The figure illustrates the major functional blocks of an impedance-based cable-testing apparatus that can locate an open or short circuit in a cable. There is no need to disconnect the cable from all other equipment in preparation for a test – an advantage in a system in which cable connections are located in places that are not readily accessible.
The cable tester is based on the concept of a cable as a transmission line, and on exploiting the impedance-transformation property of a transmission line that is a quarter wavelength long at some frequency: If one end of a quarter-wavelength-long transmission line is short-circuited, then the transmission line presents infinite impedance in the ideal case (or very high impedance in practice) to any equipment connected to the other end. If one end is open-circuited, then the transmission line presents zero impedance in the ideal case (or very low impedance in practice) to any equipment connected to the other end.
In the cable tester, a numerically controlled oscillator generates a sinusoidal signal at a frequency chosen by a microprocessor. (In the prototype tester, the frequency can lie between 500 kHz and 40 MHz.) The signal is amplified, and the resulting output signal is fed through a reference resistor (R) into the cable at an accessible point. The voltage V at the output terminal of the amplifier and the voltage Vo at the point of connection to the cable are measured. Then the impedance (Z) presented by the cable at the point of injection of the signal is given by
Z = RVo(V – Vo).
To obtain the exact value of Z, it would be necessary to measure both the magnitudes and the phases of V and Vo. In practice, it suffices to measure the magnitudes only, because under a short- or open-circuit condition, Vo must be close to zero or V, respectively.
The tester operates as follows: The microprocessor commands the oscillator to start at the lower end of its frequency range and sweep through increasing frequency until the impedance given by the above equation either falls to near zero or else rises to ≥ 10 times the nominal impedance of the cable. A near-zero-impedance indication signifies an open circuit in the cable; a high-impedance indication signifies a short circuit in the cable.
The distance d along the cable from the point of injection of the signal to the short or open circuit is then simply a quarter wavelength at the frequency (f) at which the sweep was stopped:
d = cv/(4f),
where c is the speed of light and v is the velocity factor of the cable (typical velocity factors range between 0.6 and 0.9). With the frequencies used in the prototype tester, it has been possible to locate short or open circuits at distances from about 1 to 150 m from the point of injection.
The electronic circuitry of the tester can readily be integrated into a hand-held, portable instrument that runs on batteries. Such an instrument would have great commercial potential; for example, it could reduce the time spent in diagnosing cables and electronic equipment connected to cables in airplanes.
This work was done by Pedro J. Medelius and Howard J. Simpson of Dynacs Engineering Co., Inc., for Kennedy Space Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com under the Electronic Systems category, or circle no. 172 on the TSP Order Card in this issue to receive a copy by mail ($5 charge).
This invention is owned by NASA, and a patent application has been filed. Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to Refer to KSC-11866.
the Patent Counsel, Kennedy Space Center; (407) 867-6225.
Refer to KSC-11866.