Noise Domain Reflectometry
NDR
In Noise Domain Reflectometry, the reflectometer uses significant noise or broadband signals already present on the line, rather than feeding a signal into the line to locate intermittent faults on live lines. It is therefore a passive test method that provides an estimate of the distance to a fault and allows online measurements.
Although the measurement here uses a signal that is not controlled in terms of generation, it can be used as a reflectometry test signal because impedance discontinuities are reflected regardless of the origin of that signal. For any signal with significant noise or noise-like behavior during the test period, the correlation of this signal with reflections occurring on the line results in a correlation pattern. Thereby, the peaks of this correlation pattern indicate the location of each reflection. In cable networks, multiple reflections occur, and each reflection can be observed independently of the others. The resolution of detection is directly related to the bandwidth of the tested signal.
This method is particularly useful for testing live cables, where data integrity on the cables is critical. NDR measurements can therefore be used, for example, to monitor aircraft wiring in flight or in applications where a secure connection is desired. Similar to SSTDR, in this case, the length of the line is determined by correlation. In other words, the method observes how similar or different the incident and reflected signals are and in what time intervals they are synchronous. The peaks of the determined correlation functions are used to determine the length of the wire.
NDR provides the ability to locate impedance discontinuities that are significant enough, such as substantial damage to the wire. Impedance changes of a few ohms, such as those caused by moisture, small chafe marks, or wire fraying, cannot be detected with NDR. In this regard, TDR measurement methods should be preferred. Thanks to the ability to test the cable under voltage, intermittent short circuits (arc faults) are detected by NDR as if they were short circuits (which produce a significant reflection), even if only for a short time. Intermittent open circuits can also be detected.