Things Every Electrical Engineer Should Know About EMC University Presentation October 30, 2006 7...
Transcript of Things Every Electrical Engineer Should Know About EMC University Presentation October 30, 2006 7...
Things Every Electrical Engineer Should Know about EMC
Todd H. HubingMichelin Professor of Vehicular ElectronicsClemson University
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“Signals have a current as well as a voltage.”
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Ground Loop Example
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Ground Loop Example
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Ground Loop Example
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“Currents return to their source and take the path(s) of
least impedance.”
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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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Identify Current Paths
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Identify Current Paths
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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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Where does the 56 MHz return current flow?
Identify Current Paths
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Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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Ground Loop Example 2
D/A
Trace the path of the digital and analog return currents.
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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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“Fast transition times correspond to more high-frequency content
in digital signals.”
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Digital Signal Voltages
t
t
f
f
Control transition times of digital signals!
Signal Termination
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“Efficient antennas below a few GHz are easy to spot.”
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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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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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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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“The circuit board traces with the most high-frequency current are not necessarily the high-frequency signal
traces.”
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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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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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Use common sense!
Visualize signal current paths
Locate antennas and crosstalk paths
Be aware of potential EMI sources
To Avoid EMC Problems