Non-intrusive impedance-based cable tester

• US 5,977,773 A
• Filed: 08/15/1997
• Issued: 11/02/1999
• Est. Priority Date: 08/15/1997
• Status: Expired due to Term

• Current Assignee: Administrator of The National Aeronautics and Space Administration
• Original Assignee: United States Of America As Represented By The Administrator Of The National Aeronautics And Space Administration
• Inventors: Medelius, Pedro J., Simpson, Howard J.
• Primary Examiner(s): Ballato, Josie
• Assistant Examiner(s): SUNDARAM, T R
• Application Number:
  US08/912,035
• Time in Patent Office: 809 Days
• Field of Search: 324/520, 324/521, 324/522, 324/523, 324/525, 324/527, 324/528
• US Class Current: 324/520
• CPC Class Codes: G01R 31/088 Aspects of digital computing

Abstract

A non-intrusive electrical cable tester determines the nature and location of a discontinuity in a cable through application of an oscillating signal to one end of the cable. The frequency of the oscillating signal is varied in increments until a minimum, close to zero voltage is measured at a signal injection point which is indicative of a minimum impedance at that point. The frequency of the test signal at which the minimum impedance occurs is then employed to determine the distance to the discontinuity by employing a formula which relates this distance to the signal frequency and the velocity factor of the cable. A numerically controlled oscillator is provided to generate the oscillating signal, and a microcontroller automatically controls operation of the cable tester to make the desired measurements and display the results. The device is contained in a portable housing which may be hand held to facilitate convenient use of the device in difficult to access locations.

Claims

1. An apparatus for determining the location of a discontinuity in an electrical cable comprising:

a) a variable frequency oscillator for applying a variable frequency oscillating test signal to a signal injection point at one end of an electrical cable under test;

b) a reference resistor disposed between said oscillator and said signal injection point; and

c) a microcontroller for controlling operation of said oscillator, said microcontroller including means for determining a frequency of said test signal at which an electrical impedance at said signal injection point between said oscillator and said cable is a minimum, and determining from said frequency, the distance from said injection point to a discontinuity in said cable, said microcontroller further including means for comparing a voltage magnitude at said signal injection point to a voltage magnitude at an oscillator side of said reference resistor, and determining that said discontinuity is;

     1) an open circuit if said voltages are approximately equal to one another, and

     2) a short circuit if said signal injection point voltage is approximately zero volts.

2. The apparatus of claim 1, further comprising:

d) a peak detector disposed between said signal injection point and said microcontroller for supplying a peak voltage measurement at said injection point to said microcontroller for enabling said microcontroller to determine the impedance at said injection point.

3. The apparatus of claim 2, further comprising:

e) an amplifier connected to an output of said peak detector for generating an amplified signal; and

f) an A/D converter connected to said amplifier output for converting said amplified signal into a digital value and supplying said digital value to said microcontroller.

4. The apparatus of claim 3, wherein said amplifier further comprises a three stage amplifier with first, second and third series connected amplifier stages, each generating an output;

and said A/D converter further comprises first, second and third A/D converters, each receiving a corresponding one of said outputs from said first, second and third stages.

5. The apparatus of claim 1, further comprising:

d) a display connected to said microcontroller for displaying calculated distance measurements; and

e) a keyboard connected to said microcontroller for entering control functions.

6. The apparatus of claim 5, further comprising:

f) a portable housing for containing said oscillator and said microcontroller.

7. The apparatus of claim 6, further comprising:

g) a coaxial cable of a predetermined length connected to an output of said oscillator for electrically connecting said oscillator to said cable under test.

8. A method for determining the location of a discontinuity in an electrical cable comprising the steps of:

a) applying an oscillating electrical signal through a reference resistor to a first end of said cable;

b) comparing an electrical voltage on a first side of said resistor to an electrical voltage on a second side of said resistor connected to said cable;

c) determining that an open circuit discontinuity is present in said cable if said voltages are approximately equal, and determining that a short circuit discontinuity is present in said cable if said voltage at said second side of said resistor is approximately zero volts;

d) varying a frequency of said oscillating signal until the electrical impedance at a signal injection point of said signal located at said second side of said resistor is a minimum value; and

e) determining the distance from said signal injection point to said discontinuity along said cable from the frequency at which said impedance is a minimum value.

9. The method of claim 8, wherein said step of varying said frequency further comprises:

1) varying said frequency in predetermined increments until said detected impedance is a first minimum value, and noting a first frequency at which this occurs; and

2) varying said frequency of said oscillating signal in smaller increments above and below said first frequency to determine a second frequency at which said impedance is a second lower minimum value.

10. The method of claim 8, wherein said step of determining further comprises determining the distance to said discontinuity from the formula d=(c/f)×

(vf)×

(1/4) if said detected discontinuity is an open circuit, and determining the distance to said discontinuity using the formula d=(c/f)×

(vf)×

(1/2) if the detected discontinuity is a short circuit, where d is the distance in meters from the signal injection point to said discontinuity, c is the speed of light in meters per second, f is the frequency in cycles per second of said test signal at which said minimum impedance occurs, and vf is a velocity factor of said cable.