2.EMC Fundamentals Sept 2006

7/29/2019 2.EMC Fundamentals Sept 2006

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Washington Laboratories (301) 417-0220 web: www.wll.com 7560 Lindbergh Dr. Gaithersburg, MD 20879

EMCFundamentals

Presented By:Mike Violette

Washington Laboratories, Ltd.

September 15, 2006


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Introduction

Elements of an EMI Situation

Source "Culprit"

Coupling method "Path"

Sensitive device "Victim"

SOURCEPATH

VICTIM


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Lets see how this all got started

Dead Smart Guys

First Transmitters: Spark Devices

Heinrich Hertz (1857-1894) clarifiedand expanded on

James Clerk Maxwells

Electromagnetic Theory

Marconi: first use & patent

HertzMaxwell

Marconi


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How Does EMI Affect Electronics? Radiated and conducted interference

Conducted Interference Enters and Exits Equipment throughWiring and Cabling

Radiated Interference Enters and Exits Equipment throughWiring and Enclosure Penetration

Radiated Susceptibility Radiated Emissions

Conducted Susceptibility Conducted Emissions


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Interference to TV Reception

Two Interfering Signals Injected into TV

No Interference


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Common Coupling Modes

Common and Differential Mode

Crosstalk (cabling and conductors) Field to cable (Antenna)

Conducted (direct)

Field to enclosure


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Crosstalk

(cable-to-cable coupling)

SOURCE

VICTIM


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Radiated Coupling: Field to Cable

Loop Area

Induced Current

Electromagnetic Wave

Coupling proportional to: E/H Field, Loop Area, Frequency


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COMMON and DIFFERENTIAL MODE COMMON-MODE: Line to Ground DIFFERENTIAL MODE: Line-to-Line (Normal

Mode)

VCM

VDM

INoise


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Radiated Coupling: Field to Cable

Patient Monitor

Loop AreaInduced Current

Electromagnetic WaveRadio

VCM


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Instrumentation Interference

Interference Current, If

Ideal Response

Frequency (Hz)

EKG Signal

Real Response

Frequency (MHz)

NOISE


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Effect of Modulation

Interference Current, If


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How Does EMI Affect Electronics? Electrostatic Discharge & Transient Pulses

ESD can induce glitches in circuits,

leading to false triggering, errors in address& data lines and latch-up of devices

Upset

Damage Degradation leading to future failure(s)

Gee, the humidity

is low in here.

Whats this for?


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Filtering

Interference Current

EKG Signal

C

C

Interference Current

EKG Signal

Please, Im veryticklish


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Surge Coupling

Lightning and pulse sources cause high-energy transientsinto power and data cables

IndirectDirect


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Digital Equipment Sources

Fourier Analysis

F(t)Log Ff=

1/T

2f 3f

T

A

Spectrum of a Square Wave

T

A

Log F

F(t)f=

1/pt

f=1/ptr

tr

t

Spectrum of a Trapezoidal Wave

(Characteristic of Digital Devices)


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Equipment Emissions Limits

Emissions Limits @ 3 meters

0

10

20

30

40

50

60

70

10 100 1000 10000

Frequency (MHz)

dBuV/m

FCC BCISPR B

FCC A

CISPR A


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The decibel (dB)

The dB is used in Regulatory Limits (FCC, CISPR, etc.)

The dB is a convenient way to express very big and very smallnumbers

The Bel was named after Alexander Graham Bell

Bel = LOG10(P2/P1)

deciBel provides a more realistic scale:

dB = 10LOG10(P2/P1)

Voltage & Current are expressed as follows:dB (V or I) = 20LOG10(V2/V1)

20LOG derives from the conversion from Power to Voltage

(ohms Law: P = E2/R)

Named

after me!


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dB Can have several reference units:

Watt: dB above one Watt (dBW)

Milliwatt: dB above one milliwatt(dBm)

Volt: dBV

Microvolt: dBuV Microamp: dBuA

picotesla: dBpT

Electric Field: dBuV/m

Radio Receiver Sensitivity ~ 10dBuV

E-Field Limit for FCC: ~40-60dBuV/m

Distance to moon: 107dBmile

(20LOG2.5E+5miles)


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Broadband Sources

Man-made noise dominates Intended transmissions, switching transients, motors, arcing

Intermittent operation of CW causes transient effects Digital Switching

Inductive kick

Switch bounce

Digital Signaling Broad spectrum based on pulse width & transition time HDTV

CDMA

UWB Technologies


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Pulsed Sources

Fourier Analysis

A

F(t)

Spectrum of a Pulse

t

Log Ff=

1/pt

f=1/ptr

tr

Do you like my

new shirt?


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Urban Ambient Profile

Switching noise

Cell phone

FM Radio


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Cables - Overview Major coupling factor in radiating emissions from an equipment

and coupling of emissions from other sources into an equipment

Acts as radiating antenna, receiving antenna, and cable-to-cable coupling mechanism

External cables are not typically part of the equipment design butthe installation requirements must be considered during thedesign

Problem is a function of cable length, impedance, geometry,frequency of the signal and harmonics, current in the line,distance from cable to observation point

Frequency Effects: Tied into Cable Wavelength

For example, wavelength at FM Radio Band (100 MHz) is 1

= c/f = 3X108/frequency = 300/fMHz


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Cables - Length/Impedance Efficiency as an antenna - function of length compared to

wavelength

At typical data transfer rates - length is short

At harmonics or spurs the length may become long Impedance mismatch creates a high SWR


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How very important

Frequencies of testing from 26 MHz to 1 GHz

Corresponding cable lengths:

L ~ 11 meters @ 26 MHz to 30 cm @ 1 GHz

Short cables can be large contributors toInterference Problems Power cables

Grounding wires Patient cables

Data cables

Control harnesses

Structures!


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Cables - Loops Emissions are a function of 1) Current; 2) Loop Geometry; 3) Return Path of the

Current

Current flow creates a magnetic field H=I/2pR for a single wire model

Single wire case is not realistic

Loop geometry formed by the current carrying conductor and the return line

contribute to the field strength Electric field strength:

E f AI

RV m MHz cmamps

meters

( / ) ( ) ( )

( )

( )

. * * 13 2 2

V ~

I

Area

E (& H)


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Filters - Overview Passband

High pass

Low pass

Single component, L, Pi, T

Common mode; differential mode

Placement Components

Lead length

Leakage Limitations


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Low Pass Filter

Noise Current

EKG Signal

C

C

Noise Current

EKG Signal

Frequency (Hz)

Rejection

EKG Signal

Noise

Attenuation of Noise

Filt T


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Filters - Types


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Filters - Components Discrete Component Filters

Component selection Lead length considerations

Power Filter Modules

Filtered Connectors Construction

Selective loading

Termination (bonding and grounding)


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Circuit Design Real Performance


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FiltersPower Line Filter Typical Schematic

Signal Line Filter(Screw-in Type)

Signal Line Filter


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Filter - Placement Isolate Input & Output

Establish boundaries with filters between

Input or Output interfaces and active circuitry

Digital and Analog

Compartments and Modules

Prevent bypass coupling Control line exposure on line side of filter

Use dog-house compartment

Shielded cables to control exposed cable runs Terminate - Terminate - Terminate

Low impedance to ground termination

Minimize lead length


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Filter Performance

Poor Installation =Poor Performance

Filter

Filter IN

Filter OUT


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Filter Placement


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Shield Concepts

+ -

Field Terminations on Inside

Metal Sphere

Faraday Cage

Ground 0V Potential

V+

V=0

+ -Electric Field Coupling

E-FieldV+


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Shield Concepts

Magnetic Field Shielding

Common at powerline and lowfrequencies;

High-current conditions

I

V

m >>1

Ferrous Shield

Low residual field

Magnetic Field Coupling

V

I


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Effects of Openings

+ -

Metal Sphere

Faraday Cage V=0

V+

V=?

Cable Leakage

+


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Radio Frequency Effects

VRF~

Shielded Enclosure

RF Source


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RF Leakage

VRF ~

Metal Box

RF Source

L

L ~ /2Perfect Transmission


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ShieldingThe Business Card Test

Good to about 1 GHz


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Shielding - Overview

Shields - conductive barriers Reflection

Absorption Materials

Electric field - conductivity

Magnetic field - permeability

Discontinuities Windows Vents

Seams

Panel components

Cable connections


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Shielding Effectiveness

SHIELD

Incident Field E1 Resultant FieldE2

SE = E2/E

1(dB)

ReflectedER


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Shielding -Reflection/Absorption

RR fE dB meters Hz

( )

( ) ( )

log(* *

) 322 10 2 3

m

Rf R

H dB

Hz meters

( )

( ) ( ). log(

* *) 14 5 10

2

m

R fP dB Hz( ) ( )log( * ) 168 10

m

A k t fdB Hz ( ) ( )* * * * m

Plane wave occurs when E to H wave impedance ratio = 1f RMHz meters( ) ( )

> 3002p

k = 3.4 for t in inches and k = 134 for t in meters


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Shielding - Material

Metal Conductivity - Permeability - m

Silver 1.05 1

Copper 1 1

Gold 0.7 1

Aluminum 0.61 1

Zinc 0.29 1

Brass 0.26 1

Nickel 0.2 1

Iron 0.17 1000

Tin 0.15 1

Steel 0.1 1000Hypernick 0.06 80000

Monel 0.04 1

Mu-Metal 0.03 80000

Stainless Steel 0.02 1000

All are good electric field shields Need high u for Mag Field Shield


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Shielding - Seams/Gaskets

Required openings offer no shielding in many applications

Apertures associated with covers tend to be long or require

many contact points (close screw spacing) Large opening treatment

Screens, ventilation covers, optic window treatments

WBCO formed to effectively close opening

Seam opening treatments

Overlapping flanges

Closely spaces screws or weld

Gasket to provide opening contact

Gasketed SE

SE a LdB cm( ) ( ).log( * ) 115 10 1 2 SE a LdB in( ) ( )

.log( * ) 99 10 1 2


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Shielding - Penetration

Conductors penetrating an opening negates theshielding provided by absorption and reflection

Cables penetrations require continuation of theshield.

or

Conductors require filtering at the boundary

Cable shields require termination Metal control shaftsserve as a conductor

Use non-metallic

Terminate shaft (full circle)


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Grounding - Overview

Purpose Safety protection from power faults

Lightning protection Dissipation of electrostatic charge

Reference point for signals

Reference point is prime importance for EMC

Potential problems Common return path coupling

High common impedance

High frequency performance


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Grounding - Impedance

Establish a low impedance return

Ground planes

Ground straps for high frequency performance

Establish single point or multipoint ground

Single point for low frequency or short distance

Distance(meters) < 15/f(MHz) Multipoint for high frequency or long distance

Distance(meters) > 15/f(MHz)


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Bonding

Bonds should have two basic characteristics

Low impedance < 2.5 milliohms

Mechanical & electro-chemical stability Low impedance

Avoid contamination

Provide for flush junction to maximize surface contact

Use gaskets or fingerstock for seam bonds

Provide a connecting mechanism Mechanical and electro-chemical stability

Torque to seat for the mechanical connection

Lock washers to retain bond

Allow for galvanic activity for dissimilar metals


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Galvanic Scale


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Component Selection

T

A

Log F

F(t)f=1/T

2f 3f

T

A

Log F

F(t)f=

1/pt

f=1/ptr

tr

t

Spectrum of a Square Wave

Spectrum of a Trapezoidal Wave

(Characteristic of Digital Devices)


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Circuit Design

Component Selection

Circuits available in an EMI version

Specify logic of necessary speed - not faster than required

EMI performance varies between manufacturers

MAX485 MAX487

EMI V

dV

dt

*


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Switching Power Supplies

Two Sources: Harmonics of switching power supply

Broadband emissions due to ringingwaveforms

&f

f


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Underdamped (Ringing) Waveform

Typical in switching circuits

f100 MHz+

100s

Volts

10s kHz

dV/dT = 100sMV/s

Broadband (radiated & conducted)


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Circuit Design - Summary

Consider EMI at the beginning Understand requirements

Select components

Design in protection

Circuit Design - Layout Design in ground planes, guards, segregation

EMI gains from layout has virtually zero recurring cost

Grounds and Returns Develop a ground scheme

Consider digital, analog, return, and shield terminations

Design in hooks Provide space for potential fix actions that may be required


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Decoupling &Power Distribution

Connect all ground pins of high frequencycircuits together in the same ground structure.

Do not separate, isolate, break or otherwisecut the ground plane.

Do not separate, isolate, break or otherwisecut the power plane.

Do not insert impedances into Vcc/powertraces.


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Isolated Power/Grounding

Example Trace Layout (Bad Idea!)

Exception: Analog circuit isolation


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Top 10 Common Mistakes

1. Improperly shielded cables: The principalproblem is the cable-to-backshell termination

2. Unfiltered cable penetrations

3. High Frequency sources with poor termination:High frequency sources: signals and power supplies

4. Case seams and apertures: bad/no gasket, orimproper mating surfaces

5. Poor bonding between metal parts of unit


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Top 10 Common Mistakes

5. Long ground leads on shields and bondingconductors

6. No high frequency filtering on analog inputs:Radiated and conducted immunity

7. Not accounting for the high frequency effectsof ESD

8. Inadequate filters on I/O cables for emissions9. Inadequately-installed power line filters


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The Ten Steps to

Avoiding EMI Problems

1. SignalTermination

2. Layout

3. Decoupling &Power Distribution

4. Grounding5. Bonding

6. Filtering

7. Cabling8. Shielding

9. Surge Suppression

10. CHECKLIST


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CHECKLIST

Signal Termination RC Terminations (33 ohms + 27 pF) on

per iodic sign als

Group high frequency so urces together;

minimize trace runs of high frequency

signals

Dont source/sink I/O (whether internal orexternal) through h igh frequency de vices

Position oscillators and crystals away from

I/O and openings in the chassis

Snub sw itching power supp ly waveforms to

minimize HF energy

Decoupling & Power D istribution Connect all ground pins of high frequency

circuits togethe r 0V reference (bond 0V to chassis)

Solid power and Ground planes

No impedances in Vcc/power traces.

Bonding C hecklist Bond 0V to chassis ground

Bond 0V to connector frames and shells

Bond conn ector frames to chassis

Bond m etal frames together

Filtering Filters are installed at enc losure wall

LC filter on unshielded cables

Plan for capacitor on shielded lines

Cabling Route cables to avoid coupling Use onlyfully-shielded cables

Fully-terminate shield grounds to

metal/metalized connector shels

Terminate shells to chassis

Shielding The Business Card Test

Use correctly-rated suppressor line-to-line

and line-to-ground Gas Tubes

Varistors

SAD (Silicon Avalanche Diodes)


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WLL Contact Information

www.wll.com; [email protected]: 301 216-1500Fax: 301 417-9069

[email protected]
http://www.wll.com/http://www.wll.com/

Documents

2.EMC Fundamentals Sept 2006