Modeling of EMI spectra emitted from a signal line on a digital PCB

For the EMC design of high-speed digital printed circuit boards (PCBs), it is necessary to predict electromagnetic interference (EMI) caused by the PC B. In this article, we pay particular attention to EM emission from signal lines on a PCB. First, models are made on the spectral profile of the curre nt in a signal line based on the high-frequency equivalent circuits with ca pacitive and resistive load, respectively. Next, we formulate near-field EMI emitted from the signal line current. As a result, the spectral profile of near-field EMI is proved to be the same a s that of the signal current. The far-field EMI by the signal current is al so formulated, and we derive the relation between resonance frequency and s pectra of far-field EMI. By these results, it is theoretically proved that the near-field and far-fi eld EMI have specific profiles, respectively. Consequently, the spectral pr ofile of radiated EMI can be estimated easily by simple models with circuit parameters without requiring rigorous analysis, and this will provide a ne w method of high-speed simulation of radiated emission from digital PCBs. ( C) 1999 Scripts Technica.