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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 14, 2007
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IntroductionElements of an EMI Situation
• Source "Culprit"• Coupling method "Path"• Sensitive device "Victim"
SOURCEPATH
VICTIM
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Let’s see how this all got startedDead 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
Marconihttp://www.wll.com
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
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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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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)
VC
M
VDM
INoise
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Radiated Coupling: Field to Cable
Patient Monitor
Loop AreaInduced Current
Electromagnetic WaveRadio
VC
M
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Instrumentation Interference
Interference Current, If
Ideal Response
Frequency (Hz)
EKG Signal
Real Response
Frequency (MHz)
NOISE
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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.
What’s this for?
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Filtering
Interference Current
EKG Signal
C
C
Interference Current
EKG Signal
Please, I’m very ticklish
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Surge Coupling• Lightning and pulse sources cause high-energy transients
into power and data cables
IndirectDirect
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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/
f =1/r
r
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)
dB
uV
/m
FCC B CISPR 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 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!
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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 ~ 10 dBuV• E-Field Limit for FCC: ~40-60 dBuV/m• Distance to moon: 107dBmile (20LOG2.5E+5miles)• National debt: 128dB$ (10LOG6E+12)
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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 SourcesFourier Analysis
A
F(t)
Spectrum of a Pulse
Log Ff =1/
f =1/r
r
Fourier->
Do you like my new shirt?
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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 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 (Human Body Resonance)
= 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 to Interference
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/2R 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)
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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
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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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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 PerformancePoor Installation =Poor Performance
Filter
Filter INFilter OUT
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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 low frequencies; High-current conditions
I
V
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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RF Leakage
VRF~
Metal Box
RF Source
L
L ~ /2Perfect Transmission
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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 Field E2
SE = E2/E1 (dB)
Reflected ER
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Shielding - Reflection/Absorption
RR fE dBmeters Hz
( )( ) ( )
log(* *
) 322 10 2 3
Rf R
H dB
Hz meters
( )
( ) ( ). log(
* *) 14 5 10
2
R fP dB Hz( ) ( )log( * ) 168 10
A k t fdB Hz( ) ( )* * * *
Plane wave occurs when E to H wave impedance ratio = 1
f RMHzmeters
( )( )
3002
k = 3.4 for t in inches and k = 134 for t in meters
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Shielding - Material
Metal Conductivity - Permeability -
Silver 1.05 1Copper 1 1Gold 0.7 1
Aluminum 0.61 1Zinc 0.29 1Brass 0.26 1Nickel 0.2 1
Iron 0.17 1000Tin 0.15 1
Steel 0.1 1000Hypernick 0.06 80000
Monel 0.04 1Mu-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 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)
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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/
f =1/r
r
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 dVdt *
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Switching Power Supplies
• Two Sources:• Harmonics of switching power supply• Broadband emissions due to ringing
waveforms
&f
f
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Underdamped (Ringing) Waveform
• Typical in switching circuits
f100 MHz+
100sVolts
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 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.
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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 principal problem is the cable-to-backshell termination
2. Unfiltered cable penetrations3. 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
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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 emissions9. Inadequately-installed power line filters
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The Ten Steps toAvoiding EMI Problems
1. Signal Termination2. Layout3. Decoupling & Power
Distribution4. Grounding5. Bonding
6. Filtering
7. Cabling
8. Shielding
9. Surge Suppression
10. CHECKLIST
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CHECKLIST
Signal Termination RC Terminations (33 ohms + 27 pF) on
periodic signals Group high frequency sources together;
minimize trace runs of high frequencysignals
Don’t source/sink I/O (whether internal orexternal) through high frequency devices
Position oscillators and crystals away fromI/O and openings in the chassis
Snub switching power supply waveforms tominimize HF energy
Decoupling & Power Distribution Connect all ground pins of high frequency
circuits together 0V reference (bond 0V to chassis) Solid power and Ground planes No impedances in Vcc/power traces.
Bonding Checklist Bond 0V to chassis ground Bond 0V to connector frames and shells Bond connector frames to chassis Bond metal frames together
Filtering Filters are installed at enclosure wall LC filter on unshielded cables Plan for capacitor on shielded lines
Cabling Route cables to avoid coupling Use only fully-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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