Method and apparatus for locating faults in communications networks

1. A test apparatus, said apparatus including:

a primary transceiver module including:

a high power amplifier module for providing a pair of test signals wherein at least one of the test signals is swept over a predetermined frequency range;

a combiner module for applying the test signals to a test medium; and

a primary receiver coupled to the combiner module for reception of a plurality of primary return signals produced by the test medium in response to the test signals;

a directional coupler module coupled between the combiner module and the test medium;

a reference signal generation module, said reference signal generation module including:

a filter module coupled to the directional coupler module, said directional coupler module providing a pair of reference test signals corresponding to the pair of test signals produced by said high power amplifier module; and

a reference signal generator for producing a plurality of reference signals based on the pair of reference test signals;

a reference receiver coupled to the filter module for receiving a plurality of reference signals from the reference signal generator;

an analog to digital converter coupled to the primary receiver and the reference receiver, said analog to digital converter producing a first set of digital signals from the plurality of primary return signals and a second set of digital signals from the plurality of reference signals; and

at least one processor coupled to the analog to digital converter wherein said at least one processor is adapted to:

determine amplitudes for each signal within the first set of digital signals and the second set of digital signals;

calculate phase offsets between each signal in the first set of digital signals and the second set of digital signals;

combine the amplitude and phase measurements for each signal in the first set of digital signals and the second set of digital signals into a single vector;

estimate from the single vector a number of passive intermodulation sources within the test medium;

determine for each passive intermodulation source its magnitude and location within the test medium; and

display the magnitude and location of each passive intermodulation source within the test medium.

2. The test apparatus of claim 1 , wherein said apparatus further includes a pair of frequency synthesizers coupled to the high power amplifier module, for production of test signals in different communications bands.

3. The test apparatus of claim 1 , wherein the primary receiver includes a two-stage down converter.

4. The test apparatus of claim 1 , wherein said apparatus further includes a reflectometer coupled between the directional coupler module and the at least one processor.

5. The test apparatus of claim 4 , wherein the reflectometer is a six port reflectometer.

6. The test apparatus of claim 1 , wherein the combiner module includes a hybrid combiner and a filtering element.

7. The test apparatus of claim 6 , wherein the filtering element is a diplexer.

8. The test apparatus of claim 6 , wherein the filtering element is a triplexer.

9. The test apparatus of claim 1 , wherein the combiner module comprises a filtering element, said filtering element being a quadruplexer.

10. The test apparatus of claim 1 , wherein the directional coupler module includes at least one reverse coupler and at least two forward couplers.

11. The test apparatus of claim 10 , wherein the reverse coupler and at least one forward coupler are coupled to a reflectometer.

12. The test apparatus of claim 10 , wherein at least one forward coupler is coupled to the filter module.

13. The test apparatus of claim 1 , wherein the filter module is a triplexer.

14. The test apparatus of claim 1 , wherein the filter module is a diplexer.

15. The test apparatus of claim 1 , wherein the filter module is a band pass filter.

16. The test apparatus of claim 1 , wherein the reference signal generator is coupled to the directional coupler module via a Low Noise Amplifier.

17. The test apparatus of claim 1 , wherein sweeping the at least one test signal's frequency includes incrementally increasing the frequency of the at least one test signal.

18. The test apparatus of claim 17 , wherein the frequency of the at least one test signal is incremented in frequency steps of 0.25 MHz.

19. The test apparatus of claim 1 , wherein the primary receiver and reference receiver are each coupled to the analog to digital converter by anti-aliasing filters.

20. The test apparatus of claim 19 , wherein the anti-aliasing filters are lowpass filters.

21. The test apparatus of claim 1 , wherein said apparatus further includes a bias tee, a direct current (“DC”) power supply unit and an Antenna Interface Standards Group (“AISG”) modem attached to a test port of the apparatus, whereby active devices in a communications network under test can be powered during testing.

22. The test apparatus of claim 21 wherein the AISG modem is utilized to change down-tilt and azimuth angles of at least one AISG-enabled antenna connected to the test port of the apparatus during testing.

23. The test apparatus of claim 1 wherein the at least one processor adapted to estimate of the number of passive intermodulation sources within the test medium is further adapted to:

construct a system of linear prediction equations based on the single vector to produce a linear prediction data matrix;

perform a singular value decomposition on the linear prediction data matrix to produce a set of singular values, wherein the set of singular values defines a magnitude of a largest singular value within the set of singular values; and

set any singular value within the set of singular values to zero if the singular value is less than 10% of the magnitude of the largest singular value within the set of singular values to produce a set of modified singular values.

24. The test apparatus of claim 23 , wherein the at least one processor adapted to determine for each passive intermodulation source its location and magnitude is further adapted to:

reconstitute a modified linear prediction data matrix using the set of modified singular values;

determine coefficients of a characteristic polynomial for the modified linear prediction data matrix utilizing a Total Least Squares method;

calculate roots of the characteristic polynomial using said coefficients;

calculate the location of each passive intermodulation source from said roots; and

calculate the magnitude of each passive intermodulation source via a Least Squares Prony method, using said coefficients and roots of the characteristic polynomial.

25. The test apparatus of claim 1 , wherein determination of the magnitude and location of each passive intermodulation source within the test medium is performed utilizing a Periodogram Power Spectral Density Estimator.

26. The test apparatus of claim 1 , wherein the calculation of the phase offsets includes:

determining absolute phases for each signal within the first and second set of digital signals by performing a least-squares fit of a sinusoidal function to each signal within the first and second set of digital signals; and

calculating a difference between the phases of the resultant sinusoidal functions derived from a least-squares process.

27. A method for determining the location and magnitude of sources of passive intermodulation within a test medium said method including the steps of:

applying a pair of test signals to the test medium wherein at least one of the test signals is swept over a predetermined frequency range;

receiving a plurality of primary return signals produced by the test medium in response to the test signals;

generating a pair of reference test signals corresponding to the pair of test signals;

generating a plurality of reference return signals from the pair of reference test signals;

compiling a first set of digital signals from the plurality of primary return signals and a second set of digital signals from the plurality of reference signals;

determining amplitudes for each signal within the first set of digital signals and the second set of digital signals;

calculating phase offsets between each signal in the first set of digital signals and the second set of digital signals;

combining the amplitude and phase measurements for each signal in the first set of digital signals and the second set of digital signals into a single vector;

estimating from single vector a number of passive intermodulation sources within the test medium;

determining for each passive intermodulation source its magnitude and location within the test medium; and

displaying the magnitude and location of each passive intermodulation source within the test medium.

28. The method of claim 27 , wherein the step of calculating the phase offsets includes:

determining absolute phases for each signal within the first and second set of digital signals by performing a least-squares fit of a sinusoidal function to each signal within the first and second set of digital signals; and

calculating a difference between the phases of the resultant sinusoidal functions derived from the least-squares process.

29. The method of claim 27 , wherein the step of estimating the number of passive intermodulation source includes:

constructing a system of forward linear prediction equations based on the single vector to produce a linear prediction data matrix;

performing a singular value decomposition on the linear prediction data matrix to produce a set of singular values, wherein the set of singular values defines a magnitude of a largest singular value within the set of singular values; and

setting any singular value within the set of singular values to zero if the singular value is less than 10% of the magnitude of the largest singular value within the set of singular values to produce a set of modified singular values.

30. The method of claim 29 , wherein the step of calculating the location and magnitude of each passive intermodulation source includes:

reconstituting a modified linear prediction data matrix using the set of modified singular values;

determining coefficients of a characteristic polynomial for the modified linear prediction data matrix utilizing a Total Least Squares method;

calculating roots of the characteristic polynomial using said coefficients;

calculating the location of each passive intermodulation source from said roots; and

calculating the magnitude of each passive intermodulation source via a Least Squares Prony method, using said coefficients and roots of the characteristic polynomial.

31. The method of claim 27 , wherein determining the magnitude and location of each passive intermodulation source within the test medium is performed utilizing a Periodogram Power Spectral Density Estimator.