ati's grounding and shielding for emc technical training short course sampler
DESCRIPTION
This three-day course is designed for technicians, operators, and engineers who need an understanding of all facets of grounding and shielding at the circuit, PCB, box or equipment level, cable-interconnected boxes (subsystem), system and building, facilities or vehicle levels. The course offers a discussion of the qualitative techniques for EMI control through grounding and shielding at all levels. It provides for selection of EMI suppression methods via math modeling and graphics of grounding and shielding parameters. Our instructor will use computer software to provide real world examples and case histories. The computer software simulates and demonstrates various concepts and helps bridge the gap between theory and the real world. The computer software will be made available to the attendees. One of the computer programs is used to design interconnecting equipments. This program demonstrates the impact of various grounding schemes and different "fixes" that are applied. Another computer program is used to design a shielded enclosure. The program considers the box material; seams and gaskets; cooling and viewing apertures; and various "fixes" that may be used for aperture protection. . There are also hardware demonstrations of the effect of various compromises and resulting "fixes" on the shielding effectiveness of an enclosure. The compromises that are demonstrated are seam leakage, and a conductor penetrating the enclosure. The hardware demonstrations also include incorporating various "fixes" and illustrating their impact.TRANSCRIPT
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Boost Your Skills with On-Site Courses Tailored to Your Needs The Applied Technology Institute specializes in training programs for technical professionals. Our courses keep you current in the state-of-the-art technology that is essential to keep your company on the cutting edge in today’s highly competitive marketplace. Since 1984, ATI has earned the trust of training departments nationwide, and has presented on-site training at the major Navy, Air Force and NASA centers, and for a large number of contractors. Our training increases effectiveness and productivity. Learn from the proven best. For a Free On-Site Quote Visit Us At: http://www.ATIcourses.com/free_onsite_quote.asp For Our Current Public Course Schedule Go To: http://www.ATIcourses.com/schedule.htm
What Is Ground?
• Earth Ground• Power Ground• Signal ground• Safety Ground• Return• Reference Level
Ground Means any Reference Conductor Used for a Common Return
Grounding Misuse and Myths
• Reasons for Grounding Are Not Clear
• The Word "Grounding" is Often Misusedwhen other Words are Meant, such as:
Connect to BondingReturn Path Earthing
• Many Myths Exist, such as:
"Lower Impedance is Always Better""Use Grounds for Digital-Circuit Reference""Use Separate Safety, Instrument & System Gnd.
• Reasons for Grounding Are Not Clear
Grounding for EMC
• Ground Circuits, Equipments, Systems, Cables, Shields
• Common Mode and Differential Mode Coupling
• Avoid Ground Loops• More Grounds are not Better
Interconnected Equipment Having 29 Questions = 229 = 500,000,000 Answers
Effects of Shared Ground Impedance
• Common Source or Common Ground Impedance Coupling
• EMI and signal use same impedance• Shared impedance provides path for
EMI to couple from a source to a victim.• Minimizing the shared ground
impedane will help mitigate the problem.
• A single point ground may help.
Power Source LoadCMC
CMC
EMI'
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Metallic Structure
Figure 10. Common Ground Impedence Common Mode EMI
EMI
Common Ground Impedance Common Mode EMI
Power Source Load
Figure 4. Illustration of Common Mode Currents
CMC 1
CMC 2
CMC
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Metallic Structure
Illustration of Common Mode Currents
Power Source Load
DCM1
DCM2
Figure 3. Illustration of Differential Mode Currents
Illustration of Differential Mode Currents
Power Source LoadCMC 1
CMC 2
CMC
Metallic Structure
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Figure 5. Illustration of Common and Differential Mode Currents
DMC 1
DMC 2
Illustration of Common and Differential Mode CurrentsIllustration of Common and Differential Mode Currents
Illustration of Common and Differential Mode Currents
PRINCIPAL RADIATION SOURCES ON PRINTED CIRCUIT BOARD
Logic familiesclock rates
• Large single-layer board• PCB card cage with back plane• Multi-layer board
Radiation from ribbon cables
Radiation from IC dips
Power Source Load
Radiated EMI
CMC 1
CMC 2
CMC
Metallic Structure
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Figure 11. Common Mode Radiated EMI
Common Mode Radiated EMI
Ground-Loop CouplingConverts Common Mode Voltage
to Differential-Mode Voltage
Using Ferrites to Absorb Common-Mode EMI
Added Feed-Thru Capacitors -Help Reduce ESD and RF Susceptibility
Note: Max Cap Value
Must Support
Data Bandwidth
C < 1/ωR
Coupling Rejection Offered by Twisting Wire Pair
SHIELDING APPLIES TO ALL LEVELS
• Components
• Circuits
• Functional Stages
• Equipments
• Systems
• Cables
• Platforms
• Buildings
CONCEPTUAL ILLUSTRATION OF FIELD INTENSITIES VS. SOURCE TYPE AND DISTANCE
High Current Corresponds toLow Impedance
Low Current Corresponds toHigh Impedance
High E Low ELoop
High HLow H
NearField
FarField
NearField
FarField
Hθ
Eθ
Hθ
Eθ
V
Monopole
V
I
Low - Impedance,Magnetic - Field Source and Wave
High-Impedance SourceElectric-Field Source and Wave
FIELD IMPEDANCE AS A FUNCTION OF DISTANCE FROM SOURCE
ELECTRIC FIELD VS SOURCE DISTANCE
SUMMARY
Electric Fields
Z > 377 OhmsRadiated From High Impedance Sources
Magnetic Fields
Z < 377 OhmsRadiated from Low Impedance Sources
Near Field Far FieldPlane Waves
Are Generated for all
Source Impedances for
Distance Greater Than
Approximately
1/6 of a Wavelength
REPRESENTATION OF SHIELDING PHENOMENAFOR PLANE WAVES
Outside World
Inside of Enclosure
Incident Wave
AttenuatedIncident
WaveInternal Reflecting
WaveMetalBarrier
ReflectedWave
Transmitted WaveA
B
Ey
Ey
HzEy
Ey
EyHz Hz
Hz
H
SHIELDING EFFECTIVENESS (SE)
SEdB = 20 log10(Eoutside/Einside)SEdB = 20 log10(Houtside/Hinside)
where: E = Electric-field StrengthH = Magnetic-field Strength
SEdB = RdB + AdBwhere:
RdB = Reflection Loss in dBAdB = Absorption Loss in dB
REFLECTION LOSS
σ ε ωfor σ
μ ω j ε ω j σ
μ ω j impedancebarrier Z
HE impedance wave Z
:Where
10 K ,Z 4
Z log 20
VSWR ZZ K,
4K1) K ( log 20 R
b
w
b
w10
b
W2
10dB
<<
=+
==
==
≥
≅
=+
=
REFLECTION LOSS (RdB) OF PLANE WAVES VS FREQUENCY
Copper
Iron*
Hypernick*
0
50
100
150
200
0
50
100
150
200
1kHz 10kHz 100kHz 1MHz 10MHz 100MHz
3kHz 30kHz 300kHz 3MHz 30MHz 300MHz
Radio Frequency
Valid for thickness > 3 δδ = Skin Depth * Permeability assumed constant with frequency
ABSORPTION LOSS, A
Current Density
t
mm σμ f
0.066 δrr MHz
=
copper to relative tyconductivi σcopper to relative typermeabili μ
MHz in frequencyfmm in thickness t where
σμft 131 δ t / 8.68 AdB
r
r
MHz
rr MHz
===
=
==
δ
ABSORPTION LOSS VS FREQUENCY
PRINCIPAL BOX SHIELDING COMPROMISES
Connectors
Fuse
Switch
PotentiometerPanel Meter
StatusIndicator
Lamp
Cover Platefor Access
Holes or Slotsfor Convection Cooling
Screw Spacing= Slot Radiation
Forced AirCooling
SLOT AND APERTURE LEAKAGE
L
ht
t << h Shield Material
Log Frequency
•
•λ / 2
SE (d
B)
Reducing Radiation Coupling by Shielding Cable Wires
