A system used in the characterization of cables (or PCB tracks) employed as transmission lines. A very short pulse (≅100 picoseconds) is launched into the cable at one end and the reflections of it are measured to obtain the position and impedance of any discontinuities along the length of the cable. The pulse length/edge speed are chosen to suit the type of cable and expected attenuation. Pulses as short as 100 picoseconds are used for short transmission lines carrying fast signals.