Estimating Unintentional Electromagnetic Emissions from ... · PDF fileThe software looks for EMI antennas on or off ... emission problems in printed circuit boards ... Estimating

Estimating Unintentional Electromagnetic Emissions from Printed Circuit Board Designs

Todd H. HubingMichelin Professor of Vehicular ElectronicsClemson University

2006 Freescale Technology Forum 2

How Difficult Can It Be?


Advantages – Can accurately determine fields in various situationsCan model things that can’t be measuredCan be used to verify measurementsUseful for research and “what if” analysis

Solve Maxwell’s equations to determine fields from specific sources

Disadvantages – User must have knowledge of field theory User must be aware of code limitationsHigh learning curveCannot “track down” an existing problemCan only model relatively simple geometriesCan only model well-defined geometriesGenerally expensive

Numerical Field Solvers

CAE Tools for EMC Engineers


My Meeting with Motorola


Why can’t a computer modeling tool evaluate printed circuit board designs using the same

criteria that human EMC expert use?


Advantages – Easier to understand what the software is doingEasier to use.

Scan a board layout looking for design rule violations.

Disadvantages – Design rules don’t apply in all situationsHigher board cost to meet unnecessary design rulesWill not detect problems that don’t violate a pre-defined rule

Design Rule Checkers

CAE Tools for EMC Engineers


EE371 Design Problem


Design Examples


Unlike numerical EM modeling software or EMC design rule checkers,expert systems examine boards in much the same manner as aprofessional EMC engineer. Potential common-mode and differential-mode EMI sources are identified and evaluated. The software looksfor EMI antennas on or off the board and evaluates how hard they arebeing driven. It identifies any problems found with the board layoutand estimates the impact of these problems on the radiated EMI fromthe system.

What is a PCB Expert System?


EMC Expert System software should work with automated printedcircuit board layout tools to:

review and analyze printed circuit board designs;

point out problems with the layout;

estimate levels of radiated EMI;

anticipate ESD and radiated susceptibility problems; and

provide circuit and board layout design advice.

Using the same general approach that human experts would use.



The goal is not to provide an accurate estimate ofradiated emission levels or to preclude thenecessity of testing the final product.



The goal is not to provide an accurate estimate ofradiated emission levels or to preclude thenecessity of testing the final product.

The goal is to distinguish between a good designand a bad design and identify features of a designthat are likely to result in emissions orsusceptibility problems.



PCB Expert System Structure




Identify Sources

Identify Antennas

Evaluate Coupling



Identify Sources


Good Antenna Parts Poor Antenna Parts

<100 MHz >100 MHz <100 MHz >100 MHz

Cables Heatsinks

Power planes

Tall components

Seams in shielding

enclosures

Microstrip or stripline

traces

Anything that is not

big

Microstrip or stripline

traces

Identify Antennas


Noise can be coupled from a source to an antenna by one or more of three different coupling mechanisms:

Conducted

Electric field coupled

Magnetic field coupled

For printed circuit board analysis and design, it is convenient to express these coupling mechanisms in terms of voltage and current.

Recognize Coupling Mechanisms


Differential-Mode Radiation

Coupling to I/O Radiation

Voltage-Driven Common-Mode Radiation

Current-Driven Common-Mode Radiation

Power Bus Radiation

PCB Expert System Emissions Models


Often used to estimate radiated emissions, but rarely the actual source of significant emissions.

Differential-Mode Emissions Model


Differential Mode Radiation Algorithm

PCB Expert Systems Algorithms


Signals coupled to I/O lines carry HF power off the board.

Not uncommon.

Usually obvious once detected.

Radiation by I/O Coupling Model


f < 118 MHz :

f > 188 MHz :

H. Shim et. al., “Expert system algorithms for identifying radiated emission problems in printed circuit boards,” Proc. of the 2004 IEEE International Symposium on EMC, Santa Clara, CA, USA, Aug. 2004, pp. 57-62.

Radiation by I/O Coupling Model


Signal 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:

Voltage-Driven Radiation Model


Ct-c

ICM

CDM

VDM

2 30t cDM

radboard

CVE Rr C−=

H. Shim and T. Hubing, “Model for estimating radiated emissions from a printed circuit board with attached cables driven by voltage-driven sources,” IEEE Trans. on Electromagnetic Compatibility, vol. 47, no. 4, Nov. 2005.

Voltage-Driven Radiation Model


Signal 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.

Current-Driven Radiation Model


cable-to-board2

2

1000.3651100 ( )

ret

B

VE

Cω

××

+

, ,

, 8, ,

75, MHz

4.71 10 75, MHz

p i DM ir

ret ip i DM i

r r

L I fa

VL I

fa a

ωε

ε ε

⎧ ≤⎪⎪= ⎨ × ×⎪ ≥⎪⎩

D. M. Hockanson et. al., "Quantifying EMI resulting from finite-impedance reference planes," IEEE Trans. on EMC, vol. 39, no. 4, Nov. 1997, pp. 286-297.

H. Shim et. al., “Expert system algorithms for identifying radiated emission problems in printed circuit boards,” Proc. of the 2004 IEEE International Symposium on EMC, Santa Clara, CA, USA, Aug. 2004, pp. 57-62.

Current-Driven Radiation Model


Design Examples


x

y

z

h

a

b

Ms

Ms

Ms

Ms

120 ( )min( , )

i

r

I hE Q fa b rε

= ⋅ ⋅

11 1 1( )

d c comp

Q fQ Q Q

−⎛ ⎞

= + +⎜ ⎟⎜ ⎟⎝ ⎠

H. Shim and T. Hubing, “Estimating radiated emissions from the power planes in a populated printed circuit board,” IEEE Trans. on Electromagnetic Compatibility, vol. 48, no. 1, Feb. 2006.

Power Bus Radiation Model


100 200 300 400 500 600 700 800 900 1000-120

-110

-100

-90

-80

-70

-60

-50

-40

-30S21 of populated and bare board(1-1-12 FR4),antenna vertical

Frequency [MHz]

S21

[dB

]barefull pop1copy buffers removed6copy buffers removed12copy buffers removedA test board with 12 copy buffers

23.7 cm x 22 cm, FR4Plane separation = 5 mils

Decoupling capacitors are untouched

The components damp the Q-factor of the resonances significantly!

Effect of Components on Power Bus Noise


Other Estimation Algorithms

Crosstalk between nets

ESD susceptibility

Magnetic field susceptibility

PCB Expert System Algorithms


• 8 clock buffers

• 28 load capacitors

• 32 decoupling capacitors

• Clocked at 50 MHz

• No heatsink

• Size: 3” by 2”, 6 layers

• Powered with one cable

U7U2

Layout of the NCMS Board


Measurement vs. Calculation: No Load


Measurement vs. Calculation: 1-nF Load


Validation of Software






View of left half of board

showing the problem nets.

GRESET

GRESET

DATA2

DATA2CONNECTOR P1

CONNECTORP2

U4

U20

U1U6





$1I6\CLKCPU

CONNECTOR P1

CONNECTORP2

U4

U1

$1I6\CLKCPU

U19

U37

Current-Driven Common-Mode

Problem.



The goal is to distinguish between a good design and a bad designand identify features of a design that are likely to result in emissionsor susceptibility problems.

Existing expert system tools are capable of finding manyproblems that would be difficult to locate manually.

Just as human experts continue to learn based onexperience and analysis, the expert system algorithmsare constantly being updated get better with experience.

Summary


Industry sponsors want tangible benefits from anyresearch that they sponsor.

The expert system project provided the vision thatenabled us to pursue research to investigate thefundamental sources of radiated emissions from PCBs.

e.g. Design guidelines, Software

Key Points

Documents

Estimating Unintentional Electromagnetic Emissions from ... · PDF fileThe software looks for EMI antennas on or off ... emission problems in printed circuit boards ... Estimating