1

THE USE OF AN ACTIVE TEST BOARD TO VALIDATE A METHOD

THAT PREDICTS DIFFERENTIAL MODE RADIATED EMISSIONS

FROM PRINTED CIRCUIT BOARDS

Emmanuel Leroux,

Andrea Giuliano,

Carla Giachino High Design Technology

Corso Trapani 16

10139 Torino Italy

Ronald De Smedt,

Jan De Moerloose,

Willem Temmerman Alcatel Bell Telephone

Francis Wellesplein 1

B-2018 Antwerpen Belgium

Paolo Fogliati,

Piero Belforte,

CSELT

Via Reiss Romoli 274

10148 Torino Italy

Bernard Demoulin

University of Lille

59655 Villeneuve d’ascq

France

Abstract - The design of new Printed Circuit Boards

(PCBs) could be conveniently supported by the use of a

fast and reliable estimations tool of the Radiated

Emissions (RE). In this paper an active test board is

used to validate a method aimed at predicting RE due to

differential mode currents on PCB traces. The method

takes into account effects of dielectric layer in the field

calculation and does not need to discretise traces in

short segments. It can simulate all traces of a PCB in a

reasonable computation time. The simulation results are

compared with measurements in a semi-anechoic

chamber in far field. Radiation from the routed power

net, on board batteries and batteries holders is put in

evidence though measurement in near field. The paper

shows the importance of modeling of components and of

taking into account all couplings in the measurement set-

up when comparing RE results. The method validated in

this paper is included in the Telecom Hardware

Robustness Inspection System (THRIS) for evaluation

of telecom hardware.

I. INTRODUCTION

The introduction of severe international

standards in the field of Electromagnetic Compatibility

(EMC) in the last years had as result an increasing

interest for computing the Radiated Emissions (RE)

from electronic products. In fact the manufacturers of

electronic systems are interested in optimising from

EMC point of view their products before arriving to the

compliance tests in order to reduce time to market and

contain the costs. In most of cases electronic products

are composed of Printed Circuit Boards (PCBs). The

possibility of having a fast and reliable estimation of the

radiated field of PCB at design level is so highly

desirable.

In this paper an active test board is used to validate a

method aimed at predicting RE due to differential mode

currents on PCB traces.

II. MODELING OF PHYSICAL PHENOMENA

As it will be explained in the part III, the active test

board used to validate the method is made of microstrip

structures. The proposed method is now explained as

applied to an embedded microstrip made of only one

rectilinear trace. If more rectilinear traces are present,

the result is made of the vectorial summation of the

contributions of each trace.

As dielectric layers can severely modify radiation

patterns [1], it is important to take into account for each

microstrip structure the actual medium existing between

the metal plane and the air. The presented method is

able to calculate the RE of any microstrip structure, as

shown in figure 1, which shows two dielectric layers but

the presented method can take into account an arbitrary

number of layers.

P

P'

y

x

z

R

observation point

L

w

Figure 1: Representation of an embedded microstrip

structure, L: trace length, R: ”antenna” position where

the electric field is computed

A way to take into account dielectric layers in the RE

calculation is to use the dyadic Green's function of the

actual layered structure [2]. Then Sommerfeld integrals

have to be solved in a numerical way and this leads to a

long computation time. The presented method uses some

technics to give an analytical solution to the problem.

The starting point is the determination of the actual

current distribution along each trace. The method just

needs the knowledge of voltage and current on one of the

two extremities of each rectilinear trace. This

information is given in Time Domain by PRESTO [3]

2

(Post-layout Rapid Exhaustive Simulation and Test of

Operation) environment. It is a post-layout quality check

software that performs electrical simulations of PCBs to

evaluate Signal Integrity (SI). A Fast Fourier Transform

(FFT) is then performed to obtain this information in

Frequency Domain. Then the current waveform at any

abscissa x on the trace is determined by means of the

Transmission Line Theory (TLT) assuming that only the

quasi-Transverse Electric Magnetic (TEM) mode is

present along the trace. Then, the radiated

ElectroMagnetic (EM) field can be calculated using

dyadic Green’s function.

The electric field radiated from a surface current

distribution is obtained by means of the Green Dyadic G r r( ) of the layered structure, which can be

interpreted as a transfer function between the surface

current distribution J and the electric field as shown

in (1):

E r j G r r J r d ro( ) ( ). ( ) ( ) (1)

where:

- r is the coordinate of the point where the electric

field is computed;

- r is the coordinate of a point placed on the

rectilinear trace.

In general,

G r r( ) does not admit a close form

expression. However, with the assumption of being in

far field conditions, the Green Dyadic can be

substantially simplified.

For Information Technology Equipment (ITE) the

values of limits for RE in the frequency band 30 MHz to

1 GHz are given by EN 55022 [4] standard. The far

field condition applies approximately in the whole

frequency range described in the EN 55022 standard for

RE, which justifies the use of far field Green’s function.

Being difficult to directly calculate the electric field due

to the current density of a segment buried in dielectric

layers, the far field method applies the same current

source on the observation point where the EM field has

to be calculated and exploits the theory of reciprocity

[5] to transform the radiated emission calculation in a

problem of the induction of a plane wave on the

stratified structure.

Then the method assumes that the field arriving at the

air/dielectric interface is also locally a plane wave which

can be divided into two components, the transverse

electric (TE) and transverse magnetic (TM) modes. By

the use of modal analogy [2], Transmission Line Theory

(TLT) can be then applied to the propagation of these

two modes in the embedded microstrip structure to

produce two transfer functions for the actual medium

between the metal plane and the air. The following

expression of the Electric field in far field conditions is

obtained for any rectilinear radiating trace as shown in

Figure 1.

E R j

Z

Re e P x P y P z

o jk R jk h

ex ey ezo o

2 0

cos , , ,(2)

where:

Zo = o

o

= wave impedance in the air

K o = 2

o

= propagation constant

o = wave length in the air

h = substrate thickness

Pex ( , ) , Pey( , ) and Pez( , ) are essentially

plane-wave transfer functions of the dielectric layered

medium [2], that combine TE and TM plane-wave

modes. They depend on:

the spherical coordinates of the measuring antenna

position in the local reference system placed on the

trace.

the spatial Fourier Transform of the current density

on the trace

The obtained expression of the Electric field in far field

conditions is analytical, does not need to discretize the

trace and takes into account dielectric layers in the field

calculation.

III. EXPERIMENTAL VALIDATION

III.1 Description of the active test boards

To simplify the comparison between simulations and

measurements of the radiation of a PCB, a dedicated set

of active test boards have been developed [6]. They are

constructed using a standard four layers technology,

with a full power and full ground plane at the inside.

The component side contains all the components and

signal lines. The solder side only contains the

decoupling capacitors, the batteries holders with

batteries and a routed net for power distribution. A large

and a small version exists with respective dimensions:

295mm x 240mm and 195mm x 160mm. The layout of

the large board is shown in figure 2, that of the small

board is similar. The active test board contains an

oscillator (U6) with interchangeable clock frequency

(e.g. 5MHz, 16 MHz, 18MHz, 50MHz, 75MHz, with

the restriction that the digital logic circuitry must be able

to handle such frequencies). The oscillator drives three

4-bit counters (U1, U3, U4). The output of these

counters is connected to large busses (varying from

150mm to 250mm on the large board, 55mm to 150mm

on the small board).

3

Figure 2: Layout of the large test board

Two busses are terminated with inverters (U2, U5) of

which the output is connected to loads. The third bus

ends with a symmetric line driver (U7) of which each

symmetrical signal (pair of lines) passes through a

common mode filter (which can be shunted to inhibit its

functioning) and is directed towards a connector. All

lines have controlled impedance (75 Ohms). Several

termination schemes can be implemented: SLT (Series

Line Termination) at the begin, AC PLT (Parallel Line

Termination) at the end. All four combinations of

termination have been examined (SLT present or not

and PLT present or not). Several versions of the test

board exist, which are equipped with different, but

(standard) pin-compatible logic (LS-TTL, HCMOS,

ACT). To simplify measurements and simulations, the

board is able operate without any cabling. The power

can be supplied by unshielded rechargeable batteries

fixed on the solder side of the board by batteries

holders. They are connected to the power and ground

plane by a power net and shunted by a decoupling

capacitor.

III.2 Measurement and simulation of the active test

boards

For the results reported in this paper only the large

board has been considered and equipped with a 16MHz

clock, with ACT technology where SLT and PLT are

present.

The simulation needs technological,

geometrical and physical data which are extracted from

the layout tool used to design the board. It presents 177

components and 107 nets which are microstrip

structures. Most of nets are based on the same

configuration which is described in the figure 3 (SLT

and PLT are present).

driver receiver

Zs

5075

100 pF

75

driver receiver

Zs

5050

100 pF

75

Figure 3: Configuration of most of the nets, SLT and

PLT are present

Before to compare simulation and measurement

for RE it is important to verify the quality of the

simulated signals on PCB traces and so the quality of

models for active components. The board has been

simulated with behavioural models which include for

drivers the measured unloaded output voltage. The

figure 4 shows the comparison between the spectrum,

obtained by the means of a Fast Fourier Transform

(FFT), of the measured and simulated voltage at the

output of (U6) component.

10MHz 100MHz 1GHz

-60

-40

-20

0

20 dBV

10MHz-60

-40

-20

0

20

100MHz 1GHz

dBV

FFT on the

measured voltage

FFT on the

simulated voltage

Frequency

Frequency Figure 4: FFT on measured and simulated voltage at the

output of (U6)

The agreement is quite good until 300 MHz.

The radiated fields of the active test boards have

been measured extensively. The measurements have

been performed in the semi-anechoic rooms at Alcatel

Bell, Antwerpen (Belgium), and at CSELT, Turin (Italy)

which are normally used for EMC normative tests.

Several different positions of the board on the table have

been considered. The measurements have been

performed on the large and small test boards, with the 3

logic families and with various clock frequencies (as

long as supported by the logic). Several combinations of

line termination have been considered as well.

4

The comparisons reported on this paper are relative to

the measurements made in CSELT for which the setup is

shown in figures 5 and 6.

H=1.01 mL=10 m

h=2 m

Metal floor of semi-anechoic chamber

on board

batteries

front side of the board

Figure 5: Measurement set-up in CSELT

Figure 6: Far field measurement in CSELT

The test board is placed on a wooden table in vertical

position, at a fixed height of 1.01 m above the ground

plane of the room. At 10 m distance the receiving

antenna is placed at a fixed height of 2 m. The board is

supplied by 4 nickel cadmium batteries placed on the

solder side of the board itself. No shielding structures

(for example boxes, conducting ribbon, etc.) have been

used to limit the radiation of batteries as it is shown in

the figure 7.

Figure 7: Back of the board, presence of battery holders

without e.m. shield

The envelope of measured and simulated maximum

electric field are shown in figure 8. The measurement in

vertical polarisation has been obtained with a 120 kHz

bandwidth spectrum analyser.

108

109

0

5

10

15

20

25

30

35

40

measurement

Simulation

Frequency in Hz

Ele

ctri

c fi

eld i

n d

B

V/m

Figure 8: Comparison between the envelope of

maximum measured and simulated electric field

The envelopes of simulated and measured radiated

emissions are similar and show a minimum at around

400 MHz. But between 100 MHz and 350 MHz, the

simulation underestimates the measurement results of

5

some 15 dB. In order to investigate this difference, some

measurements in near field have been performed. The

test bench shown in figure 9 has been used to measure

the magnetic field radiated at a distance of 1cm from the

back of the board. It uses a spectrum analyser HP

8595EM, a near field probe HP 11940A and a three-axis

table for scanning the test board.

Figure 9: Three-axis scanning set-up for near field RE

measurements, the top layer of the board is shown

In figure 10 the magnetic field measured at 1

cm from the back side of the board near the batteries is

shown.

102

103

20

30

40

50

60

70

Frequency [MHz]

Hfield [dB a/m]

Figure 10: Measured magnetic field at 1 cm from the

back side of the board near the batteries

It is important to notice that the same measurement

made far away from the batteries on the back side of the

board shows emissions due to edges effects that are

much lower than the ones of the batteries themselves.

The radiation measured from the batteries is not

produced by the batteries themselves but by the power

and ground noise ( I noise) which is re-injected from

the active components over the power and ground planes

into the batteries, batteries connections and the power

routed net. These measurements show that the radiation

of the signal nets is present along the whole frequency

range, whereas the radiation due to the batteries is

restricted to the 100 MHz, 350 MHz frequency range.

This range is exactly the same where the prediction

underestimates the far field measurements. As no near

field to far field interpolation can be practically used, we

can not make any quantitative explanation but we can

reasonably justify the impact of the batteries emission

on the far field measurement, looking at its spectral

distribution in near field.

On the first designed board, due to the absence of

accessible ground plane, it was impossible to introduce a

shield. So a new version of the board has been produced

with an “EMI approach”. On the back (solder side) of

the new board a ground metallization has been brought

to the surface through several plated-through holes as it

is shown in the figure 11 and the batteries have been

mounted on it.

By this way a flexible EMI shield has been soldered as

shown in the figure 12: it covers all the batteries and

cuts the spurious radiation from the back of the board.

By this way the radiated field from the new test board

can be reasonably supposed coming only from the front

of the board (signal traces side). The figure 13 shows the

comparison between the envelop of maximum measured

radiated emissions by the original board and the new

one.

.

Figure 11: Top and bottom view of the new board

6

Figure 12: EMI shield soldered to the ground

metallization on the back of the board

108

109

0

5

10

15

20

25

30

35

40

Original board

New board

Frequency in Hz

Ele

ctri

c fi

eld

in

dB

V

/m

Original board

New board

Frequency in Hz

Ele

ctri

c fi

eld

in d

B

V/m

Figure 13: Comparison between the envelopes of

maximum measured radiated emissions by the original

board and the new one

A comparison between the radiation spectrum of the old

and new implementation shows a significant difference

of emission levels in the frequency range between 70

MHz and 350 MHz where the radiation of the batteries,

batteries connections and the power routed net of the

original board plays its role. This confirms the

hypothesis made above.

We can now compare the simulation results already

obtained above to the measurement of radiated

emissions due to the new board as shown in figure 14:

Measurement

Simulation

Frequency in Hz

Ele

ctri

c fi

eld

in

dB

V

/m

Figure 14: comparison between measurement and

simulation of the envelope of maximum radiated

emissions by the new board

The envelope of measured radiation spectrum is well

reproduced by the simulation and the gap between

measurement and simulation is less than 4 dB until 300

MHz. After this frequency the gap increases. It can be

partially due to the validity limit of models used for

active components as an example has been shown in

figure 4. This limit could be extended by the means of

Time Domain Reflectometer measurements made on a

sample of the devices. Such measurements permit to

replace the value of the output capacitance of a driver

that often suffers from uncertainty due to the dispersion

of this capacitance value by a Scattering parameter in

time domain. This topic has been described in [7].

It is important to notice that only 15 minutes on a SUN

ULTRA 1 workstation were required to simulate the

currents and RE of all the 107 nets of the board that

presents 177 components.

IV. CONCLUSIONS

An analytical method to predict RE due to

differential mode currents on PCB traces has been

presented, it takes into account effects of dielectric layer

in the field calculation and does not need to discretise

traces in short segments. It can simulate all traces of a

PCB in a reasonable computation time. The comparison

between measurements and simulations made on an

active test board shows that all couplings involved in the

measurement set-up must be well understood to be able

to explain the results of the comparison. Measurements

in near field permitted to show the radiation of the

batteries, batteries connections and the power routed net

of the board. A new design of the board was

implemented to cut this spurious effect and permitted to

validate the method used to predict the differential RE

from PCB traces.

But this experience also shows how much can be

important to include the radiation from the

power/ground distribution. The 4-ports models for

7

digital drivers used in PRESTO post-layout analysis tool

allows the simulation of Simultaneous Switching Noise

on power and ground pins of active components taking

into account the actual loads. It is then possible to

predict I noise on routed traces and on planes by the

means the Transmission Line Method (TLM) . These

features are being used through a european ESPRIT

ESD project in order to predict the RE from the power

distribution network of a board.

A good modelling of components which can avoid

uncertainty due to the dispersion of their electrical

parameters is also necessary for reliable simulations,

especially in the higher frequency ranges.

The simulation software used for signal integrity and

e.m. predictions as well the near field equipment used

for measurements are included in the THRIS [8]

environment. On the basis of results coming from this

kind of experimental validation, further improvements

of radiated field prediction will be included in the future

release of this tool.

REFERENCES

[1] E. Leroux, F. Canavero, G. Vecchi, "Prediction of

radiated electromagnetic emissions from PCB

tracks based on Green dyadics," Proc. EURO-

DAC, Brighton (UK), Sept. 18-22, 1995, pp. 354-

359.

[2] C. Felsen, N. Marcaviz, Radiation and scattering

of waves, Chp. 5, Prentice - Hall, Eaglewood

Cliffs, 1973

[3] S. Forno, S. Rochel, "Advanced simulation and

modeling techniques for hardware quality

verification of digital systems," Proc. EURO-DAC,

Grenoble (F), 1994 [4] EN55022, “Limits and methods of measurement of

radio interference characteristics of information technology equipment”, 1985

[5] Monteath, Applications of the Electromagnetic

Reciprocity Principle, Pergamon Press, 1973.

[6] R. De Smedt, W. Temmerman, “Using an Active

Test Board for Evaluation of EMC Tools on PCB

Level”, COST Action 243 EMC Workshop,

Paderborn (Germany), 7-8 April 1997

[7] E. Leroux “Conception et validation d’une

méthode numérique hybride appliquée à la

prédiction du rayonnement d’une carte

électronique connectée à son câblage”, PhD thesis,

University of Lille, june 1998

[8] P.Belforte, G.Guaschino, F.Maggioni, “A new

system for the evaluation of telecom hardware

robustness”, EMC’96 Roma