Digital frequency response compensator and arbitrary response generator system

Abstract

A digital signal processing system capable of compensating for frequency response variations and generating a response characteristic that complies with a provided specification. The system automatically generates digital filters to provide this compensation with almost any form of channel frequency response information and with user defined specifications. The capability of this system to trade-off noise performance, pulse response, and frequency response flatness in order to provide an optimized response is demonstrated. The system also provides feedback to the user on the final response characteristics.

1. A signal processing system capable of compensating for the channel response characteristics of an input waveform, comprising:
 input means for inputting input specifications for specifying the design of a filter, including: 
 channel response characteristics defining the response characteristics of a channel used to acquire said input waveform; and  
 user specifications for specifying a desired frequency response and a degree of compliance to the desired frequency response;  
 
 a filter builder for generating coefficients for said filter and outputting final performance specifications, having: 
 a compensation filter generator for generating coefficients corresponding to a compensation response on the basis of the inverse of the channel response characteristics; and  
 a response filter generator for generating coefficients corresponding to a combination of an ideal response and a noise reduction response on the basis of the user specifications; and  
 
 said filter for filtering said input waveform and outputting an overall response waveform having said desired frequency response, comprising: 
 a filter coefficient cache for storing the coefficients generated by said filter builder;  
 a compensation filter portion for filtering said input waveform using the coefficients stored in said filter coefficient cache corresponding to said compensation response; and  
 a response filter portion having a response filter stage and a noise reduction stage for filtering the compensated waveform output from said compensation filter portion and outputting said overall response waveform; said response filter portion filtering using the coefficients stored in said filter coefficient cache corresponding to said combination of said ideal response and said noise reduction response.  
 
 
     
     
       2. The signal processing system according to  claim 1 , wherein said filter is implemented as an infinite impulse response (IIR) filter. 
     
     
       3. The signal processing system according to  claim 1 , wherein said filter is implemented as a finite impulse response (FIR) filter. 
     
     
       4. The signal processing system according to  claim 1 , wherein said channel response characteristics are predetermined based on a reference signal and the reference signal as acquired by said channel. 
     
     
       5. The signal processing system according to  claim 1 , wherein said user specifications comprise a bandwidth, a response optimization, a compensation compliance, and  or a filter implementation type. 
     
     
       6. The signal processing system according to  claim 5 , wherein said response optimization is a pulse response optimization implemented using a Besselworth filter. 
     
     
       7. The signal processing system according to  claim 5 , wherein said response optimization is a noise performance optimization implemented using a Butterworth filter. 
     
     
       8. The signal processing system according to  claim 5 , wherein said response optimization is a flatness optimization implemented using a Butterworth filter. 
     
     
       9. The signal processing system according to  claim 5 , wherein said filter implementation type is finite impulse response (FIR) or infinite impulse response (IIR). 
     
     
       10. The signal processing system according to  claim 1 , wherein said user specifications default to predetermined values. 
     
     
       11. A signal processing element for filtering an input digital waveform, comprising:
 a filter builder for generating filter coefficients on the basis of a channel frequency response and a user response characteristics; said channel frequency response being determined on the basis of a response input and a correction input;  
 an infinite impulse response (IIR) filter having an IIR input for said input digital waveform and an IIR coefficient input connected to said filter builder; said IIR filter producing an IIR filtered waveform from the input digital waveform on the basis of the filter coefficients generated by said filter builder;  
 a finite impulse response (FIR) filter having an FIR input for said input digital waveform and a FIR coefficient input connected to said filter builder; said FIR filter producing a FIR filtered waveform from the input digital waveform on the basis of the filter coefficients generated by said filter builder; and  
 an output selector switch for selecting either said FIR filtered waveform or said FIR  IIR filtered waveform for output.  
 
     
     
       12. The signal processing element according to  claim 11 , wherein said filter builder detects changes in the sampling rate of said input digital waveform that require the filter coefficients to be generated. 
     
     
       13. The signal processing element according to  claim 11 , wherein said filter builder generates filter coefficients for said FIR filter or said IIR filter on the basis of said output selector switch. 
     
     
       14. The signal processing element according to  claim 11 , wherein said filter builder has channel, compensation, shaper, and noise reduction outputs for evaluating the performance of the filtering. 
     
     
       15. The signal processing element according to  claim 11 , wherein said response input is a known input response and said correction input is a measured input response as acquired by an input channel. 
     
     
       16. The signal processing element according to  claim 11 , wherein said user response characteristics are used to generate filter coefficients corresponding to an arbitrary response portion of the filter. 
     
     
       17. The signal processing element according to  claim 11 , wherein said user response characteristics comprise a bandwidth, a response optimization, a compensation compliance, and a filter implementation type. 
     
     
       18. The signal processing element according to  claim 17 , wherein said response optimization is a pulse response optimization implemented using a Besselworth filter. 
     
     
       19. The signal processing element according to  claim 17 , wherein said response optimization is a noise performance optimization implemented using a Butterworth filter. 
     
     
       20. The signal processing element according to  claim 17 , wherein said response optimization is a flatness optimization implemented using a Butterworth filter. 
     
     
       21. The signal processing element according to  claim 17 , wherein said filter implementation type is FIR or IIR. 
     
     
       22. The signal processing element according to  claim 11 , wherein said user response characteristics default to predetermined values. 
     
     
       23. A method of filtering an input digital waveform to compensate for the response characteristics of an acquisition channel, comprising the steps of:
 generating a compensation portion of a filter on the basis of an input channel response, using the steps of: 
 pre-warping said input channel response;  
 designing an analog filter emulating the pre-warped input channel response by making an initial filter guess and iterating the coefficients of said initial filter guess to minimize a mean-squared error;  
 inverting said analog filter; and  
 digitizing the inverted analog filter to produce said compensation portion of said filter using a bilinear transformation; and  
 
 filtering said input digital waveform using said compensation portion of said filter.  
 
     
     
       24. The method according to  claim 23 , further comprising the step of generating an arbitrary response portion of said filter on the basis of an input user specifications, wherein said input digital waveform is filtered using said arbitrary response portion of said filter, thereby producing a filter digital waveform having the desired response characteristics. 
     
     
       25. The method according to  claim 24 , wherein said input user specifications comprise a bandwidth, a response optimization, a compensation compliance, and a filter implementation type. 
     
     
       26. The method according to  claim 24 , wherein said arbitrary response portion of said filter comprises a shaper and a noise reducer. 
     
     
       27. The method according to  claim 24 , wherein said input user specifications default to predetermined values. 
     
     
       28. The method according to  claim 23 , wherein said filter is implemented as an infinite impulse response (IIR) filter. 
     
     
       29. The method according to  claim 23 , wherein said filter is implemented as a finite impulse response (FIR) filter. 
     
     
       30. The signal processing system according to  claim 23 , wherein said input channel response is predetermined based on a reference signal and the reference signal as acquired by said channel. 
     
     
       31. The method according to  claim 23 , wherein said filter type is FIR or IIR. 
     
     
       32. The method according to  claim 23 , wherein the coefficients of said initial filter guess are iterated until said mean-squared error is less than a compensation compliance specified in an input user specifications . 
     
     
       33. A method for filtering an input waveform, comprising the steps of:
   determining a plurality of parameters for a digital filter to substantially flatten a frequency response of a channel of a digital oscilloscope across at least one frequency range based upon at least i )  response characteristics of the channel and ii )  a user input concerning a desired bandwidth;        building the digital filter according to the plurality of parameters;        receiving an input waveform on the channel;        converting the input waveform to a digital representation;        applying the digital filter to the digital representation; and        generating a filtered digital waveform,        wherein the channel has a nominal bandwidth and application of the digital filter yields an effective channel bandwidth substantially equal to the desired bandwidth.     
     
     
       34. The method of  claim 33 , wherein the parameters are further determined based upon a specified noise compensation on the channel. 
     
     
       35. The method of  claim 33 , wherein the digital filter improves a pulse response of the channel. 
     
     
       36. The method of  claim 33 , wherein the order of the digital filter is determined based on user input concerning at least one desired channel response characteristic. 
     
     
       37. The method of  claim 33 , wherein the order of the digital filter has a variable order. 
     
     
       38. The method of  claim 33 , wherein a frequency response of the digital filter comprises a substantial inverse of the channel response characteristics in a frequency range within the effective channel bandwidth. 
     
     
       39. The method of  claim 33 , further comprising changing the frequency scale of the channel frequency response. 
     
     
       40. The method of  claim 33 , wherein building the digital filter comprises converting a parameterized analog filter into a digital domain. 
     
     
       41. The method of  claim 33 , wherein determining the plurality of parameters comprises iteratively determining the parameters to minimize a mean- squared error between the channel frequency response and the filter frequency response.   
     
     
       42. The method of  claim 33 , wherein the digital filter comprises two or more cascaded digital filter elements. 
     
     
       43. The method of  claim 33 , wherein the digital filter comprises a Butterworth or Besselworth filter. 
     
     
       44. The method of  claim 33 , further comprising calibrating the channel without a probe coupled to the channel. 
     
     
       45. The method of  claim 33 , further comprising calibrating for the characteristics of a probe coupled to the channel according to parameters stored in non- volatile memory on the probe.   
     
     
       46. The method of  claim 33 , wherein determining the plurality of parameters comprises receiving user input through a graphical user interface. 
     
     
       47. The method of  claim 33 , wherein the digital oscilloscope comprises a digital sampling oscilloscope. 
     
     
       48. A method for filtering an input waveform, comprising the steps of:
   determining a plurality of parameters for a digital filter to flatten a frequency response of a channel of a digital oscilloscope across at least one frequency range based upon response characteristics of the channel, said channel having a nominal bandwidth and a nominal pulse response;        building the digital filter according to the plurality of parameters, the digital filter being configured to flatten the channel frequency response and improve the pulse response of the channel;        receiving an input waveform on the channel;        converting the input waveform to a digital representation;        applying the digital filter to the digital representation; and        generating a filtered digital waveform,        wherein application of the digital filter substantially flattens the channel frequency response across the frequency range and substantially improves the channel pulse response.     
     
     
       49. The method of  claim 48 , wherein the parameters are further determined based upon a specified noise compensation on the channel. 
     
     
       50. The method of  claim 48 , wherein the parameters are determined according to a user input concerning a desired bandwidth for the channel. 
     
     
       51. The method of  claim 50 , wherein application of the digital filter yields an effective channel bandwidth substantially equal to the desired bandwidth. 
     
     
       52. The method of  claim 48 , wherein the order of the digital filter is determined based on at least one user specified response characteristic. 
     
     
       53. The method of  claim 48 , wherein the order of the digital filter is variable. 
     
     
       54. The method of  claim 48 , wherein a frequency response of the digital filter comprises a substantial inverse of the channel response characteristics in a frequency range within the effective channel bandwidth. 
     
     
       55. The method of  claim 48 , further comprising changing the frequency scale of the channel frequency response. 
     
     
       56. The method of  claim 48 , wherein building the digital filter comprises converting a parameterized analog filter into a digital domain. 
     
     
       57. The method of  claim 48 , wherein determining the plurality of parameters comprises iteratively determining the parameters to minimize a mean- squared error between the channel frequency response and the filter frequency response.   
     
     
       58. The method of  claim 48 , wherein the digital filter comprises two or more cascaded digital filter elements. 
     
     
       59. The method of  claim 48 , wherein the digital filter comprises a Butterworth or Besselworth filter. 
     
     
       60. The method of  claim 48 , further comprising calibrating the channel without a probe coupled to the channel. 
     
     
       61. The method of  claim 48 , further comprising calibrating for the characteristics of a probe coupled to the channel according to parameters stored in non- volatile memory on the probe.   
     
     
       62. The method of  claim 48 , wherein determining the plurality of parameters comprises receiving user input through a graphical user interface. 
     
     
       63. The method of  claim 48 , wherein the digital oscilloscope comprises a digital sampling oscilloscope. 
     
     
       64. A digital oscilloscope comprising:
   a channel to receive an input waveform;        a converter to convert the input waveform to a digital representation; and        a digital filter to receive the digital representation, the digital filter having a frequency response to substantially flatten a frequency response of the channel across at least one frequency range based upon at least i )  response characteristics of the channel and ii )  a user input concerning a desired bandwidth;        wherein the channel has a nominal bandwidth and application of the digital filter yields an effective channel bandwidth substantially equal to the desired bandwidth.     
     
     
       65. The apparatus of  claim 64  wherein the digital filter is built according to a plurality of parameters that are determined to substantially flatten the response of the channel across the at least one frequency range. 
     
     
       66. The apparatus of  claim 65 , wherein the parameters are further determined based upon a specified noise compensation specification. 
     
     
       67. The apparatus of  claim 65 , wherein the plurality of parameters are further determined iteratively by minimizing mean- squared error between the channel frequency response and the digital filter frequency response.   
     
     
       68. The apparatus of  claim 64 , wherein the digital filter improves pulse response of the channel. 
     
     
       69. The apparatus of  claim 64 , wherein the digital filter has an order determined based on user input concerning at least one desired channel response characteristic. 
     
     
       70. The apparatus of  claim 64 , wherein the digital filter has a variable order. 
     
     
       71. The apparatus of  claim 64 , wherein the frequency response of the digital filter comprises a substantial inverse of the channel response characteristics in a frequency range within the effective channel bandwidth. 
     
     
       72. The apparatus of  claim 64 , wherein the digital filter is built by converting a parameterized analog filter into a digital domain. 
     
     
       73. The apparatus of  claim 64 , wherein the digital filter comprises two or more cascaded digital filter elements. 
     
     
       74. The apparatus of  claim 64 , wherein the digital filter comprises a Butterworth or Besselworth filter. 
     
     
       75. The apparatus of  claim 64 , wherein the channel comprises a probe. 
     
     
       76. The apparatus of  claim 75 , wherein the probe contains parameter information to build the digital filter. 
     
     
       77. The apparatus of  claim 64 , further comprising a graphical user interface for user input of parameters for building the digital filter. 
     
     
       78. The apparatus of  claim 64 , wherein the digital filter comprises an infinite impulse response filter. 
     
     
       79. The apparatus of  claim 64 , wherein the digital filter comprises a finite impulse response filter. 
     
     
       80. The apparatus of  claim 64 , wherein the digital oscilloscope comprises a digital sampling oscilloscope. 
     
     
       81. A digital oscilloscope comprising:
   a channel to receive an input waveform, the channel having a nominal bandwidth and a nominal pulse response;        a converter to convert the input waveform to a digital representation; and        a digital filter to receive the digital representation, the digital filter having a frequency response to substantially flatten a frequency response of the channel across at least one frequency range based upon response characteristics of the channel and to improve the pulse response of the channel;        wherein application of the digital filter yields i )  an improved pulse response of the channel and ii )  a substantially flattened frequency response of the channel across the at least one frequency range.     
     
     
       82. The apparatus of  claim 81 , wherein the digital filter is built according to a plurality of parameters that are determined to substantially flatten the response of the channel across the at least one frequency range. 
     
     
       83. The apparatus of  claim 82 , wherein the parameters are further determined based upon a specified noise compensation on the channel. 
     
     
       84. The apparatus of  claim 82 , wherein the parameters are determined according to a user input concerning a desired bandwidth. 
     
     
       85. The apparatus of  claim 84 , wherein application of the digital filter yields an effective channel bandwidth substantially equal to the desired bandwidth. 
     
     
       86. The apparatus of  claim 84 , wherein a frequency response of the digital filter comprises a substantial inverse of the channel response characteristics in a frequency range within the effective channel bandwidth. 
     
     
       87. The apparatus of  claim 82 , wherein the parameters are further determined based upon a specified noise compensation on the channel and according to a user input concerning a desired bandwidth. 
     
     
       88. The apparatus of  claim 81 , further comprising a graphical user interface for user input of parameters for building the digital filter. 
     
     
       89. The apparatus of  claim 81 , wherein the digital filter comprises an infinite impulse response filter. 
     
     
       90. The apparatus of  claim 81 , wherein the digital filter comprises a finite impulse response filter. 
     
     
       91. The apparatus of  claim 81 , wherein the digital oscilloscope comprises a digital sampling oscilloscope.