Common-Mode-Current Generation Caused by Difference of Unbalance of Transmission Lines on a Printed Circuit Board with Narrow Ground Pattern

Tetsushi WATANABE  Osami WADA  Takuya MIYASHITA  Ryuji KOGA  

IEICE TRANSACTIONS on Communications   Vol.E83-B   No.3   pp.593-599
Publication Date: 2000/03/25
Online ISSN: 
Print ISSN: 0916-8516
Type of Manuscript: Special Section PAPER (Special Issue on Recent Progress in Electromagnetic Compatibility Technology)
Category: EMC Design of PCB
EMC technology,  digital circuit,  printed circuit board,  microstrip line,  common mode current,  narrow ground,  

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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.