Definition

Characteristic impedance (Z₀) is the ratio of voltage to current for an electromagnetic wave traveling along a transmission line. For PCB traces, Z₀ depends on trace geometry (width, thickness), reference plane spacing, and dielectric constant. Common values are 50Ω single-ended (standard for RF), 75Ω (video), and 90-100Ω differential (high-speed digital). Z₀ is frequency-independent to first order (though material properties may vary with frequency). Impedance mismatch causes reflections that degrade signal integrity at high speeds.