things every electrical engineer should know about emc university presentation october 30, 2006 7...
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Things Every Electrical Engineer Should Know about EMC
Todd H. HubingMichelin Professor of Vehicular ElectronicsClemson University
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Kyoto University PresentationOctober 30, 2006 2
“Signals have a current as well as a voltage.”
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Kyoto University PresentationOctober 30, 2006 3
Ground Loop Example
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Kyoto University PresentationOctober 30, 2006 4
Ground Loop Example
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Kyoto University PresentationOctober 30, 2006 5
Ground Loop Example
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Kyoto University PresentationOctober 30, 2006 6
“Currents return to their source and take the path(s) of
least impedance.”
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Kyoto University PresentationOctober 30, 2006 7
EMC course final exam question
After passing through a CMOS input, signal current always ultimately flows:
a.) to digital groundb.) to the earthc.) back to the sourced.) all of the abovee.) it doesn’t matter.
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Kyoto University PresentationOctober 30, 2006 8
Identify Current Paths
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Kyoto University PresentationOctober 30, 2006 9
Identify Current Paths
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Kyoto University PresentationOctober 30, 2006 10
Current takes the path of least impedance!
> 100 kHz this is generally the path of least inductance
< 10 kHz this is generally the path(s) of least resistance
Identify Current Paths
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Kyoto University PresentationOctober 30, 2006 11
Where does the 56 MHz return current flow?
Identify Current Paths
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Kyoto University PresentationOctober 30, 2006 12
Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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Kyoto University PresentationOctober 30, 2006 13
Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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Kyoto University PresentationOctober 30, 2006 14
Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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Kyoto University PresentationOctober 30, 2006 15
1. Don’t do it!
2. If you must do it, never ever allow a trace or another plane to cross over the gap.
3. If you must do it, never ever place a gap between two connectors.
4. See Rule #1!
Rules for Gapping a Ground Plane
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Kyoto University PresentationOctober 30, 2006 16
“Fast transition times correspond to more high-frequency content
in digital signals.”
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Kyoto University PresentationOctober 30, 2006 17
Digital Signal Voltages
t
t
f
f
Control transition times of digital signals!
Signal Termination
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Kyoto University PresentationOctober 30, 2006 18
“Efficient antennas below a few GHz are easy to spot.”
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Kyoto University PresentationOctober 30, 2006 19
λ/2
λ/4 Quarter-Wave Monopole
Half-Wave Dipole
• Size
• Two Halves
Electrically Small Loop
What makes an efficient antenna?
Identify Antennas
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Kyoto University PresentationOctober 30, 2006 20
Voltage DrivenSignal or component voltage appears between two good antenna parts.
metersmmVEMHzvoltV
rad
s
3@/360500@1
≈=
More than 60 dB above the FCC Class B limit!
Example:
Recognize Coupling Mechanisms
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Kyoto University PresentationOctober 30, 2006 21
Current DrivenSignal current loop induces a voltage between two good antenna parts.
- Vcm +
Current driven voltage tend to be 3 or 4 orders of magnitude smaller than voltage driven voltages. However, antenna
efficiencies can be 5 or 6 orders of magnitude higher.
Recognize Coupling Mechanisms
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Kyoto University PresentationOctober 30, 2006 22
“The circuit board traces with the most high-frequency current are not necessarily the high-frequency signal
traces.”
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Kyoto University PresentationOctober 30, 2006 23
Active Devices (Power Pins)
For some ICs, the high-frequency currents drawn from the power pins can be much greater than the high-frequency currents in the signals!
Identify Sources
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Kyoto University PresentationOctober 30, 2006 24
Noise on the low-speed I/O
For some ICs, significant high-frequency currents appear on low-speed I/O including outputs that never change state during normal operation!
Identify Sources
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Kyoto University PresentationOctober 30, 2006 25
Use common sense!
Visualize signal current paths
Locate antennas and crosstalk paths
Be aware of potential EMI sources
To Avoid EMC Problems