Method and system for testing a signal path having an operational signal
A technique for testing a signal path while an operational signal is present is disclosed. The technique may be performed without injecting a test signal into the signal path by using an operational signal already present in the signal path. A method of testing a signal path includes receiving an operational signal from the signal path and estimating a correlation of the operational signal. A system for testing a signal path includes an extractor configured to extract a sample of the operational signal when coupled to a signal path and a correlator configured to estimate a correlation of the operational signal. Various properties of the signal path may be determined using the technique.
1. A method of testing a signal path comprising:
a) receiving an operational signal from the signal path, wherein the operational signal is present within the system during operation and is not an injected test signal;
b) estimating a correlation of the operational signal at a time-offset wherein the correlation is the expectation of the product of the operational signal and a reference signal; and
c) determining a characteristic of the signal path from the correlation.
2. The method of claim 1 wherein the operational signal has reflections caused by at least one impedance discontinuity in the signal path.
3. The method of claim 1 wherein:
receiving an operational signal comprises separating a main portion of the operational signal from a reflected portion of the operational signal; and
estimating a correlation of the operational signal comprises estimating a cross-correlation of the main portion of the operational signal using the reflected portion of the operational signal as the reference signal.
4. The method of claim 1 wherein estimating a correlation of the operational signal comprises estimating an autocorrelation of the operational signal using the operational signal as the reference signal.
5. The method of claim 1 wherein estimating a correlation of the operational signal comprises estimating a plurality of correlation values corresponding to a plurality of time-offsets.
6. The method of claim 1 wherein estimating a correlation of the operational signal comprises:
sampling the operational signal at a plurality of first sample times to form a plurality of first samples;
sampling the operational signal at a plurality of second sample times to form a plurality of corresponding second samples wherein each corresponding second sample time is delayed by the time-offset from each first sample time;
multiplying each of the plurality of first samples by the corresponding second samples to form a plurality of products; and
summing the plurality of products to form the correlation.
7. The method of claim 1 wherein determining a characteristic of the signal path further comprises determining a length of the signal path based on a reflection corresponding to an end of the signal path.
8. The method of claim 1 wherein determining a characteristic of the signal path further comprises comparing the correlation to a baseline correlation.
9. The method of claim 1 further comprising frequency-shifting a spectrum of the operational signal to produce a frequency-shifted signal and wherein estimating the correlation is estimated from the frequency-shifted signal.
10. The method of claim 1 further comprising modifying the operational signal to improve a ratio of a nominal peak of the correlation to a nominal sidelobe level of the correlation.
11. The method of claim 10 , wherein modifying the operational signal comprises adding a noise-like signal to the operational signal.
12. The method of claim 1 wherein the operational signal is distorted by reflections from an anomaly in the signal path.
13. The method of claim 12 wherein determining a characteristic of the signal path further comprises determining a distance to the anomaly based on the time-offset for which the correlation is at an extremum.
14. The method of claim 12 wherein determining a characteristic of the signal path further comprises determining an impedance of the anomaly based on an extremum of the correlation.
15. A method of testing a signal path having at least one anomaly comprising:
extracting from the signal path a forward portion of an operational signal within the signal path wherein the operational signal is present within the system during operation and is not an injected test signal;
extracting from the signal path a reflection of the operational signal introduced by the at least one anomaly;
estimating a plurality of cross-correlation values of the forward portion of the operational signal and the reflection of the operational signal corresponding to a plurality of relative time-offsets between the forward portion of the operational signal and the reflection of the operational signal wherein the cross-correlation is the expectation of the product of the forward portion of the operational signal and the reflection of the operational signal; and
finding a first extremum of the plurality of cross-correlation values corresponding to a first one of the at least one anomaly.
16. The method of claim 15 , further comprising finding a second extremum of the plurality of correlation values corresponding to a second one of the at least one anomaly.
17. A method of testing a signal path having a plurality of anomalies comprising:
extracting a portion of an operational signal from the signal path, the operational signal having distortion introduced by the plurality of anomalies wherein the operational signal is present within the system during operation and is not an injected test signal;
a. estimating an autocorrelation of the operational signal at a plurality of time offsets wherein the autocorrelation is the expectation of the product of the operational signal with a time offset version of the operational signal;
b. determining a number of peaks of the autocorrelation; and
c. calculating a number of anomalies from the number of peaks.
18. The method of claim 17 , further comprising determining a distance to one of the plurality of anomalies.
19. The method of claim 17 , wherein determining a distance to one of the plurality of anomalies comprises pattern-matching the autocorrelation with an expected distribution of peaks to determine a location of one of the plurality of anomalies.
20. A system for testing a signal path having an operational signal comprising:
an extractor configured to extract a sample of the operational signal when coupled to a signal path wherein the operational signal is present within the system during operation and is not an injected test signal;
a correlator coupled to the extractor and configured to estimate a correlation of the sample of operational signal at an at least one time-offset;
a processor coupled to the correlator and configured to determine a characteristic of the signal path from the correlation.
21. The system of claim 20 , wherein the correlator is further configured to estimate an auto-correlation of the operational signal.
22. The system of claim 20 , wherein the extractor comprises:
a first directional coupler configured to extract a sample of a forward portion of the operational signal when coupled to the signal path; and
a second directional coupler configured to extract a sample of a reflected portion of the operational signal when coupled to the signal path;
wherein the correlator is further configured to estimate a cross-correlation of the sample of the reflected portion and the sample of the forward portion of the operational signal.
23. The system of claim 20 , wherein the correlator is further configured to estimate the correlation at a plurality of time-offsets.
24. The system of claim 20 , further comprising a time-offset control coupled to the correlator and configured to vary the time-offset from a minimum value to a maximum value corresponding to a range of time-offsets for which the correlation is to be estimated.
25. The system of claim 24 , further comprising a rate control coupled to the time-offset control and configured to control a rate at which the time-offset is varied so that the rate is reduced when the correlation indicates that the current time-offset is near a delay corresponding to an extremum of the correlation.
26. The system of claim 20 , where the correlator comprises:
a variable delay element coupled to the extractor and configured to introduce a variable time delay to produce a delayed sample of the operational signal;
a multiplier coupled to the variable delay element and coupled to the extractor and configured to multiply the delayed sample by the delayed operational signal and output a multiplication product;
an integrator coupled to the multiplier and configured to integrate the multiplication product to form the correlation.
27. The system of claim 20 , further comprising a plurality of correlators configured to estimate the correlation for a plurality of time-offsets substantially simultaneously.
28. The system of claim 20 , further comprising a media interface coupled to the extractor and configured to modify an electrical property of the operational signal chosen from the group of electrical properties consisting of frequency, voltage, current, and impedance.
29. The system of claim 20 , further comprising
a memory component configured to store a baseline correlation; and
a comparator configured to compare the correlation to the baseline correlation whereby changes in the signal path can be detected.
30. The system of claim 20 , wherein the correlator comprises:
a first sample and hold coupled the extractor and configured to take a first sample of the operational signal;
a second sample and hold coupled to the extractor and configured to take a second sample of the operational signal;
a variable trigger delay connected between the first and second sample and hold and configured to cause the first sample and hold to be triggered with time-offset relative to the second sample and hold;
a multiplier connected to the first and second sample and hold and configured to form a product from the first sample and the second sample;
a third sample and hold configured to sample the product when the product is stable to produce a held product; and
an integrator configured to accept and integrate the held product to form the correlation.
31. The system of claim 20 , wherein said system is integrated into a system element chosen from the group of system elements consisting of a circuit breaker, a connector, a connector saver, a wiring integration unit, and a junction box.
32. The system of claim 20 , further comprising a noise-generator coupled to the extractor and configured to superimpose a noise-like signal on the operational signal when coupled to the signal path.
33. A system providing self-test capability comprising:
at least one signal path carrying an operational signal used by the system during operation wherein the operational signal is not an injected test signal;
at least one noise domain reflectometer coupled to the at least one signal path adapted to monitor the signal path during operation of the system wherein the noise domain reflectometer comprises:
a correlator coupled to the signal path and configured to estimate a correlation of the operational signal;
a processor coupled to the correlator and configured to determine a characteristic of the signal path.
34. The system of claim 33 , further comprising an indicator coupled to the processor and configured to provide an indication when an anomaly is present.
35. The system of claim 33 , wherein the processor is coupled to the system in a configuration such that operation of the system is modified when the anomaly is present.