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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
http://www.wll.com/http://www.wll.com/