TDR and FDR Identification of Bad Splices in Telephone Cables

To facilitate the widespread deployment of DSL Internet access technicians must be able to identify and locate even minor discontinuities in transmission lines. Discontinuities cause a portion of the signal to be reflected backward and this leads to intersymbol interference and impairment of high speed digital transmission. In addition, discontinuities introduce signal loss that can limit the distance of transmission. Telephone line technicians identify corroded splices as a frequent "trouble" that impairs DSL video service. The paper first reviews the frequency domain reflectometry (FDR) method and how the reflection phase angle can be determined through use of the FFT. We are able to detect a bad splice because it introduces a small series resistance that increases the apparent impedance of the remaining cable and causes reflections. Sensitive coherent detection allows the FDR method to detect the very small reflections caused by 10-ohm series resistance at a distance of 2900 m. In contrast, commercial TDR instruments are not able to detect this discontinuity at distances beyond 1200 m. Telephone cable characteristic impedance is slightly capacitive in the DSL frequency range and the 10-ohm series resistance makes the apparent impedance somewhat more real, resulting in a reflection with positive phase angle (~10 degrees). This reflection angle can be used to distinguish the bad splice discontinuity from other types of impairments and knowledge of the type of fault allows effective dispatch of a repair crew. Previous work has shown reflection angles for water in the cable (~160deg), and bridged taps (180deg). Through measurements, this paper compares bad splice reflection angles (~10deg;) with those from gauge changes (~135deg)