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66 DESIGN007 MAGAZINE I MAY 2025 and to conducting planes restricts the amount of field energy that radiates from any individual transmission line. For traces situated between conducting planes in a stripline configuration, board radiation is almost nonexistent (with the exception of edge fringing fields). However, on a typical outer (microstrip) layer, where hundreds of transmission lines may be active simultaneously, even a small portion of energy escaping from each trace can collectively pro- duce a significant radiated field. In Figure 1, electric field lines are plotted in blue. ese can be thought of as the "lines of force." Note that they begin and end on conduc- tor surfaces (where physical charges reside). ey refract (change direction) at boundaries between different dielectrics. Electric fields, which capacitively couple signals into a nearby trace, are absorbed by the plane but still tend to radiate noise outward. Magnetic fields are plotted in red. ese existed in circular form around the trace along which the electric potential (voltage) is a con- stant. ey form closed contours around one or more conductors. e inductance of the trace depends on the geometry of the circuit and the magnetic properties of the media con- taining the field. Electromagnetic fields are totally blocked by a solid plane. e differential pair in Figure 2 illustrates the electromagnetic field coupling between the two trace segments. Magnetic fields tend to radiate into air, which is actually just another dielectric with a dielectric constant of 1. e energy carried along a transmission line travels within the electromagnetic field between the conductors. Radiation or field coupling occurs only in areas where the wave amplitude is changing. is happens at the leading edge as it moves down the line. When the rise time increases, the radiating area grows larger. For example, during a rise time of 100 ps, a wave can travel 1.5 cm. If the transmission line is series-terminated at the logic source and measures 6 cm in length, the radiation pattern persists for a full round trip of the wave, approx- imately 800 ps. e radiating pattern is identi- cal for both the outgoing and reflected waves. For fast leading edges, the radiation observed from a distance appears to come from a mov- ing current element, effectively behaving like a moving dipole. Radiation and cross-coupling happen wher- Figure 2: Microstrip differential pair coupling.