Tests for non-linear distortion using digital signal processing

Abstract

A system for determining a composite signal level at which a signal path begins to generate non-linear distortion. A uses a reference test signal, which is preferably a short-duration burst of repeatable broadband energy, that is passed through the signal path and received on a digital signal acquisition unit. An impaired received reference test signal is comprised of is formed from the transmitted reference test signal, linear distortion components, and non-linear distortion components. The impaired received reference test signal is digitally processed to reveal the non-linear distortion components. The impaired received reference test signal may be processed with a stored reference test signal to find a time-domain impulse response from which the uncorrelated distortion energy can be measured. Alternately, a reference test signal, such as an orthogonal frequency division multiplex (OFDM) reference signal with spectral holes, can be processed in the frequency domain to find the non-linear distortion energy that enters the spectral holes. Alternately, a transfer function of a signal path, showing an output voltage as a function of an input voltage, can be generated from a two-burst waveform comprised of a clipping high-level sinewave and non-clipping low-level sinewave. As the reference test signals are elevated in level, the magnitude of the non-linear distortion products can typically be observed to increase.

1. A test system for measuring a non-linear distortion created by a reference test signal transported on a signal path comprising:
 a transmitter for transmitting the reference test signal;  
 a receiver, for receiving an impaired reference test signal, wherein the impaired reference test signal comprises the reference test signal as impaired while passing between the transmitter and receiver through the signal path;  
 a digital signal processor connected to an output of the receiver to receive the impaired reference test signal from the receiver, the digital signal processor: 
 storing a copy of the reference test signal;  
 computing an impulse response of the signal path using the copy of the reference test signal and the impaired reference test signal;  
 calculating the energy in the impulse response during time periods of the impulse response that contain primarily non-linear distortion energy.  
 
 
     
     
       2. A test system according to  claim 1  wherein the stored copy of the reference test signal contains a linear distortion created by the signal path. 
     
     
       3. A test system according to  claim 1  wherein the transmitter sends additional reference test signals at multiple power levels and
 further wherein the digital signal processor determines the non-linear distortion energy for each power level.  
 
     
     
       4. A test system according to  claim 1  wherein the reference test signal comprises a sequence of single burst reference test signals, each having a different power level. 
     
     
       5. A test system according to  claim 1  wherein the receiver further captures background noise within the signal path and
 further wherein the digital signal processor measures the energy contained within the background noise and  
 still further wherein the digital signal processor determines the level of non-linear distortion as compared to the background noise.  
 
     
     
       6. A test system according to  claim 1  wherein the digital signal processor averages the results of a plurality of repeated tests to reduce the effect of background noise. 
     
     
       7. A test system according to  claim 1  wherein the reference test signal comprises either of a quadratic chirp, a Koo pulse, a pseudonoise sequence, stepped-frequency waveform, or an OFDM signal with flat spectral energy. 
     
     
       8. A test system according to  claim 1  wherein the transmitter is located in a home terminal device and the receiver is located in a headend or a hub site. 
     
     
       9. A test system according to  claim 1  wherein the digital signal processor determines the impulse response by performing a frequency-domain division of the reference test signal into the impaired reference test signal, followed by inverse Fourier transform. 
     
     
       10. A test system according to  claim 1  wherein the digital signal processor determines the impulse response by performing a convolution in a time domain. 
     
     
       11. A test system as per  claim 1 , the digital signal processor further:
 calculating one of: a total energy in the impulse response, or the energy in the impulse response during time periods of the impulse response that contain primarily signal energy correlated to the reference test signal;  
 calculating a ratio based on the energy during time periods that contain primarily non-linear distortion energy and one of: the total energy in the impulse response, or the energy during time periods that contain primarily signal energy.  
 
     
     
       12. A test system for measuring a non-linear distortion created by a signal path comprising:
 a transmitter for transmitting a reference test signal wherein the reference test signal comprises an OFDM reference test signal having at least one spectral hole;  
 a signal path;  
 a receiver for receiving an impaired reference test signal comprising the reference test signal as impaired while passing between the transmitter and receiver through the signal path;  
 a digital signal processor connected to an output of the receiver for measuring a total amount of energy within the impaired reference test signal and for measuring an amount of energy within the at least one spectral hole;  
 wherein the non-linear distortion energy is determined from the energy contained within the at least one spectral hole.  
 
     
     
       13. A test system according to  claim 12  wherein the digital signal processor averages the results of several repeated tests to reduce the effect of background noise. 
     
     
       14. A test system for determining a transfer function of a signal path comprising:
 a transmitter for transmitting a two-burst waveform comprised of a low-level sinewave part and a high-level sinewave part;  
 a signal path;  
 a receiver for receiving the two-burst waveform as impaired while passing between the transmitter and receiver through the signal path; and  
 a digital signal processor connected to an output of the receiver for determining a non-linear transfer function of the signal path by plotting the time samples of the low-level sinewave on a first axis and delayed time samples of the high-level sinewave on a second axis.  
 
     
     
       15. A test system according to  claim 14  wherein a set of samples comprising the transfer function is averaged to reduce noise. 
     
     
       16. A test system according to  claim 14  wherein the coefficients of the Taylor series expansion is determined from the transfer function. 
     
     
       17. A test system according to  claim 14  wherein the linear distortion created after the creation of a non-linear distortion is characterized. 
     
     
       18. A test system according to  claim 17  wherein the linear distortion created after the creation of the non-linear distortion is removed. 
     
     
       19. A system for measuring a non- linear distortion created by a signal path, the system comprising:      a receiver configured to receive an impaired reference test signal, wherein the impaired reference test signal comprises a reference test signal as impaired while traversing a signal path; and        a digital signal processor connected to receive the impaired reference test signal, wherein the digital signal processor is configured to:      compute an impulse response of the signal path using a copy of the reference test signal and the impaired reference test signal; and        calculate the energy in the impulse response during time periods of the impulse response that contain primarily non - linear distortion energy.       
     
     
       20. The system according to  claim 19 , wherein the digital signal processor is configured to store a previously received reference test signal as the copy of the reference test signal. 
     
     
       21. The system according to  claim 19 , further comprising a transmitter for transmitting the reference test signal. 
     
     
       22. The system according to  claim 21 , wherein the transmitter sends additional reference test signals at multiple power levels and further wherein the digital signal processor determines the non- linear distortion energy for each power level.   
     
     
       23. The system according to  claim 21 , wherein the transmitter is located in a home terminal device and the receiver is located in a headend or a hub site. 
     
     
       24. The system according to  claim 19 , wherein the reference test signal comprises a sequence of single burst reference test signals, each having a different power level. 
     
     
       25. The system according to  claim 19 , wherein the receiver is further configured to capture background noise within the signal path, and wherein the digital signal processor is configured to measure the energy contained within the background noise and determine the level of non- linear distortion as compared to the background noise.   
     
     
       26. The system according to  claim 19 , wherein the digital signal processor is configured to average the results of a plurality of repeated tests to reduce the effect of background noise. 
     
     
       27. The system according to  claim 19 , wherein the reference test signal comprises either of a quadratic chirp, a Koo pulse, a pseudonoise sequence, stepped- frequency waveform, or an OFDM signal with flat spectral energy.   
     
     
       28. The system according to  claim 19 , wherein the digital signal processor is configured to determine the impulse response by performing a frequency- domain division of the reference test signal into the impaired reference test signal, followed by inverse Fourier transform.   
     
     
       29. The system according to  claim 19 , wherein the digital signal processor is configured to determine the impulse response by performing a convolution in a time domain. 
     
     
       30. The system as per  claim 19 , wherein the digital signal processor is further configured to:
   calculate one of:    ( i )  a total energy in the impulse response, or      ( ii )  the energy in the impulse response during time periods of the impulse response that contain primarily signal energy correlated to the reference test signal;          calculate a ratio based on the energy during time periods that contain primarily non - linear distortion energy and one of:    ( i )  the total energy in the impulse response, or      ( ii )  the energy during time periods that contain primarily signal energy.       
     
     
       31. A system for measuring a non- linear distortion created by a signal path comprising:      a receiver for receiving an impaired reference test signal comprising a reference test signal as impaired while traversing a signal path, wherein the reference test signal comprises an OFDM reference test signal having at least one spectral hole; and        a digital signal processor configured to determine the non - linear distortion created by the signal path from energy contained within the at least one spectral hole.     
     
     
       32. The system according to  claim 31 , wherein the digital signal processor averages the results of several repeated tests to reduce the effect of background noise. 
     
     
       33. The system according to  claim 31 , further comprising a transmitter for transmitting the reference test signal to the receiver via the signal path. 
     
     
       34. A system for determining a transfer function of a signal path comprising:
   a receiver for receiving a two - burst waveform as impaired while traversing a signal path, wherein the two - burst waveform comprises a low - level sinewave part and a high - level sinewave part; and        a digital signal processor configured to determine a non - linear transfer function of the signal path by plotting the time samples of the low - level sinewave on a first axis and delayed time samples of the high - level sinewave on a second axis.     
     
     
       35. The system according to  claim 34 , wherein a set of samples comprising the transfer function is averaged to reduce noise. 
     
     
       36. The system according to  claim 34 , wherein the coefficients of the Taylor series expansion is determined from the transfer function. 
     
     
       37. The system according to  claim 34 , wherein the linear distortion created after the creation of a non- linear distortion is characterized.   
     
     
       38. The system according to  claim 37 , wherein the linear distortion created after the creation of the non- linear distortion is removed.   
     
     
       39. The system according to  claim 34 , further comprising a transmitter for transmitting the two- burst waveform to the receiver via the signal path.   
     
     
       40. A method for measuring non- linear distortion created by a signal path, the method comprising:      receiving an impaired reference test signal, wherein the impaired reference test signal comprises a reference test signal as impaired while traversing a signal path;        computing an impulse response of the signal path using a copy of the reference test signal and the impaired reference test signal; and        calculating the energy in the impulse response during time periods of the impulse response that contain primarily non - linear distortion energy.     
     
     
       41. The method according to  claim 40 , further comprising storing a previously received reference test signal as the copy of the reference test signal. 
     
     
       42. The method according to  claim 40 , further comprising transmitting the reference test signal via the signal path. 
     
     
       43. The method according to  claim 42 , further comprising:
   sending additional reference test signals at multiple power levels; and        determining the non - linear distortion energy for each power level.     
     
     
       44. The method according to  claim 42 , wherein transmitting the reference test signal comprises transmitting the reference test signal from a home terminal device to a headend or a hub site. 
     
     
       45. The method according to  claim 40 , wherein the reference test signal comprises a sequence of single burst reference test signals, each having a different power level. 
     
     
       46. The method according to  claim 40 , further comprising:
   capturing background noise within the signal path;        measuring the energy contained within the background noise; and        determining the level of non - linear distortion as compared to the background noise.     
     
     
       47. The method according to  claim 40 , further comprising averaging the results of a plurality of repeated tests to reduce the effect of background noise. 
     
     
       48. The method according to  claim 40 , wherein the reference test signal comprises either of a quadratic chirp, a Koo pulse, a pseudonoise sequence, stepped- frequency waveform, or an OFDM signal with flat spectral energy.   
     
     
       49. The method according to  claim 40 , further comprising determining the impulse response by performing a frequency- domain division of the reference test signal into the impaired reference test signal, followed by inverse Fourier transform.   
     
     
       50. The method according to  claim 40 , further comprising determining the impulse response by performing a convolution in a time domain. 
     
     
       51. The method as per  claim 40 , further comprising:
   calculating one of    ( i )  a total energy in the impulse response, or      ( ii )  the energy in the impulse response during time periods of the impulse response that contain primarily signal energy correlated to the reference test signal;          calculating a ratio based on the energy during time periods that contain primarily non - linear distortion energy and one of:    ( i )  the total energy in the impulse response, or      ( ii )  the energy during time periods that contain primarily signal energy.       
     
     
       52. A method for measuring non- linear distortion created by a signal path, the method comprising:      receiving an impaired reference test signal comprising a reference test signal as impaired while traversing a signal path, wherein the reference test signal comprises an OFDM reference test signal having at least one spectral hole;        determining the non - linear distortion created by the signal path from energy contained within the at least one spectral hole.     
     
     
       53. The method according to  claim 52 , further comprising averaging the results of several repeated tests to reduce the effect of background noise. 
     
     
       54. The method according to  claim 52 , further comprising transmitting the reference test signal via the signal path. 
     
     
       55. A method for determining a transfer function of a signal path comprising:
   receiving a two - burst waveform as impaired while traversing a signal path, wherein the two - burst waveform comprises a low - level sinewave part and a high - level sinewave part; and        determining a non - linear transfer function of the signal path by plotting the time samples of the low - level sinewave on a first axis and delayed time samples of the high - level sinewave on a second axis.     
     
     
       56. The method according to  claim 55 , further comprising a set of samples comprising the transfer function to reduce noise. 
     
     
       57. The method according to  claim 55 , further comprising determining the coefficients of the Taylor series expansion from the transfer function. 
     
     
       58. The method according to  claim 55 , further comprising characterizing the linear distortion created after the creation of a non- linear distortion.   
     
     
       59. The method according to  claim 58 , further comprising removing the linear distortion created after the creation of the non- linear distortion.   
     
     
       60. The method according to  claim 55 , further comprising transmitting: the two- burst waveform via a signal path.   
     
     
       61. A device for measuring non- linear distortion created by a signal path, the device comprising:      receiving circuitry configured to receive an impaired reference test signal, wherein the impaired reference test signal comprises a reference test signal as impaired while traversing a signal path; and        signal processing circuitry connected to receive the impaired reference test signal, wherein the signal processing circuitry is configured to:      compute an impulse response of the signal path using: a copy of the reference test signal and the impaired reference test signal; and        calculate the energy in the impulse response during time periods of the impulse response that contain primarily non - linear distortion energy.       
     
     
       62. The device according to  claim 61 , wherein the signal processing circuitry is configured to store a previously received reference test signal as the copy of the reference test signal. 
     
     
       63. The device according to  claim 61 , wherein the signal processing circuitry comprises a digital signal processing; integrated circuit, a personal computer, a workstation, or an embedded processor. 
     
     
       64. The device according to  claim 63 , wherein the receiving circuitry is configured to receive additional reference test signals at multiple power levels and the signal processing circuitry is configured to determine the non- linear distortion energy for each power level.   
     
     
       65. The device according to  claim 61 , wherein the reference test signal comprises a sequence of single burst reference test signals, each having a different power level. 
     
     
       66. The device according to  claim 61 , wherein the receiving circuitry is further configured to capture background noise within the signal path, and wherein the signal processing circuitry is configured to measure the energy contained within the background noise and determine the level of non- linear distortion as compared to the background noise.   
     
     
       67. The device according to  claim 61 , wherein the signal processing circuitry is configured to average the results of a plurality of repeated tests to reduce the effect of background noise. 
     
     
       68. The device according to  claim 61 , wherein the reference test signal comprises either of a quadratic chirp, a Koo pulse, a pseudonoise sequence, stepped- frequency waveform, or an OFDM signal with flat spectral energy.   
     
     
       69. The device according to  claim 61 , wherein the processing circuitry is configured to perform a frequency- domain division of the reference test signal into the impaired reference test signal, followed by inverse Fourier transform, in order to determine the impulse response.   
     
     
       70. The device according to  claim 61 , wherein the processing circuitry is configured to perform a convolution in a time domain in order to determine the impulse response. 
     
     
       71. The device as per  claim 61 , wherein the signal processing circuitry is further configured to:
   calculate one of:    ( i )  a total energy in the impulse response, or      ( ii )  the energy in the impulse response during time periods of the impulse response that contain primarily signal energy correlated to the reference test signal;          calculate a ratio based on the energy during time periods that contain primarily non - linear distortion energy and one of:    ( i )  the total energy in the impulse response, or      ( ii )  the energy during time periods that contain primarily signal energy.       
     
     
       72. A device for measuring non- linear distortion created by a signal path, the device comprising:      receiving circuitry configured to receive an impaired reference test signal comprising a reference test signal as impaired while traversing a signal path, wherein the reference test signal comprises an OFDM reference test signal having at least one spectral hole; and        signal processing circuitry configured to determine non - linear distortion created by the signal path from energy contained within the at least one spectral hole.     
     
     
       73. The device according to  claim 72 , wherein the signal processing circuitry is configured to average the results of several repeated tests to reduce the effect of background noise. 
     
     
       74. A device for determining a transfer function of a signal path, the device comprising:
   receiving circuitry configured to receive a two - burst waveform as impaired while traversing a signal path, wherein the two - burst waveform comprises a low - level sinewave part and a high - level sinewave part; and        signal processing circuitry configured to determine a non - linear transfer function of the signal path by plotting the time samples of the low - level sinewave on a first axis and delayed time samples of the high - level sinewave on a second axis.     
     
     
       75. The device according to  claim 74 , wherein the signal processing circuitry is configured to average a set of samples comprising the transfer function in order to reduce noise. 
     
     
       76. The device according to  claim 74 , wherein the signal processing circuitry is configured to determine the coefficients of the Taylor series expansion from the transfer function. 
     
     
       77. The device according to  claim 74 , wherein the signal processing circuitry is configured to characterize the linear distortion created after the creation of a non- linear distortion.   
     
     
       78. The device according to  claim 77 , wherein the signal processing circuitry is configured to remove the linear distortion created after the creation of the non- linear distortion.