Path Interrupt
Absence of a continuous conductive copper reference area beneath a high-speed transmission line alters the return current flow path and increases electromagnetic radiation. In these configurations, a ground plane discontinuity forces return currents to detour around the gap.
Signal Distortion
Severe impedance variations occur when a transmission line crosses these splits in the reference plane. The impedance spike at the crossing point causes signal reflections that distort the rising and falling edges of high-speed digital pulses. In high-density assemblies, this localized mismatch produces trace crosstalk and elevates the overall electromagnetic emissions of the system.
This distortion is particularly critical in differential signaling, where the split can convert differential mode signals into common mode noise.
Remediation Strategy
Layout constraints must enforce continuous copper planes beneath high-frequency routing to maintain a uniform characteristic impedance. When a split plane is unavoidable, stitching capacitors are positioned across the gap to provide a low-impedance AC return path for the high-frequency current. These capacitors must be located immediately adjacent to the crossing point to minimize the loop area.
Time-domain reflectometry tests can scan the finished board to pinpoint the exact location and magnitude of any ground plane discontinuity that escaped design rule checks.
