Purpose

• This course discusses techniques for analyzing and eliminating noise in microcontroller (MCU) and microprocessor (MPU) based embedded systems.

Objectives

• Learn about a method for performing a system-level EMI test.

• See how to evaluate current balance.

• Gain a basic knowledge of tests for measuring the emissions fromLSI devices that can be used for product selection.

Content

• 18 pages

Learning Time

30 minutes

Course Introduction

Reducing EMI

EMI reduction is a goal shared by both

the semiconductor experts who design

MPUs and other LSI devices, and by the engineers who apply those chips in embedded systems

ECU Electronic Control Unit

EMI Electromagnetic Interference

Explanation of Terms

A microcontroller chip is composed of a core, I/O ports, and power supply circuitry. The core consists of the CPU, ROM, RAM, and blocks implementing timers, communication, and analog functions.

Power supply

Two power supplies are applied to the LSI: Vcc and Vss. The core power supply internal to the LSI is VCL (internal step-down). The Vss-based power supply routed through the LSI is VSL.

Harness Cables (wires) connecting a board and power supply or connecting one unit in a system to another

Balun

LISN

TEM Cell

WBFC

Core

A room designed to block radiation from the outside and to minimize reflections off the room’s walls, ceiling, and floor

A passive electronic device that converts between balanced and unbalanced electrical signals

CISPR 25 International Special Committee on Radio Interference (CISPR) publication 25: “Limits and methods of measuring radio disturbance characteristics for the protection of receivers on board vehicles.” CISPR is a sub-committee of the International Electrotechnical Commission (IEC).

Line Impedance Stabilization Network

Transverse Electromagnetic Cell

Workbench Faraday Cage

Anechoicchamber

• System-level evaluation

- Example: performed on three test boards

- Test method for measuring emissions from wiring harness: (CISPR 25

equivalent)

• Radiation levels ranged from high for board A to low for board C

Radiation from Wiring Harness

Test setup

LISN

Antenna

Circuit board with MPU

Harness

Anechoic chamber Board B

Board C

Board A

• Near-field distribution was measured also, using an EMV-200 test system

- A sensor coil on a probe rotatesand moves with precision in three dimensions to scan and recordthe EMI radiated from the circuit board

• Data from the CISPR 25 test and the EMV-200 scan was used to examine the correspondence between the field strength and system level evaluation at the connector position

EMI from Circuit Board

EMV-200

Probe with sensor coil

Data from near-field scan

MPU

Power supply connector

Circuit board

f = 80MHz

Emission Measurement Results

• With probe above the harness connector, there is a direct relationship between the antenna and near-field probe readings

- Using a low-emissions MCU reduces emissions at the wiring harness

connector on the board

• Moving from a 2-layer board to a 4-layer board further reduces emissions at the wiring harness connector

Harness mounting area

MCU

Board B

@80 MHzHarness mounting

area MCU

Board C

@80MHz

Harness mounting area

MCU

Board A

Directly above MCU

Above harness mounting area

Evaluating Current Balance in PCB

- Test board provides extra pads to which 470Ωresistors can be connected to divert current through loops on left and right, creating differences between the signal and return currents in the area highlighted by the pink oval

- Near-field scan data of the entire board was obtained for three test cases:

• Case 1: No resistors were connected, so currents in measurement area were

balanced

• Case 2: a 470Ω resistor was connected on left

side of board, creating a 10% current unbalance

in the measurement area

• Case 3: Two 470Ω resistors were connected on

the left and right sides of the board, creating

Near-field measurements show the common-mode radiation caused by unbalanced currents flowing in the circuit board

Left loop Right loop

Line width = 1.3mm

Termination(50Ω)

Pads

100, 90 or 80%

100%

Area in which a difference between the signal current and return current can be created

h = 2.5mm

f = 80MHz

Case 1: Current Balanced

With no 470Ω resistors

connected, current was balanced, so minimum levels of EMI were detected when the EMV-200’s probe scanned the measurement area of the printed circuit board

Case A

100%

100%

No 470Ω resistors(Both loops open)

Case 2: Current Unbalanced by 10%

With a 470Ω resistor connected,

a 10% current unbalance was created, which caused the EMI to grow to moderate levels in the area of the unbalance

Case BCase A

h = 2.5mm

f = 80MHz

100%

100%

100%

90%

Additional resistor(470Ω)

1/10

Case 3: Current Unbalanced by 20%

With both 470Ω resistors

connected, a 20% current unbalance created; this caused the EMI to becomes high in the area of the unbalance

Case B Case CCase A

h = 2.5mm

f = 80MHz

100%

100%

100%

90%

100%

80%

Two additional resistors (470Ω)

1/10

1/10

Near-field scans can help locate the cause of EMI

problems

Board Layout Affects Emissions

Terminated

Microstrip line

Reference Microstrip Line

Signal input:

100MHz sine wave, 1.0Vp-p )

An ideal microstrip line shows a fairly uniform current distribution and minimum emissions

Pitch: 5mm; Scan height: 10mm

Scanned from bottom side (reference plane)

@100MHz

Layout Affects Emissions — 2

Symmetric Pattern

Emissions increase as the width of the pc board becomes more narrow

Pitch: 5mm; Scan height: 10mm

Scanned from bottom side (reference plane)

@100MHz

Layout Affects Emissions — 3

Asymmetric Pattern

The asymmetric pc board causes even more emissions

Pitch: 5mm; Scan height: 10mm

Scanned from bottom side (reference plane)

@100MHz

Emission Measurement Standards

The international standards listed here are used to measure electromagnetic emissions* from MCUs and other ICs

Standard No.: Title Latest StandardDocument Issue

Date Remarks

IEC 61967-1: General conditions and definitions 2002-03-12

IS[IEC 61967-1]

IEC 61967-2: Measurement of radiated emissions,

TEM-cell and wideband TEM-cell

Method

2005-09 IS[IEC 61967-2]

2005-06 TS[IEC TS 61967-3]

2002-04-30

IS[IEC 61967-4]

2002-06-25

IS[IEC 61967-6]

IEC 61967-3: Measurement of radiated emissions,

Surface Scan Method (Technical

Specifications)IEC 61967-4: Measurement of conducted emissions,

1-ohm/50-ohm Direct Coupling Method [IEC 61967-4 Ed. 1.1]

2006-2007Edition 1.1

2003-01-17

IS[IEC 61967-5]IEC 61967-5: Measurement of conducted emissions,

Workbench Faraday Cage Method

IEC 61967-6: Measurement of conducted emissions,

Magnetic Probe Method

IS: IEC International Standard TS: Technical Specification*Measurement range: 150kHz to 1GHz

IC

Vcc

49Ω

1Ω 50Ωin IC

Vcc

50Ω

in

IC

Vcc

950Ω

50Ω1K Ohmex.

vn

IS IS

Supply Current Measurement

The VDE probe and magnetic probe methods are international standards; the resistor-divider probe method is not

VDE Probe Magnetic Probe Resistor-Divider Probe

[IEC TS 61967-6][IEC 61967-4]

IS

EM Radiation, CM Voltage Testing

These three methods are also good for emissions testing; the Faraday Cage method can measure common-mode voltage for each part of the circuit board

Faraday Cage Loop Probe

u

Vcc

vn

[IEC 61967-5]

[IEC TS 61967-3]

TS

50Ω 50Ω

TEM Cell

[IEC 61967-2]

IS

Problem with Normal TEM Cell

When measuring emissions from LSI devices, the combined EM field data are almost identical to that of the magnetic field measurement alone; the electric field data is difficult to see

Magnetic field

Electric field + Magnetic field

(combined result produced

by a normal TEM cell measurement)

TEM cell output level (dB)

Frequency (MHz)

TEM cell method (normal)

Terminator Measuring system

50Ω 50Ω

TEM Cell

Electric field

Electric field coupling

(50Ω terminator)

50Ω terminator

(magnetic field coupling)

Output

Renesas “Hybrid Balun”

Applying the TEM Cell Method

With the “hybrid balun” that Renesas has adopted, voltages proportional to a pure electric field and a pure magnetic field can be obtained• Photo shows an electric field coupling

• Changing the terminator and output port results in a magnetic field coupling

System-level evaluation techniques

Importance of circuit board layout

Methods for evaluating emissions from LSI devices

Course Summary

For more information on specific devices and related

support products and material, please visit our Web site:

http://america.renesas.com