Spectral Selectivity
Constructive wave interference occurs when signals encounter periodic impedance discontinuities along high-speed transmission lines at a wavelength corresponding to twice the physical spacing of the perturbations. This periodic reflection behavior, known as bragg resonance, attenuates the signal rapidly at the matching frequency band. It creates a stopband where propagation ceases and signal integrity degrades.
Grating Periodicity
Transmission lines in dense multi-layer boards suffer from reflections when they pass a series of regularly spaced vias or structural features. The reflection coefficient rises sharply when the phase shift between consecutive reflections equals a multiple of pi radians. High-frequency boards running at gigahertz rates exhibit measurable signal loss at these critical frequencies because each small discontinuity adds constructively to the reflected wave.
Attenuation depends on the spacing of the structures and the dielectric constant of the laminate. Design rules prevent this behavior by shifting via patterns, varying the line impedance, or adjusting the pitch.
Signal Attenuation
Trace routing through periodic copper voids or ground grid patterns also triggers this phenomenon. The insertion loss plots display a sharp dip at the tuned frequency which limits the usable bandwidth. Board designers must simulate the return loss profile of such channels to locate these drops.
