The HP Model 1415A Time Domain Reflectometer, a double-sized plug-in unit for the 140-series oscilloscopes, is a completely integrated transmission line testing system. The unit includes both the time base and vertical deflection function of a high speed sampling oscilloscope, and a fast rise step generator. The model 1415A locates and determines the magnitude and nature of discontinuities in high frequency and transmission systems and can be used to measure the quality of broad band systems.