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RF Current Density Distribution
Within a transmission line, electromagnetic energy propagates within a transmission line from a source to a load. Faraday’s law states that when a time variant current travels within a transmission line, magnetic flux is created that surrounds the transmission line (Right Hand Rule), and exist within the surrounding dielectric (free space or the physical substrate; core and prepreg).
Current distribution in traces tends to spread out within a return path, as illustrated in the figure, provided courtesy of Dr. Howard Johnson, “High Speed Digital Design-A Handbook of Black Magic.” This distribution exists in both the forward direction as well as the return path. RF current distribution shares a common impedance between trace and plane (or trace-to-trace), which results in mutual coupling due to the current spread in the reference plane away from the trace. The peak current density lies directly beneath the trace and falls off sharply from each side of the trace into the reference structure.
When the physical distance spacing (H) between a transmission line and reference plane increases, the inductance of the loop area of both the forward and return path also increases. The current distribution present at a physical distance from the center of the transmission line is described in the equation below.
When the physical distance spacing (H) between a transmission line and reference plane increases, the inductance of the loop area of both the forward and return path also increases. The current distribution present at a physical distance from the center of the transmission line is described in the equation below.
where I(d) = signal current density (A/inch or A/cm)
Io = total current (A)
H = height of the trace above the ground plane (in. or cm)
D = perpendicular distance from the centerline of the trace (in. or cm)
RF currents exist based on the topologies shown in the figure below. The physical distance that the current spreads away from the trace into a reference plane is approximately one transmission line width distance away. For example, if a trace is 0.008 in. (0.002 mm) wide, flux coupling to an adjacent trace may occur if the adjacent trace is less than or equal to 0.008 in. (0.002 mm) away. If the adjacent trace is routed greater than one trace width away, coupling of RF flux will generally be minimal between the two transmission lines.
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Microstrip only one reference plane or stripline topology (both reference planes) |
Coplanar (parallel lines) |
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