Things Every Electrical Engineer Should Know about EMC

Todd H. HubingMichelin Professor of Vehicular ElectronicsClemson University

Kyoto University PresentationOctober 30, 2006 2

“Signals have a current as well as a voltage.”

Kyoto University PresentationOctober 30, 2006 3

Ground Loop Example

Kyoto University PresentationOctober 30, 2006 4

Ground Loop Example

Kyoto University PresentationOctober 30, 2006 5

Ground Loop Example

Kyoto University PresentationOctober 30, 2006 6

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

least impedance.”

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.

Kyoto University PresentationOctober 30, 2006 8

Identify Current Paths

Kyoto University PresentationOctober 30, 2006 9

Identify Current Paths

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

Kyoto University PresentationOctober 30, 2006 11

Where does the 56 MHz return current flow?

Identify Current Paths

Kyoto University PresentationOctober 30, 2006 12

Ground Loop Example 2

D/A

Trace the path of the digital and analog return currents.

Kyoto University PresentationOctober 30, 2006 13

Ground Loop Example 2

D/A

Trace the path of the digital and analog return currents.

Kyoto University PresentationOctober 30, 2006 14

Ground Loop Example 2

D/A

Trace the path of the digital and analog return currents.

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

Kyoto University PresentationOctober 30, 2006 16

“Fast transition times correspond to more high-frequency content

in digital signals.”

Kyoto University PresentationOctober 30, 2006 17

Digital Signal Voltages

t

t

f

f

Control transition times of digital signals!

Signal Termination

Kyoto University PresentationOctober 30, 2006 18

“Efficient antennas below a few GHz are easy to spot.”

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

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

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

Kyoto University PresentationOctober 30, 2006 22

“The circuit board traces with the most high-frequency current are not necessarily the high-frequency signal

traces.”

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

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

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