The paper shows the experimental validation of predictive results of radiated emissions of a multilayer pcb. The radiated field is calculated from simulated results of pcb signals obtained from DWN analysis of interconnects.

1. THE USE OF AN ACTIVE TEST BOARD TO VALIDATE A METHOD
THAT PREDICTS DIFFERENTIAL MODE RADIATED EMISSIONS
FROM PRINTED CIRCUIT BOARDS
Emmanuel Leroux, Ronald De Smedt, Paolo Fogliati, Bernard Demoulin
Andrea Giuliano, Jan De Moerloose, Piero Belforte,
Carla Giachino Willem Temmerman
High Design Technology Alcatel Bell Telephone CSELT University of Lille
Corso Trapani 16 Francis Wellesplein 1 Via Reiss Romoli 274 59655 Villeneuve d’ascq
10139 Torino Italy B-2018 Antwerpen Belgium 10148 Torino Italy France
Abstract - The design of new Printed Circuit Boards As it will be explained in the part III, the active test
(PCBs) could be conveniently supported by the use of a board used to validate the method is made of microstrip
fast and reliable estimations tool of the Radiated structures. The proposed method is now explained as
Emissions (RE). In this paper an active test board is applied to an embedded microstrip made of only one
used to validate a method aimed at predicting RE due to rectilinear trace. If more rectilinear traces are present,
differential mode currents on PCB traces. The method the result is made of the vectorial summation of the
takes into account effects of dielectric layer in the field contributions of each trace.
calculation and does not need to discretise traces in As dielectric layers can severely modify radiation
short segments. It can simulate all traces of a PCB in a patterns [1], it is important to take into account for each
reasonable computation time. The simulation results are microstrip structure the actual medium existing between
compared with measurements in a semi-anechoic the metal plane and the air. The presented method is
chamber in far field. Radiation from the routed power able to calculate the RE of any microstrip structure, as
net, on board batteries and batteries holders is put in shown in figure 1, which shows two dielectric layers but
evidence though measurement in near field. The paper the presented method can take into account an arbitrary
shows the importance of modeling of components and of number of layers.
taking into account all couplings in the measurement set- z
up when comparing RE results. The method validated in observation point
this paper is included in the Telecom Hardware P
Robustness Inspection System (THRIS) for evaluation
of telecom hardware.
R
L
I. INTRODUCTION y
The introduction of severe international P'
standards in the field of Electromagnetic Compatibility w
(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 x
electronic systems are interested in optimising from
Figure 1: Representation of an embedded microstrip
EMC point of view their products before arriving to the
structure, L: trace length, R: ”antenna” position where
compliance tests in order to reduce time to market and
the electric field is computed
contain the costs. In most of cases electronic products
are composed of Printed Circuit Boards (PCBs). The
A way to take into account dielectric layers in the RE
possibility of having a fast and reliable estimation of the
calculation is to use the dyadic Green's function of the
radiated field of PCB at design level is so highly
actual layered structure [2]. Then Sommerfeld integrals
desirable.
have to be solved in a numerical way and this leads to a
In this paper an active test board is used to validate a
long computation time. The presented method uses some
method aimed at predicting RE due to differential mode
technics to give an analytical solution to the problem.
currents on PCB traces.
The starting point is the determination of the actual
current distribution along each trace. The method just
II. MODELING OF PHYSICAL PHENOMENA
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]
1

2. (Post-layout Rapid Exhaustive Simulation and Test of obtained for any rectilinear radiating trace as shown in
Operation) environment. It is a post-layout quality check Figure 1.
software that performs electrical simulations of PCBs to
evaluate Signal Integrity (SI). A Fast Fourier Transform   Zo   
ER j e jko R
e jkoh cos Pex , x Pey , y Pez , z (2)
(FFT) is then performed to obtain this information in 2R 0
Frequency Domain. Then the current waveform at any
abscissa x on the trace is determined by means of the where:
Transmission Line Theory (TLT) assuming that only the
o
quasi-Transverse Electric Magnetic (TEM) mode is Zo = = wave impedance in the air
present along the trace. Then, the radiated o
ElectroMagnetic (EM) field can be calculated using 2
dyadic Green’s function. Ko = = propagation constant
The electric field radiated from a surface current o
distribution is obtained by means of the Green Dyadic = wave length in the air
o
   h = substrate thickness
G( r r ) of the layered structure, which can be
interpreted as a transfer function between the surface
Pex ( , ) , P ( , ) and P ( ,
ey ez ) are essentially
 plane-wave transfer functions of the dielectric layered
current distribution J and the electric field as shown
medium [2], that combine TE and TM plane-wave
in (1): modes. They depend on:
    the spherical coordinates of the measuring antenna
   
E(r ) j o G( r r ). J ( r )d ( r ) (1) position in the local reference system placed on the
trace.
where: the spatial Fourier Transform of the current density
on the trace

- r is the coordinate of the point where the electric
The obtained expression of the Electric field in far field
field is computed;
 conditions is analytical, does not need to discretize the
- r is the coordinate of a point placed on the trace and takes into account dielectric layers in the field
rectilinear trace.
   calculation.
In general, G( r r ) does not admit a close form
expression. However, with the assumption of being in III. EXPERIMENTAL VALIDATION
far field conditions, the Green Dyadic can be
substantially simplified. III.1 Description of the active test boards
For Information Technology Equipment (ITE) the
values of limits for RE in the frequency band 30 MHz to To simplify the comparison between simulations and
1 GHz are given by EN 55022 [4] standard. The far measurements of the radiation of a PCB, a dedicated set
field condition applies approximately in the whole of active test boards have been developed [6]. They are
frequency range described in the EN 55022 standard for constructed using a standard four layers technology,
RE, which justifies the use of far field Green’s function. with a full power and full ground plane at the inside.
Being difficult to directly calculate the electric field due The component side contains all the components and
to the current density of a segment buried in dielectric signal lines. The solder side only contains the
layers, the far field method applies the same current decoupling capacitors, the batteries holders with
source on the observation point where the EM field has batteries and a routed net for power distribution. A large
to be calculated and exploits the theory of reciprocity and a small version exists with respective dimensions:
[5] to transform the radiated emission calculation in a 295mm x 240mm and 195mm x 160mm. The layout of
problem of the induction of a plane wave on the the large board is shown in figure 2, that of the small
stratified structure. board is similar. The active test board contains an
Then the method assumes that the field arriving at the oscillator (U6) with interchangeable clock frequency
air/dielectric interface is also locally a plane wave which (e.g. 5MHz, 16 MHz, 18MHz, 50MHz, 75MHz, with
can be divided into two components, the transverse the restriction that the digital logic circuitry must be able
electric (TE) and transverse magnetic (TM) modes. By to handle such frequencies). The oscillator drives three
the use of modal analogy [2], Transmission Line Theory 4-bit counters (U1, U3, U4). The output of these
(TLT) can be then applied to the propagation of these counters is connected to large busses (varying from
two modes in the embedded microstrip structure to 150mm to 250mm on the large board, 55mm to 150mm
produce two transfer functions for the actual medium on the small board).
between the metal plane and the air. The following
expression of the Electric field in far field conditions is
2

3. driver receiver
Zs
75 75
50
100 pF
driver receiver
Zs
75 50
50
100 pF
Figure 3: Configuration of most of the nets, SLT and
Figure 2: Layout of the large test board PLT are present
Before to compare simulation and measurement
Two busses are terminated with inverters (U2, U5) of for RE it is important to verify the quality of the
which the output is connected to loads. The third bus simulated signals on PCB traces and so the quality of
ends with a symmetric line driver (U7) of which each models for active components. The board has been
symmetrical signal (pair of lines) passes through a simulated with behavioural models which include for
common mode filter (which can be shunted to inhibit its drivers the measured unloaded output voltage. The
functioning) and is directed towards a connector. All figure 4 shows the comparison between the spectrum,
lines have controlled impedance (75 Ohms). Several obtained by the means of a Fast Fourier Transform
termination schemes can be implemented: SLT (Series (FFT), of the measured and simulated voltage at the
Line Termination) at the begin, AC PLT (Parallel Line output of (U6) component.
Termination) at the end. All four combinations of 20
dBV
termination have been examined (SLT present or not
and PLT present or not). Several versions of the test 0 FFT on the
board exist, which are equipped with different, but measured voltage
(standard) pin-compatible logic (LS-TTL, HCMOS, -20
ACT). To simplify measurements and simulations, the
board is able operate without any cabling. The power -40
can be supplied by unshielded rechargeable batteries
-60
fixed on the solder side of the board by batteries 10MHz
20
100MHz Frequency 1GHz
dBV
holders. They are connected to the power and ground
FFT on the
plane by a power net and shunted by a decoupling 0 simulated voltage
capacitor.
-20
III.2 Measurement and simulation of the active test
boards -40
For the results reported in this paper only the large -60
10MHz 100MHz Frequency 1GHz
board has been considered and equipped with a 16MHz Figure 4: FFT on measured and simulated voltage at the
clock, with ACT technology where SLT and PLT are output of (U6)
present.
The simulation needs technological, The agreement is quite good until 300 MHz.
geometrical and physical data which are extracted from The radiated fields of the active test boards have
the layout tool used to design the board. It presents 177 been measured extensively. The measurements have
components and 107 nets which are microstrip been performed in the semi-anechoic rooms at Alcatel
structures. Most of nets are based on the same Bell, Antwerpen (Belgium), and at CSELT, Turin (Italy)
configuration which is described in the figure 3 (SLT which are normally used for EMC normative tests.
and PLT are present). 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.
3

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.
front side of the board
on board
batteries
h=2 m
H=1.01 m L=10 m
Metal floor of semi-anechoic chamber
Figure 5: Measurement set-up in CSELT
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.
40
measurement
35
Electric field in dB V/m
30
25
20
15
Simulation
10
5
0
8 9
10 Frequency in Hz 10
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 Figure 8: Comparison between the envelope of
used to limit the radiation of batteries as it is shown in maximum measured and simulated electric field
the figure 7.
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
4

5. some 15 dB. In order to investigate this difference, some This range is exactly the same where the prediction
measurements in near field have been performed. The underestimates the far field measurements. As no near
test bench shown in figure 9 has been used to measure field to far field interpolation can be practically used, we
the magnetic field radiated at a distance of 1cm from the can not make any quantitative explanation but we can
back of the board. It uses a spectrum analyser HP reasonably justify the impact of the batteries emission
8595EM, a near field probe HP 11940A and a three-axis on the far field measurement, looking at its spectral
table for scanning the test board. 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.
Figure 9: Three-axis scanning set-up for near field RE By this way the radiated field from the new test board
measurements, the top layer of the board is shown can be reasonably supposed coming only from the front
of the board (signal traces side). The figure 13 shows the
In figure 10 the magnetic field measured at 1 comparison between the envelop of maximum measured
cm from the back side of the board near the batteries is radiated emissions by the original board and the new
shown. one.
70
Hfield [dB a/m]
60
50
40
.
30
20
2 3
10 10
Frequency [MHz]
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. Figure 11: Top and bottom view of the new board
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.
5

6. Electric field in dB V/m
Measurement
Simulation
Figure 12: EMI shield soldered to the ground
Frequency in Hz
metallization on the back of the board
Figure 14: comparison between measurement and
40
simulation of the envelope of maximum radiated
35
Original board emissions by the new board
Electric field in dB V/m
30
The envelope of measured radiation spectrum is well
25 reproduced by the simulation and the gap between
20 measurement and simulation is less than 4 dB until 300
New board MHz. After this frequency the gap increases. It can be
15
partially due to the validity limit of models used for
10 active components as an example has been shown in
5 figure 4. This limit could be extended by the means of
Time Domain Reflectometer measurements made on a
0
10
8
10
9 sample of the devices. Such measurements permit to
Frequency in Hz
replace the value of the output capacitance of a driver
that often suffers from uncertainty due to the dispersion
Original board
of this capacitance value by a Scattering parameter in
Electric field in dB V/m
time domain. This topic has been described in [7].
It is important to notice that only 15 minutes on a SUN
New board 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
Frequency in Hz
Figure 13: Comparison between the envelopes of An analytical method to predict RE due to
maximum measured radiated emissions by the original differential mode currents on PCB traces has been
board and the new one presented, it takes into account effects of dielectric layer
in the field calculation and does not need to discretise
A comparison between the radiation spectrum of the old traces in short segments. It can simulate all traces of a
and new implementation shows a significant difference PCB in a reasonable computation time. The comparison
of emission levels in the frequency range between 70 between measurements and simulations made on an
MHz and 350 MHz where the radiation of the batteries, active test board shows that all couplings involved in the
batteries connections and the power routed net of the measurement set-up must be well understood to be able
original board plays its role. This confirms the to explain the results of the comparison. Measurements
hypothesis made above. in near field permitted to show the radiation of the
We can now compare the simulation results already batteries, batteries connections and the power routed net
obtained above to the measurement of radiated of the board. A new design of the board was
emissions due to the new board as shown in figure 14: 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
6

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
7