Increase of Common-Mode Radiation due to Guard Trace Voltage and Determination of Effective Via-Location

Tohlu MATSUSHIMA  Tetsushi WATANABE  Yoshitaka TOYOTA  Ryuji KOGA  Osami WADA  

IEICE TRANSACTIONS on Communications   Vol.E92-B   No.6   pp.1929-1936
Publication Date: 2009/06/01
Online ISSN: 1745-1345
DOI: 10.1587/transcom.E92.B.1929
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Section on 3rd Pan-Pacific EMC Joint Meeting -- PPEMC'08--)
common-mode radiation,  imbalance difference model,  printed circuit board,  guard trace,  ground via,  

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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.