An EMC Experimental Kit for Demonstrating
The Effect of Printed Circuit Board Layout on the Radiated Emissions
Fatih Üstüner1, Ersan Baran2
1
TÜBİTAK BİLGEM UEKAE Gebze, Kocaeli, Turkey, fatih.ustuner@uekae.tubitak.gov.tr
2
TÜBİTAK BİLGEM UEKAE Gebze, Kocaeli, Turkey, ersanbaran@uekae.tubitak.gov.tr
Abstract
An EMC experimental kit is developed to illustrate the fundamental mechanisms of radiated emissions from
printed circuit boards. The kit is then expanded to cover and demonstrate some of the EMC design techniques such as
shielding and filtering that are useful for reducing radiated emissions. In this paper, the experimental kit and the
experiment configurations are described. Representative results are given in order to show the PCB layout effect on the
differential and common mode radiated emissions. Moreover, the need for integral application of shielding and filtering
is emphasized as the outcome of the experiments.
1. Introduction
One of the primary environmental requirements imposed on the electrical-electronics equipment is the restriction
of the radiated emissions. The fundamental rationale behind this requirement is to protect the electromagnetic spectrum
that is a valuable resource for the radio frequency communication and other wireless activities. The source of radiated
emissions from the equipment is the interconnecting wires on the printed circuit boards (PCB) and the cables that are
connected to them. The mechanisms of radiated emissions from the circuit boards were studied and understood in
several papers [1-3]. Radiation from PCBs can occur as either differential mode or common mode. Differential-mode
radiation is the result of the normal operation of the circuit and results from current flowing around loops formed by the
conductors of the circuit, as shown in Fig. 1a. Common-mode radiation, however, is the result of parasitics in the circuit
and results from undesired voltage drops in the conductors. The differential mode current that flows through the ground
impedance produces a voltage drop in the circuit ground system. When cables are then connected to the system, they are
driven by this common-mode ground potential, forming antennas, which radiate as shown in Fig. 1b. Because these
parasitic impedances are not intentionally designed into the system or shown in the documentation, common-mode
radiation is often harder to understand and control [4]. Moreover, the common mode radiation is the dominant radiated
emission mechanism in the PCBs [1].
(a)
(b)
Figure 1 Radiation Mechanisms from Printed Circuit Boards; (a) Differential Mode (b) Common Mode
Although the theory is well established, electronics design engineers especially having no electromagnetic theory
background, encounter some difficulties in fully grasping the fundamentals of radiated emissions from PCBs and this
may result in the application of poor techniques for reducing these emissions. This is especially true for the common
978-1-4244-6051-9/11/$26.00 ©2011 IEEE
mode radiation. In order to illustrate the fundamental radiation mechanisms in PCBs, layout effects on radiated
emissions and the ways to suppress them, an experimental kit is developed. The experiments around this kit are
designed in such a way to emphasize the useful techniques in reducing the radiated emissions.
2. Description of the Experimental Kit
The experimental kit basically consists of four PCBs having the same circuit topology but different layouts.
These boards are then configured in five different ways by adding wires, metal enclosure and feed-through filters; basic
board, wires connected board, wires connected board inside a metal enclosure, basic board inside a metal enclosure and
wires connected to the board through filters inside a metal enclosure. In the basic boards, an oscillator at 20 MHz drives
one of the inverter gates of an 7404 integrated circuit (IC) and the gate output is terminated by a 10Kohm resistor as
seen from the schematic of the circuit given in Fig.2. Overall circuit is fed from a 9V battery which is regulated to 5 V
level via a linear voltage regulator. In the PCB layout, all circuit components except the resistor are grouped together,
their positions and the interconnections between them are fixed for all different layouts. However, the interconnection
layout between the output of the logic gate and the resistor is changed depending on the layout configuration (red line
area). This is the only changing parameter between different layouts. In the first layout (Board A), the resistor is placed
at the far side of the PCB and the signal path between the logic gate and the resistor is wired far away from the return
path in such a way to maximize the loop area formed between the signal and return paths (see Fig.3a). In the second
layout (Board B), the resistor is kept at its original location but the signal path is made as close as possible to the return
path, thereby the loop area is minimized (see Fig. 3b). In the third layout (Board C), the resistor is placed as close as
possible to the driving logic gate and thereby, the loop area is decreased further (see Fig.3c). In the fourth layout (Board
D), the second layout is principally maintained but the return paths are replaced by a ground plane covering one side of
the PCB (see Fig. 3d). In all these configurations, the differential mode radiation is expected to be the dominant
radiation mechanism. These basic board layout configurations are labeled as A1, B1, C1 and D1. Then radiated
emissions measurement is carried out on each of these configurations. In the measurement, the boards are placed on a
non-conducting table at 80 cm height inside a semi-anechoic chamber and a biconical antenna is placed in horizontal
polarization at 1 meter away from the board at 120 cm height. The measurement is taken between 30 MHz and 200
MHz.
Afterwards, two separate 50 cm long wires are connected to the pads located at the opposite sides of the each
board (see Fig. 3a). Electrically, the pads are parts of the return path or ground of the circuit. Thereby a dipole antenna
configuration is obtained and the driving source will be the voltage drop on the return path (ground) of the circuit.
These wires connected boards are labeled as A2, B2, C2 and D2 depending on their original layout. In all these
configurations, it is expected that the common mode radiation will be more effective. The previously mentioned
measurement procedure is repeated for all these configurations.
Thirdly, the boards are placed inside a metal enclosure acting as a Faraday cage. The resulting configurations are
labeled as A3, B3, C3, D3 (wires connected boards inside a metal enclosure). As an extension to these configurations,
the wires connecting to the boards are removed and the boards are fully covered by the metal enclosure. These
configurations are labeled as A4, B4, C4, D4 (basic boards inside a metal enclosure). All these configurations are
measured again in this case.
Finally, the wires connected boards inside the metal enclosure are modified in such a way that the wires are
connected to the boards by passing feed-through filters placed at the entrance of the metal enclosure. In this case, the
configurations are labeled as A5, B5, C5 and D5. The previous measurement is repeated for all these configurations
also.
Figure 2 Experimental Circuit Schematic
Figure 3 Board Layouts
3. Measurement Results
The measurements results present several outcomes that are helpful in understanding the radiated emissions from
PCBs. For example, the measurement results of test boards A1, B1, C1 and D1 given in Fig. 4 emphasizes the fact that
the radiated emissions can be decreased by decreasing the loop area formed by the signal and return paths. Moreover it
emphasizes also the importance of the ground plane in reducing the loop area. Another striking comparison can be
observed by looking at the measurement results of test boards B1 and B2. As seen in Fig. 5, the B2 configuration (wires
connected board) results in higher emissions than the B1 configuration (basic board). This result shows that the
common mode radiation is much more dominant than the differential mode radiation. The comparison between the test
boards A1 and A4 shows the effectiveness of shielding offered by the metal enclosure when no connecting wires are
present (see Fig. 6). In order to see the effect of ground plane in reducing common mode radiation, the comparison
between the test boards B2 and D2 is suitable. As seen from Fig. 7, the ground plane reduces the common mode
radiation around 20 dB. The comparison between A type boards with wires shows the importance of filtering integrated
with shielding. As seen from Fig. 8, shielding couldn’t reduce the radiated emissions alone if there are cables connected
to the board. However, once the wires are filtered then the emission decreases. Finally, the combination of shielding,
filtering and the use of ground plane reduces the emissions further as illustrated in Fig. 9 showing the results obtained in
D boards with wires.
4. Conclusion
In this work, an EMC experimental kit is presented along with some striking example results. Although the
radiation mechanisms in PCBs are well established, there are some difficulties in understanding these mechanisms. The
kit illustrates these mechanisms using different board configurations and also shows how the remedies work. The kit
emphasizes the dominance of the common mode radiation. It also shows that the basic EMC measures such as
grounding, shielding and filtering should be applied in an integrated way.
5. References
1. C. R. Paul, “A Comparison of the Contributions of Common-Mode and Differential-Mode Currents in Radiated
Emissions”, IEEE Transactions on EMC, Vol.31, No:2, May 1989, pp. 189-193
2. R. German, H. Ott, C. Paul, “Effect of an Image Plane on Printed Circuit Board Radiation”, 1990 IEEE Intern. Symp.
on EMC, pp.284-291, Washington DC, August 21-23, 1990
3. J. L. Drewniak, T. H. Hubing, T. P. Van Doren, “Investigation of Fundamental Mechanisms of Common-Mode
Radiation from Printed Circuit Boards with Attached Cables”, 1994 IEEE Intern. Symp. on EMC, pp. 110-115, Chicago,
August 22-26, 1994.
4. H. Ott, Electromagnetic Compatibility Engineering, New Jersey, John Wiley & Sons, 2009, pp. 464-465
A1
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Figure 4 A1, B1, C1 and D1
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Figure 8 A2, A3 and A5
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Figure 9 D2, D3 and D5