Electromagnetic Behavior
Energy oscillations within the space between conductive power and ground planes occur when the dimensions of the board match a multiple of the signal wavelength. Occurrence of plane cavity resonance leads to substantial voltage fluctuations and increased electromagnetic emissions at specific frequencies. This effect is similar to the standing waves that form in an acoustic chamber or a waveguide.
At these resonant frequencies, the impedance of the power distribution network peaks, making it difficult to maintain a stable supply voltage for high-speed components.
Voltage Fluctuation
Noise generated by switching circuits can trigger these resonances, causing the planes to radiate energy like an antenna. This radiation often leads to failures in electromagnetic compatibility testing because the emissions are difficult to contain with standard shielding. The peak voltages can also interfere with the operation of nearby sensitive circuits by inducing noise onto signal traces.
Designers use simulations to predict these frequencies based on the board size and the dielectric constant of the laminate. Adding decoupling capacitors or changing the board shape can shift these resonances away from the operating frequencies of the system.
Suppression Method
Thin dielectric layers between the planes help to dampen these oscillations by increasing the capacitance and reducing the quality factor of the cavity. Strategic placement of stitching vias also breaks up the resonance modes.
