Boundary Emission
The emission of electromagnetic energy occurs from the open boundaries of a printed circuit board where the power and ground planes terminate. This power plane edge radiation is driven by the high frequency voltage fluctuations generated by switching integrated circuits on the board. The parallel planes act as a parallel-plate waveguide that guides these waves toward the outer edge of the substrate where they launch into free space.
This radiation mechanism is mitigated by pulling the power plane boundaries inward relative to the ground plane boundaries.
Radiation Mitigation
Extending the ground planes beyond the edges of the power plane creates a shielding effect that directs the electric field lines back into the substrate rather than into the surrounding environment. This layout strategy reduces power plane edge radiation by capturing the fringing fields before they can escape. Another approach involves placing a row of vias connected to ground along the perimeter of the board to form a Faraday cage.
This via barrier terminates the horizontal electromagnetic wave propagation and prevents the board from acting as an efficient patch antenna.
Far Field Scan
Detection involves measuring the radiated emissions in an anechoic chamber or using a near field scanner along the board perimeter. This scan identifies the hot spots where the shielding barrier is incomplete.
