Evaluation of EMI Reduction Effect of Guard Traces Based on Imbalance Difference Model

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 14dB achieved by placing guard traces on both sides of the signal line, and the calculated reduction agreed with the measured one within 1dB. According to the CDF and characteristic impedance calculations, common-mode radiation can be reduced by about 25dB 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.