Common-mode excitation caused by an imperfect ground plane on a printed circuit board (PCB) has been conventionally explained with the 'current driven' scheme, in which the common-mode current is driven by the ground voltage across the unintentional inductance of the ground plane. We have developed an alternative method for estimating common-mode excitation that is driven by the difference of the common-mode voltages for two connected transmission lines. A parameter called current division factor (CDF) that represents the degree of imbalance of a transmission line explains the common-mode voltage. In this paper, we calculate the CDF with two-dimensional (2-D) static electric field analysis by using the boundary element method (BEM) for asymmetric transmission lines with an arbitrary cross-section. The proposed 2-D method requires less time than three-dimensional simulations. The EMI increase due to a signal line being close to the edge of the ground pattern was evaluated through CDF calculation. The estimated increase agreed well--within 2 dB--with the measured one.

A guard trace placed near a signal line reduces common-mode radiation from a printed circuit board. The reduction effect is evaluated by the imbalance difference model, which was proposed by the authors, when the guard trace has exactly the same potential as the return plane. However, depending on interval of ground connection of the guard trace, the radiation can increase when the guard trace resonates. In this paper, the authors show that the increase of radiation is caused by the common mode, and extend the imbalance difference model to explain a mechanism of increase of common-mode radiation. Additionally, the effective via location of the guard trace is proposed to reduce the number of vias. The guard trace voltage due to the resonance excites the common mode at the interface where the cross-sectional structure of the transmission line changes since the common-mode excitation is expressed by the product of the voltage and the difference of current division factors. To suppress the common-mode excitation, the guard trace should be grounded at the point where the cross-sectional structure changes. As a result, the common-mode radiation decreases even when the guard trace resonates.

Placing a guard trace next to a signal line is the conventional technique for reducing the common-mode radiation from a printed circuit board. In this paper, the suppression of common-mode radiation from printed circuit boards having guard traces is estimated and evaluated using the imbalance difference model, which was proposed by the authors. To reduce common-mode radiation further, a procedure for designing a transmission line with guard traces is proposed. Guard traces connected to a return plane through vias are placed near a signal line and they decrease a current division factor (CDF). The CDF represents the degree of imbalance of a transmission line, and a common-mode electromotive force depends on the CDF. Thus, by calculating the CDF, we can estimate the reduction in common-mode radiation. It is reduced not only by placing guard traces, but also by narrowing the signal line to compensate for the variation in characteristic impedance due to the guard traces. Experimental results showed that the maximum reduction in common-mode radiation was about 14 dB achieved by placing guard traces on both sides of the signal line, and the calculated reduction agreed with the measured one within 1 dB. According to the CDF and characteristic impedance calculations, common-mode radiation can be reduced by about 25 dB while keeping the characteristic impedance constant by changing the gap between the signal line and the guard trace and by narrowing the width of the signal line.

A connector model expressed as an inductance is proposed for use in a previously reported common-mode antenna model. The common-mode antenna model is an equivalent model for estimating only common-mode radiation from a printed circuit board (PCB) more quickly and with less computational resources than a calculation method that fully divides the entire structure of the PCB into elemental cells, such as narrow signal traces and thin dielectric layers. Although the common-mode antenna model can estimate the amount of radiation on the basis of the pin configuration of the connector between two PCBs, the calculation results do not show the peak frequency shift in the radiation spectrum when there is a change in the pin configuration. A previous study suggested that the frequency shift depends on the total inductance of the connector, which led to the development of the connector model reported here, which takes into account the effective inductance of the connector. The common-mode antenna model with the developed connector model accurately simulates the peak frequency shift caused by a change in the connector pin configuration. The results agree well with measured spectra (error of 3 dB).

The purpose of this report is to predict far-field EMI spectrum emitted from a signal line on a digital PCB based on near-field EMI measurement. The relation between near magnetic field and far electric field is shown. A method of predicting far electric field from near magnetic field is proposed. Current flowing along a signal line is calculated from measured near magnetic field. Far electric field is estimated from the current. Measurement and prediction of EM emission are carried out using a simple PCB. The result of prediction and measurement of far-field EMI spectrum coincide within the error of 3 dB.

In a differential transmission line, a large common-mode radiation is excited due to its asymmetry. In this paper, the imbalance difference model, which was proposed by the authors for estimation of common-mode radiation, is extended to apply to the differential signaling systems. The authors focus on a differential transmission line with asymmetric property, which consists of an adjacent return plane and two signal lines which are placed close to an edge of the return plane. Three orthogonal transmission modes, a normal mode, a primary common mode and a secondary common mode, are defined. Among these transmission modes, the secondary common mode is dominant in radiation, and a mechanism of the secondary common-mode generation is explained. The radiated emission which was calculated using the imbalance difference model was in good agreement with that obtained by full wave calculation.

We propose a novel technique of matching at both ends of the guard trace to suppress resonance. This approach is derived from the viewpoint that the guard trace acts as a transmission line. We examined that matched termination suppresses guard-trace resonance through simulating a circuit and measuring radiation. We found from these results that the proposed method enables guard-trace voltages to remain low and hence avoids increases in radiation. In addition, we demonstrated that "matched termination at the far end of the guard trace" could suppress guard-trace resonance sufficiently at all frequencies. We eventually found that at least two vias at both ends of the guard trace and only one matching resistor at the far end could suppress guard-trace resonance. With respect to fewer vias, the method we propose has the advantage of reducing restrictions in the printed circuit board layout at the design stage.

This paper explains a mechanism of common-mode generation on a printed circuit board with a narrow ground pattern. A transmission line has its value of degree of unbalance. At a connection point of two transmission lines having different degrees of unbalance, common mode voltage is generated proportional to the difference, and it drives common mode current. The authors propose a method to evaluate common mode current distribution and verify it by measurement. Although calculated common mode current is larger than measured one by a few dBs, both of them are proportional to the degree of unbalance. An EMI reduction technique, 'unbalance matching,' is also proposed.