Spread Spectrum TDR

Overview:

LiveWire Test Labs, Inc. has developed cutting edge technology that detects and locates intermittent fault conditions in high noise environments (e.g. live electrical power circuits, various types of concrete/steel structures) in real time.

Why It’s Important:

LiveWire’s Spread Spectrum Time Domain Reflectometry technology reduces maintenance/troubleshooting/monitoring costs, increase system reliability, and can extend the useful life of systems and structures by providing accurate monitoring of system health environments.

Core Technology: Spread Spectrum Reflectometry

Today’s time domain reflectometry methods are not able to locate the tiny faults left after an arc fault event, because their impedance discontinuity is too small to create a measurable reflection. On the other hand, if the fault could be found during the few milliseconds the arc occurs, it would be an actual short circuit, which returns plenty of reflected power! This is the concept that led to the development of Spread Spectrum Time Domain Reflectometry (SSTDR) at LiveWire Test Labs.

Spread Spectrum signals have been used in communication and radar for over 50 years. Direct Sequence Spread Spectrum (DSSS) communication uses a high speed pseudo noise (PN) code multiplexed with existing digital data to spread the spectrum, increase the number of simultaneous users on the line, and reduce the effects of noise and jamming. This same ability to reduce interference with other “users” and to resist “jamming” provides the ability to test live wires in flight without either interfering with the avionics signals or being corrupted by them.

The basic spread spectrum system is shown in Figure 1. In order to guarantee no interference, the PN code is very small (25-70 dB down) compared to the data signal. In fact, it is below the noise margin of the data. The PN code is added to the data/noise signal, and the combined signal is transmitted down the wire, where it reflects from the end of the wire. The combined incident/reflected signal is correlated with the PN code. This correlation is high if the two codes are synchronized, and low if they are not. Thus the system is capable of running live, with the test signal completely buried within the system noise. It can locate intermittent faults a few milliseconds long to within a few centimeters over tens to hundreds of meters of wire.

Our research in conjunction with our industrial partners is working towards integrating this technology directly into the arc fault circuit breakers, into the connectors between the wires, and eventually into the wires themselves.

Spread Spectrum System, Figure 1.

For more in-depth technical information, please contact us.