Spread spectrum time domain reflectometry

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Spread spectrum time domain reflectometry is based on STDR reflectometry ("Sequence Time Domain Reflectometry") and is used to locate intermittent faults in current-carrying wires, e.g. to detect intermittent short circuits on current-carrying cabling or in power line networks. In contrast to STDR, SSTDR provides a solution to overcome EMC constraints by adding pseudo-noise to the STDR sequence or modulating it with sinusoidal carriers. The modulation frequency is selected so that the modulated signal does not fall within the forbidden frequency bands. Frequency spreading is used to generate and feed complex signals. Reflexions from the conductor are detected by correlating the reflected signal with a copy of the transmitted signal. Consequently, mathematical algorithms are applied to both the shape and timing of the signals to locate discontinuities. As the spread spectrum signals are isolated from system noise and activity in the analysis, this approach largely eliminates interference that might occur during the cable measurement. An SSTDR system can therefore be operated on a live cable and also allows the test signal spectrum to be modified to better meet EMC regulations.

SSTDR is applied for preventive maintenance and in the context of fault location, especially in aeronautical systems and in high voltage cable monitoring of overhead lines and underground cables. It is beneficial whenever it would be impractical to remove or excavate miles of cable.