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Course Introduction
Purpose
• This course discusses techniques for analyzing and eliminating
noise in microcontroller (MCU) and microprocessor (MPU) based
embedded systems.
Objectives
• Learn what EMI is and why it should be minimized.
• Understand decoupling capacitors and how they should be used.
• Find out how to measure noise currents and near-field emissions.
• Discover a way to evaluate the effectiveness of EMI prevention
measures.
Content
• 16 pages
Learning Time
30 minutes
Design Goal: Reduce 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
- It includes techniques for decreasing
the noise generated by a specific
system, circuit, or device that might cause
problems in other electronic systems,
circuits, and devices
Explanation of Terms
Anechoic
chamber
A room designed to block radiation from the outside and to minimize
reflections off the room’s walls, ceiling, and floor
Balun
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).
Core
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.
ECU
Electronic Control Unit
EMI
Electromagnetic Interference
Harness
Cables (wires) connecting a board and power supply
or connecting one unit in a system to another
LISN
Line Impedance Stabilization Network
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.
TEM Cell
Transverse Electromagnetic Cell
WBFC
Workbench Faraday Cage
Why Is EMI Reduction Important?
Example: In an automobile, noise radiated and
conducted from the ECU to the radio
and its antenna can disturb FM reception
FM
station
FM band
radio signals
Radiated emissions
from ECU
Conducted emissions
from power wiring
FM Radio
Battery
ECU
Wiring harness
(power line)
MPU
Antenna
Use a Decoupling Capacitor
• One way to reduce EMI is to minimize rapid variations in the current that
the main power source has to deliver to the MPU or other LSI device
• By using a decoupling capacitor (Cdc) to provide the fluctuations of the
current drawn by the MPU, the current drawn from the power supply will be
more stable and, thus, generate less EMI
A
C
C=A+B
Package
Chip
B
CPG
Power
supply
Decoupling
capacitor
(Cdc)
Vcc* current
current
Vss*
(measured by
1Ω resistor)
Module
Main clock
Measuring point (VDE method)
Minimize Inductance of Cdc Wires
Decoupling
capacitor
• The external decoupling capacitor
(Cdc1) supplies the fast transients in
the Vss current flowing into the chip;
the main power supply charges this
decoupling capacitor gradually
1
Power
supply
• Current variations flowing in wiring
inductances can produce unwanted
induced voltages (V = L x di/dt)
2
Lb, Lbg = 10nH
• Lowering the inductances (Lb, Lbg)
of the wiring to the external
decoupling capacitor reduces its
impedance, enabling it to achieve
better EMI suppression
• On-chip decoupling capacitors
(Cchip, etc.) are very effective at
suppressing EMI, even if their
capacitance is small.
Different Design Approaches
EMI decreases when . . .
- the traces to the external decoupling capacitor are redesigned,
reducing the values of Lb and Lbg from 10nH to 1nH
- a 3,000pF internal decoupling capacitor is built into the chip
Lb and Lbg decreased to 1nH
On-chip capacitor = 3,000pF
Measuring Noise Current
• Noise current can be evaluated quantitatively (using the VDE method [1Ω], etc.)
• Efforts to reduce EMI can be evaluated by comparing circuit performance
with and without various decoupling capacitors
Vcc
Vss
R
(1Ω)
LSI
Chip
VDE method
measuring point
VDE Measurement Method
Vcc
IC
49Ω
in
R=1Ω
Spectrum
Analyzer
50Ω
Vcc
Vss
Measuring Near-field Emissions
Probe with
sensor coil
EMV-200
Details of EMV-200:
• Sensor coil (magneticfield sensor) is at the
tip of a probe mounted
on a robotic arm that
moves with precision in three dimensions.
• Sensor coil is directional, so the probe rotates
to accurately detect magnetic fields generated
by noise currents, as the arm moves in a pattern
close above the board without making contact.
Shielded room
• Tester accurately maps the magnitude of the
circuit’s magnetic-field emissions at a specific
measurement frequency.
Noise-current Measurement — 1
VDE method (IEC 61967-4) and NF-probe analysis (at 80MHz)
Pitch: 2mm
Height above board: 5mm
Scanned Area
58dB
46dB
MPU: SH7055RF
40MHz (10MHz x 4) program execute
Vcc = 3.3V, PVcc = 5.0V
FM
Decoupling capacitors
Without decoupling capacitors
FM
With decoupling capacitors
Noise-current Measurement — 2
Pitch: 2mm
Height above board: 5mm
VDE method (IEC 61967-4) and NF-probe analysis (at 80MHz)
Scanned Area
58dB
46dB
42dB
MPU: SH7055SF
40MHz (10MHz x 4) program execute
Vcc = 3.3V, PVcc = 5.0V
VCL capacitors
Decoupling capacitors
24dB
FM
Without decoupling capacitors
FM
With decoupling capacitors
Noise-current Measurement — 3
Pitch: 2mm
Height above board: 5mm
VDE method (IEC 61967-4) and NF-probe Analysis (at 80MHz)
Scanned Area
58dB
46dB
MPU: SH7058FCC
80MHz (10MHz x 8) program execute
Vcc = 3.3V, PVcc = 5.0V
VCL capacitors
Decoupling capacitors
20dB
FM
With VCL capacitors, but
without decoupling capacitors
22
28dB
FM
With VCL capacitors and
decoupling capacitors
Evaluating Supply Decoupling
Area under device
showing pads for
decoupling capacitors
Current measurement points
(Vcc, PVcc, Vss)
Power
supply
connections
Top of evaluation board
Typical
decoupling
capacitor
Pads for inductors
(ferrite beads)
Bottom of evaluation board
Evaluation Example
Near-field tests* using the
evaluation board allow
comparisons of levels
of RF current in the
power supply lines
No filter components
12 bypass capacitors added
* MPU: SH7055R (40MHz)
Measurement frequency: 80MHz
Ferrite bead + 12 caps
Supply Decoupling Test Results
Decoupling
capacitors
Using a ferrite bead and multiple decoupling
capacitors is an effective way to reduce EMI
Typical Circuit Board Example
Slit
(moat)
Ferrite
bead
Ground plane
(no slit)
Vcc
Noise Current (dBµV)
70
f = 80MHz
60
Capacitors added
50
40
30
-20dB
20
10
Capacitor
Target level
Capacitors +
ferrite bead
0
I/O
current
GND
Core current
0
2
4
6
8
10
12
14
16
18
Number of Decoupling Capacitors
20
Course Summary
• Importance of EMI reduction
• Decoupling capacitors
• EMI measurements
• Evaluating EMI reduction techniques
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