Definition

Characteristic impedance (Z₀) is the ratio of voltage to current for a wave traveling along a transmission line. It's determined by trace geometry (width, thickness), dielectric thickness to reference plane, and dielectric constant. For PCB traces, common structures include microstrip (trace on outer layer referenced to plane below) and stripline (trace between two planes). Fabricators typically guarantee controlled impedance within ±10% (e.g., 50Ω ±5Ω). They achieve this by adjusting trace widths during manufacturing to compensate for material and etch variations. Specify your target impedance in your fabrication notes and the fabricator will optimize trace geometry.