Definition

Time Domain Reflectometry (TDR) measures transmission line impedance by sending a fast-rise pulse down the line and analyzing reflections. Impedance discontinuities cause partial reflections - the magnitude and polarity indicate whether impedance increased or decreased, and the timing reveals the location. PCB fabricators use TDR to verify controlled impedance on test coupons, typically guaranteeing ±10% (standard) or ±5% (tight tolerance). Industry-standard equipment includes sampling oscilloscopes like the Tektronix DSA8300 series with TDR modules, achieving rise times under 20 ps for sub-millimetre spatial resolution. TDR results appear in impedance test reports delivered with controlled-impedance boards.