A cable-testing apparatus has been designed for use in detecting a short or open circuit in a cable, subject to a special frequency requirement as explained in the next paragraph. This cable tester is based on the injection of a signal at one end of the cable and the use of a pickup coil placed adjacent to the cable to detect the signal. The pickup coil is moved along the outside of the cable until the signal is lost; the loss of signal indicates the location of a short or open circuit.
Commercial cable testers based on this principle inject signals with frequencies of several hundred kilohertz. In the specific application for which the present cable tester was developed, the cable is shielded, and there is a special requirement to use a frequency of 10 Hz; this requirement is dictated by the frequency responses of signal processors connected to the cable. The use of this frequency in a shielded cable poses two challenges to effective signal-pickup cable testing: (1) at such a low frequency, the shield becomes almost 100 percent effective, so that there is very little signal power that leaks through to the outside; and (2) for a given magnetic-field signal amplitude, the output of a pickup coil is directly proportional to the signal frequency.
These challenges are overcome by use of a narrow-band-pass filter followed by a high-gain amplifier. The filter is needed to minimize the noise that enters the amplifier along with the weak signal that coil picks up from the cable; the amount of this noise is proportional to the bandwidth of the filter, and thus one should make the bandwidth as narrow as practicable. The filter is of 16th order and exhibits a bandwidth of 0.05 Hz with a center frequency of 10 Hz. Both the signal frequency and the center frequency are crystal-controlled to prevent drifts that would degrade signal-pickup performance.
The output of the amplifier is detected, using a digital quadrature demodulator. Optionally, the output of the amplifier can also be displayed on an oscilloscope. Like the tester described in the preceding article, this one has commercial potential, especially for use in the aircraft industry.
This work was done by Pedro J. Medelius of Dynacs Co., Inc., for Kennedy Space Center . For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Systems category, or circle no. 165 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
the Patent Counsel, Kennedy Space Center; (407) 867-6225.
Refer to KSC-11865.