emc fundamentals

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Things Every Electrical Engineer

Should Know about EMC

Todd H. HubingMichelin Professor of Vehicular Electronics

Clemson University



Kyoto University Presentation

October 30, 2006 3

Ground Loop Example




October 30, 2006 4

Ground Loop Example




October 30, 2006 5

Ground Loop Example




October 30, 2006 6

“Currents return to theirsource and take the path(s) of

least impedance.”




October 30, 2006 7

EMC course final exam question

After passing through a CMOS input, signalcurrent always ultimately flows:

a.) to digital ground

b.) to the earth

c.) back to the sourced.) all of the above

e.) it doesn’t matter.




October 30, 2006 8

Identify Current Paths




October 30, 2006 9





October 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





October 30, 2006 11

Where does the 56 MHz return current flow?





October 30, 2006 12

Ground Loop Example 2

D/A

Trace the path of the digital and analog return currents.




October 30, 2006 13


D/A





October 30, 2006 14


D/A





October 30, 2006 15

1. Don’t do it!

2. If you must do it, never ever allow a trace oranother 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




October 30, 2006 16

“Fast transition times correspondto more high-frequency content

in digital signals.”




October 30, 2006 17

Digital Signal Voltages

t

t

f

f

Control transition times of digital signals!

Signal Termination




October 30, 2006 18

“Efficient antennas below afew GHz are easy to spot.”




October 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




October 30, 2006 20

Voltage DrivenSignal or component voltage appears between two good antenna parts.

metersmmV E

MHzvolt V

rad

s

3@ / 360

500@1

≈

=

More than 60 dB above the FCC Class B limit!

Example:

Recognize Coupling Mechanisms




October 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 magnitudesmaller than voltage driven voltages. However, antenna

efficiencies can be 5 or 6 orders of magnitude higher.

Recognize Coupling Mechanisms




October 30, 2006 22

“The circuit board traces with themost high-frequency current are not

necessarily the high-frequency signaltraces.”




October 30, 2006 23

Active Devices (Power Pins)

For some ICs, the high-frequency currents drawn from the power pinscan be much greater than the high-frequency currents in the signals!

Identify Sources




October 30, 2006 24

Noise on the low-speed I/O

For some ICs, significant high-frequency currents appear on low-speedI/O including outputs that never change state during normal operation!

Identify Sources




October 30, 2006 25

Use common sense!

Visualize signal current paths

Locate antennas and crosstalk pathsBe aware of potential EMI sources

To Avoid EMC Problems

emc fundamentals

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