DSL line tester using wideband frequency domain reflectometry
Celaya de la Torre, Bernardo
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Digital subscriber line (DSL) technology is used to provide high speed Internet access and, more recently, video services over twisted pair lines. Telephone lines have impairments that hinder DSL transmission, and some examples are coils and bridge taps that were introduced to aid voice telephony. Other faults are caused by physical damage that results in open circuits, short circuits or water in the line. Telephone companies have to locate and repair these faults to enable high-speed data services. It is also useful to “qualify” lines for future DSL service so that a new customer can be promptly informed whether high-speed service can be supplied. This thesis proposes a novel technique called Wideband frequency domain reflectometry (W-FDR) to accurately locate impairments in telephone lines and estimate the reflection magnitude caused by faults. The measurement produces a result similar to the well-known time domain reflectometer (TDR), however, digital signal processing techniques are now applied to provide enhanced resolution and range. In addition to magnitude, the new technique is able to measure reflection angle which can help to determine the nature of the fault (wire in the cable, broken wires, etc). The measurement technique consists of energizing the line with a sinusoid that increases from 50 kHz to 1300 kHz in discrete frequency steps after coherent detection. The amplitude of the reflected signal is recorded as a function of frequency for 2500 equally spaced frequencies. The Fourier transform and some signal processing are then used to estimate the complex reflection coefficient location of the faults in the telephone line. Lines with up to 4 reflection points have been accurately analyzed. Results show distance measurement accuracy better than 1% and phase measurement accuracy better than 10 degrees for line lengths up to 5 km. These measurements exceed the performance of currently available TDR instruments.
DegreeMaster of Science (M.Sc.)
SupervisorDodds, David E.
Copyright DateSeptember 2004
Transmssion line testing