Definition

Time Domain Reflectometry (TDR) measures transmission line impedance by sending a fast pulse down the line and analyzing reflections. Impedance changes cause partial reflections; the magnitude and polarity indicate whether impedance increased or decreased. TDR is used to verify controlled impedance on PCB test coupons, with fabricators guaranteeing specified impedance ±10%. TDR also locates faults: opens, shorts, and impedance discontinuities show as reflections at specific distances. Modern TDR equipment achieves sub-nanosecond resolution for fine spatial accuracy.