Loop fault sectionalization
AssigneeBell Telephone Laboratories, Incorporated
InventorJohn T. Peoples
Life of this patent
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Abstract
A fault on a nonloaded telephone subscriber loop is sectionalized by a frequency domain distance detection system. The derivative with respect to frequency of the phase angle of the complex input impedance of a cable pair is continuously monitored over a swept-frequency range beyond the voice-frequency band. The phase itself is obtained indirectly as the Hilbert transform of the logarithmic differences between the instantaneous impedance and a delayed version thereof. Thus, circuitry which is less sensitive to noise, parameter variation, and measurement error than prior techniques becomes practical for generating waves having energy peaks corresponding to the distance from the measurement end of a loop under test to impedance irregularities therealong.
Claims9
Apparatus for determining distances to impedance irregularities along a nonloaded communications cable from the input terminals thereof, having connected to such input terminals a swept-frequency signal source and a spectrum analyzer for displaying the frequencies of the maxima of the power spectrum of the derivative with respect to frequency of the phase angle of the complex impedance manifested at such input terminals, wherein the distances to impedance irregularities are proportional to the frequencies of these maxima characterized in that
- means responsive to said swept-frequency signal source measures the magnitude of the input impedance of said communications cable, and
- means for Hilbert transforming the difference between natural logarithms of a direct and delayed impedance magnitude from said measuring means yields the equivalent of said derivative with respect to frequency of the phase angle of said impedance.
A method for determining distance to impedance irregularities along a nonloaded communications cable from the input terminals thereof, having connected to such input terminals a swept-frequency signal source and a spectrum analyzer for displaying the frequencies of the maxima of the power spectrum of the derivative with respect to frequency of the phase angle of the complex impedance manifested at such input terminals, wherein the distances to impedance irregularities are proportional to the frequencies of the maxima characterized in that
- in a first step the magnitude of said complex impedance of the cable under test is determined from an amplitude component of the signal measured at said input terminals independently of any phase component thereof resulting from the application of the swept-frequency signal to said input terminals,
- in a second step an intermediate signal is obtained from the natural logarithms of direct and delayed replicas of said impedance magnitude from said first step, and
- in a third step a further signal proportional to the derivative of the phase-angle component with respect to frequency is obtained from a Hilbert transform operation on the intermediate signal from said second step.
Dependencies are inferred from the claim text.