FDTD modeling of EMI antennas

The FDTD (finite-difference time-domain) method is used for modeling EMI antenna geometries to anticipate EMI problems in high-speed digital designs. FDTD is well-suited to modeling large-scale geometries such as cables that might be driven against PCB ground planes as a result of a noise voltage that appears at the connector. Three specific cases are addressed herein including a simple cable driven against a PCB ground, coupling between a high-speed digital and I/O line that drives a cable against the PCB, and the finite impedance of the PCB reference plane that results in an effective noise source that drives the PCB ground against the cable. The FDTD modeling is compared with measurements. Guidelines for coupling to I/O lines, and a keep-out distance around the board periphery for high-speed digital lines can then be developed.