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

EMC Fundamentals

Presented By:Mike Violette

Washington Laboratories, Ltd.September 15, 2006

Introduction

Elements of an EMI Situation

• Source "Culprit"

• Coupling method "Path"

• Sensitive device "Victim"

SOURCEPATH

VICTIM

Let’s see how this all got started

Dead Smart Guys

• First Transmitters: Spark Devices

• Heinrich Hertz (1857-1894) clarified and expanded on

• James Clerk Maxwell’s Electromagnetic Theory

• Marconi: first use & patent

HertzMaxwell

Marconi

How Does EMI Affect Electronics?• Radiated and conducted interference

• Conducted Interference Enters and Exits Equipment through Wiring and Cabling

• Radiated Interference Enters and Exits Equipment through Wiring and Enclosure Penetration

Radiated Susceptibility Radiated Emissions

Conducted Susceptibility Conducted Emissions

Interference to TV Reception

Two Interfering Signals Injected into TV

No Interference

Common “Coupling Modes”

Common and Differential Mode

• Crosstalk (cabling and conductors)

• Field to cable (“Antenna”)

• Conducted (direct)

• Field to enclosure

Crosstalk

(cable-to-cable coupling)

SOURCE

VICTIM

Radiated Coupling: Field to Cable

Loop Area

Induced Current

Electromagnetic Wave

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

COMMON and DIFFERENTIAL MODE• COMMON-MODE: “Line to Ground”

• DIFFERENTIAL MODE: “Line-to-Line” (Normal Mode)

VCM

VDM

INoise

Radiated Coupling: Field to Cable

Patient Monitor

Loop AreaInduced Current

Electromagnetic WaveRadio

VCM

Instrumentation Interference

Interference Current, If

Ideal Response

Frequency (Hz)

EKG Signal

Real Response

Frequency (MHz)

NOISE

Effect of Modulation

Interference Current, If

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.

What’s this for?

Filtering

Interference Current

EKG Signal

C

C

Interference Current

EKG Signal

Please, I’m very ticklish

Surge Coupling

• Lightning and pulse sources cause high-energy transients into power and data cables

IndirectDirect

Digital Equipment SourcesFourier Analysis

F(t)Log Ff=

1/T2f 3f

T

A

Spectrum of a Square Wave

T

A

Log F

F(t)f=1/πτπτπτπτ

f=1/πτπτπτπτr

ττττr

ττττ

Spectrum of a Trapezoidal Wave(Characteristic of Digital Devices)

Equipment Emissions Limits

Emissions Limits @ 3 meters

0

10

20

30

40

50

60

70

10 100 1000 10000

Frequency (MHz)

dBuV/m

FCC B CISPR B

FCC A

CISPR A

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 small numbers

• 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

(ohm’s Law: P = E2/R)

Named

after me!

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 ~ 10 dBuV

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

• Distance to moon: 107dBmile (20LOG2.5E+5miles)

• National debt: 128dB$

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

Pulsed SourcesFourier Analysis

A

F(t)

Spectrum of a Pulse

ττττ

Log Ff=1/πτπτπτπτ

f=1/πτπτπτπτr

ττττr

Do you like my

new shirt?

Urban Ambient Profile

Switching noise

Cell phone

FM Radio

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 but the installation requirements must be considered during the design

• 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 meter

λ= c/f = 3X108/frequencyλ = 300/fMHz

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

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 to Interference Problems• Power cables

• Grounding wires

• Patient cables

• Data cables

• Control harnesses

• Structures!

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/2πR 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 cm

amps

meters

( / ) ( ) ( )

( )

( )

. * *= 13 22

V ~

I

Area

E (& H)

Filters - Overview• Passband

• High pass

• Low pass

• Single component, L, Pi, T

• Common mode; differential mode

• Placement

• Components

• Lead length

• Leakage Limitations

Low Pass Filter

Noise Current

EKG Signal

C

C

Noise Current

EKG Signal

Frequency (Hz)

Rejection

EKG Signal

Noise

Attenuation of Noise

Filters - Types

Filters - Components

• Discrete Component Filters• Component selection

• Lead length considerations

• Power Filter Modules

• Filtered Connectors• Construction

• Selective loading

• Termination (bonding and grounding)

Circuit Design – Real Performance

FiltersPower Line Filter Typical Schematic

Signal Line Filter(Screw-in Type)

Signal Line Filter

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

Filter Performance

Poor Installation =

Poor Performance

Filter

Filter INFilter OUT

Filter Placement

Shield Concepts

+ -

Field Terminations on Inside

Metal Sphere“Faraday Cage”

“Ground” 0V Potential

V+

V=0

+ -Electric Field Coupling

E-FieldV+

Shield Concepts

Magnetic Field Shielding

Common at powerline and low frequencies;

High-current conditions

I

V

µ >>1

Ferrous Shield

Low residual field

Magnetic Field Coupling

V

I

Effects of Openings

+ -

Metal Sphere“Faraday Cage” V=0

V+

V=?

Cable Leakage

+

Radio Frequency Effects

VRF~

Shielded Enclosure

RF Source

RF Leakage

VRF ~

Metal Box

RF Source

L

L ~ λλλλ/2Perfect Transmission

Shielding

The Business Card TestGood to about 1 GHz

Shielding - Overview

• Shields - conductive barriers• Reflection

• Absorption

• Materials• Electric field - conductivity

• Magnetic field - permeability

• Discontinuities• Windows

• Vents

• Seams

• Panel components

• Cable connections

Shielding Effectiveness

SHIELD

Incident Field E1 Resultant Field E2

SE = E2/E1 (dB)

Reflected ER

Shielding -Reflection/Absorption

RR fE dBmeters Hz

( )( ) ( )

log(* *

)= +322 10 2 3

σ

µ

Rf R

H dB

Hz meters

( )

( ) ( ). log(

* *)= +14 5 10

2σ

µ

R fP dB Hz( ) ( )log( * )= +168 10σ

µ

A k t fdB Hz( ) ( )* * * *= µ σ

Plane wave occurs when E to H wave impedance ratio = 1

f RMHzmeters

( )( )

> 3002π

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

Shielding - Material

Metal Conductivity - σ Permeability - µ

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 1000

Hypernick 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

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

Shielding - Penetration

• Conductors penetrating an opening negates the shielding provided by absorption and reflection

• Cables penetrations require continuation of the shield or

• Conductors require filtering at the boundary

• Cable shields require termination

• Metal control shafts serve as a conductor

• Use non-metallic

• Terminate shaft (full circle)

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

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)

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

Galvanic Scale

Component Selection

T

A

Log F

F(t)f=1/T

2f 3f

T

A

Log F

F(t)f=1/πτπτπτπτ

f=1/πτπτπτπτr

ττττr

ττττ

Spectrum of a Square Wave

Spectrum of a Trapezoidal Wave(Characteristic of Digital Devices)

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 dVdt⇒ *

Switching Power Supplies

• Two Sources:• Harmonics of switching power supply

• Broadband emissions due to ringing waveforms

&f

f

Underdamped (Ringing) Waveform

• Typical in switching circuits

f100 MHz+

100sVolts

10s kHz

dV/dT = 100sMV/s

Broadband (radiated & conducted)

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

Decoupling &Power Distribution

• Connect all ground pins of high frequency circuits together in the same ground structure.

• Do not separate, isolate, break or otherwise “cut” the ground plane.

• Do not separate, isolate, break or otherwise “cut” the power plane.

• Do not insert impedances into Vcc/power traces.

Isolated Power/Grounding

• Example Trace Layout (Bad Idea!)

Exception: Analog circuit isolation

Top 10 Common Mistakes

1. Improperly shielded cables: The principal problem 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, or improper mating surfaces

5. Poor bonding between metal parts of unit

Top 10 Common Mistakes

5. Long ground leads on shields and bonding conductors

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

7. Not accounting for the high frequency effects of ESD

8. Inadequate filters on I/O cables for emissions

9. Inadequately-installed power line filters

The Ten Steps toAvoiding EMI Problems

1. Signal Termination

2. Layout

3. Decoupling & Power Distribution

4. Grounding

5. Bonding

6. Filtering

7. Cabling

8. Shielding

9. Surge Suppression

10. CHECKLIST

CHECKLIST

Signa l Termination◊ RC Term inations (33 ohms + 27 pF) on

per io dic s ignals

◊ Group high frequency sources togeth er;

m inim ize trace runs of h ig h frequencys ign als

◊ Don ’t source/s ink I/O (w heth er interna l or

ex ternal) throu gh high frequen cy devices

◊ Position oscilla tors a nd c rys tals away fromI/O and openings in the ch assis

◊ Snub switching power sup ply w aveforms tom inim ize HF e nergy

Decoupling & Power Distribution◊ Connect all ground pins of high frequency

c ircuits together

◊ 0V reference (bond 0V to chass is )

◊ Solid pow er a nd Ground planes

◊ No impedance s in Vcc/power traces .

Bonding Checklis t◊ Bond 0V to ch assis ground

◊ Bond 0V to co nnector frames an d shells

◊ Bond conne cto r frames to ch assis

◊ Bond metal frames to geth er

Filtering◊ Filters are installed a t enclosure wall

◊ LC filte r on unshielded cables

◊ Plan for capac itor on shielded lin es

Cabling◊ Route cables to avoid coupling◊ Use only fully -shielded ca ble s

◊ Fully -te rminate shield grounds to

metal/metalized conne ctor shels◊ Termina te shells to chass is

Shielding◊ The Business Card Test

◊ Use co rrectly-rated su ppressor line -to -line

and line -to-ground

◊ Gas Tubes

◊ Var is to rs

◊ SAD (Silicon Avalanche Diode s)

WLL Contact Information

www.wll.com; info@wll.comPhone: 301 216-1500Fax: 301 417-9069

INFO@WLL.COM