Common-mode currents and printed circuit boards

Common-mode (CM) currents through cables attached to printed circuit boards (PCB's) are often the main cause of interference and may even dominate the direct radiation of the board. In this paper, the authors predict the coupling between the differential-mode (DM) circuit on the board and the CM circuit in a Bersier setup, by means of a rapid transmission-line model. This coupling is expressed as the ratio of the CM current and the DM current. Several boards with different complexity were studied in the frequency domain. For demonstration purposes a board with modern digital electronics was developed. Measurements between 100 kHz-1 GHz agree with the TL model