Electromagnetic Field Suppression in Multilayer Printed Circuit Power Plane Stackups
Thin dielectric power plane pairs suppress high frequency cavity modes and radiated emissions by lowering target impedance and shifting resonant frequencies.
Perimeter RC Snubber
Termination Device
Passive electronic networks placed at the edge of a power plane structure dissipate high frequency energy and reduce edge radiation. A standard perimeter RC snubber consists of a series resistor and capacitor connected between the power and ground planes. It acts as a termination for the parallel plate waveguide formed by the board layers.
By matching the snubber impedance to the characteristic impedance of the planes, the designer prevents signals from reflecting off the board edges. This component prevents the circuit board from acting as an efficient antenna.
Implementation Rule
Placement of these snubbers occurs at the furthest points from the power source or at regular intervals along the boundary. The value of the resistor is typically low, while the capacitor provides a low impedance path at the target resonance frequency. If the snubber is omitted, the edge of the board becomes a source of electromagnetic interference that can fail compliance testing.
Engineers calculate the values based on the plane capacitance and the expected noise spectrum of the switching power supplies. The physical layout must minimize the loop inductance of the connection to ensure effectiveness at gigahertz frequencies.
Performance Evaluation
Evaluation of the snubber effectiveness happens in a semi anechoic chamber during radiated emissions testing. A significant reduction in peak emissions at specific frequencies confirms the proper selection of component values. The heat generated by the resistor is rarely a concern in low power signals but must be checked in high current applications.