexer 1 to 10 Mhz

Lactching Contactor Coil pulsesContact cycling

1 to 25 Mhz50khz to 25Mhz

Transfer Switch 0.1 to 25 Mhz

Power Supply Switching Circuit 0.5 to 25Mhz

Power Controller 2 to 15 khz

Power Transfer controller Constant noisetransients

10 to 25Mhz50 khz to 25 Mhz

Magnet Armatures 2 to 4 Mhz 250v transient spike

Circuit breaker cam Contacts 10 to 20Mhz

Corona 0.1 to 10Mhz 100microvolt/khz

Vaccum Cleaner 0.1 to 1.0Mhz

/V khz

As a source ,a path and a Susceptor must all be present for an interference situation to exist, so there are design particles relevant to each of these entities.

Conducted inerference problems,design particles deals with the treatment of noise sources ,with transmission paths,and with sensitive devices

1.Noise Source Treatment

1.1 modes of operation

1.2 pulse Design

1.3 Arc Discharges

Noise Source Treatment :

Noise Source Treatment must consider the source’s modes of operation as well as pulse design and arc discharges

Pulse Design :

Pulses produce the least EMI when they have slow rise rates and are of long duration

Do not operate oscillatory circuits and switching devices at a faster speed than that necessary to perform the desired function.

Arc Discharge :

Minimize the occurrence of arc discharges. Arcs result when the potential difference between two bodies is great enough to breakdown the intermediate insulation.

Radiated interference is any interference transferred through a medium by an electromagnetic field. The electromagnetic field constituted energy which actually escapes from a source and spreads out in space according to the laws of wave propagation.

Radiated energy will degrade the performance of a nearby device depend upon:

1.The directivity of the energy as it leaves the source 2.Losses in propagating to the device 3.The susceptibility of the device to the characteristics of the energy.

Electromagnetic radiation can escape from equipment in a variety of ways:

1.Through component case:

The case material is not designed to attenuate adequately the enclosed electric and magnetic fields Example: plastic case or a partially open metal case .

2. Through component case discontinuities: The case is not mechanically tight to EMI(many component cases are not designed to prevent radiation) Example: connector holes, fuse holes, control shaft holes, mating surfaces

Shielding effectiveness:

The component case in which the shielding effectiveness is not fully utilized because of inadequate case grounding

Via interconnecting cabling attached to a component

Via unusal conditions

Example: corroded surfaces, corona discharges ,charge build-up between components, and insulation breakdown.

Source Spectrum Magnitude

Bistable circuits 15khz-400mhz

Harmonic generator 30 khz-100 0 khz

A standard represents a consensus of those substantially concerned with scope and provisions of the particular standard.It is intended as a guide to aid the manufacturer, the user, and others who are likely to be effected.