Method and apparatus for increasing bandwidth in sampled systems

Abstract

Systems that sample a continuous analog time-domain signal.The invention is particularly applicable to systems that sample a continuous analog time-domain signal and whose analog components have a bandwidth limit below that desired or specified. The invention has been created to address particular problems in the design of digital sampling oscilloscopes (DSOs) that require more bandwidth than that which is easily achievable through traditionally analog techniques. A method and apparatus are provided in the form of a digital filter that is capable of surgically increasing the bandwidth of the system beyond the bandwidth achievable in an analog system. Furthermore, it is demonstrated that this system can perform this bandwidth increase without degradation in the time-domain performance of the system such as pulse or step response. In some cases, the time-domain performance is improved by flattening of the frequency response. Additionally, the system, while boosting the bandwidth, is capable of simultaneously removing noise and therefore producing a digitized signal of higher fidelity than that obtained without the filter in place.

1. A digital filter for increasing the bandwidth of a sampled system, comprising:
 a first response portion providing no adjustment to a signal of the sampled system; 
 a second response portion providing a gain rising substantially log-linearly to said signal of the sampled system; 
 a third response portion providing a substantially steady gain to said signal of the sampled system; 
 a fourth response portion providing a gain dropping substantially log-linearly to said signal of the sampled signal; and 
 a fifth response portion providing predetermined attenuation to said signal of said sampled system. 
 
     
     
       2. The digital filter of  claim 1 , wherein said bandwidth of said sampled system is increased to approximately 2 GHz. 
     
     
       3. The digital filter of  claim 1 , wherein said first response portion extends substantially from 0 to 1.8 GHz. 
     
     
       4. The digital filter of  claim 1 , wherein said second response portion extends from approximately 1.8 GHz to 2.0 GHz. 
     
     
       5. The digital filter of  claim 1 , wherein said fifth response portion extends substantially above 3 GHz. 
     
     
       6. The digital filter of  claim 5 , wherein said predetermined attenuation of said fifth response portion is −5 dB. 
     
     
       7. The digital filter of  claim 1 , wherein said third and fourth response portions are unspecified. 
     
     
       8. A digital filter for increasing the bandwidth of a sampled system, comprising:
 a first response portion providing no adjustment to a signal of the sampled system; 
 a second response portion providing a gain rising substantially log-linearly to said signal of the sampled system; and 
 a third response portion providing a predetermined attenuation to said signal of said sampled system. 
 
     
     
       9. The digital filter of  claim 8 , wherein said digital filter further flattens the frequency response of said sampled system. 
     
     
       10. The digital filter of  claim 8 , wherein said digital filter further reduces out-of-band noise. 
     
     
       11. The digital filter of  claim 8 , wherein said digital filter controls a pulse/step response of the sampled system. 
     
     
       12. The digital filter of  claim 11 , comprising multiple poles and zeros. 
     
     
       13. The digital filter of  claim 11 , wherein the magnitude in the second response portion is specified as the inverse frequency response of the original system being boosted. 
     
     
       14. The digital filter of  claim 8 , wherein the digital difference equation is implemented as biquad sections or any other filter topology. 
     
     
       15. The digital filter of  claim 8 , wherein said digital filter is represented as a Finite Impulse Response (FIR) filter, either by sampling the impulse response, or any other similar method. 
     
     
       16. A digital filter for use with a digital sampling oscilloscope, comprising:
 a first response portion providing no adjustment to a signal of the sampled system; 
 a second response portion providing a gain rising substantially log-linearly to said signal of the sampled system; and 
 a third response portion providing a predetermined attenuation to said signal of said sampled system. 
 
     
     
       17. The digital filter of  claim 16 , wherein said digital filter is implemented in hardware realizations including FPGAs, Gate Arrays and IP Cores. 
     
     
       18. The digital filter of  claim 16 , wherein a stabilizing zero is removed. 
     
     
       19. A method for increasing the bandwidth of a digital oscilloscope, comprising:
 providing the digital oscilloscope having a frequency response with an upper bandwidth limit; and   applying a digital filter to the digital oscilloscope frequency response to obtain a filtered frequency response, the digital filter having a frequency response in a frequency range proximate the upper bandwidth limit, the frequency response of the digital filter comprising:
 a first response portion providing rising gain in a first frequency range, 
 a second response portion at a second frequency range higher than said first frequency range, said second response portion providing substantially constant gain, and 
 a third response portion at a third frequency range higher than said second frequency range, said third response portion providing decreasing gain; 
 wherein the filtered frequency response has an increased upper bandwidth limit. 
   
     
     
       20. The method of claim 19, further comprising generating filter coefficients for the digital filter based at least upon the inverse of the digital oscilloscope frequency response proximate the upper bandwidth limit. 
     
     
       21. The method of claim 19, wherein the digital filter is a finite impulse response filter. 
     
     
       22. The method of claim 19, wherein the digital filter is combined with one or more other digital filters to form a composite digital filter. 
     
     
       23. The method of claim 19, wherein the digital filter further comprises a fourth response portion at a fourth frequency range higher than said third frequency range, said fourth response portion providing substantially constant magnitude response. 
     
     
       24. The method of claim 23, wherein the substantially constant magnitude response is attenuating. 
     
     
       25. The method of claim 19, wherein the digital filter provides substantially no gain or attenuation below the frequency range of the first response portion. 
     
     
       26. The method of claim 19, wherein a magnitude response of the digital filter is the substantial inverse of the digital oscilloscope amplitude response at one or more frequencies proximate the upper bandwidth limit. 
     
     
       27. The method of claim 19, wherein the gain in the first response portion rises substantially log-linearly. 
     
     
       28. The method of claim 19, wherein the second response portion includes a first region in which the gain rises and a second region in which the gain falls. 
     
     
       29. The method of claim 19, wherein the third response portion includes a first region that provides gain and a second region that provides attenuation. 
     
     
       30. The method of claim 19, wherein the step response of the digital oscilloscope after application of the digital filter is substantially consistent with the upper bandwidth limit of the filtered frequency response. 
     
     
       31. The method of claim 19, wherein the digital filter attenuates out-of-band noise. 
     
     
       32. The method of claim 19, wherein the digital filter substantially flattens the frequency response of the digital oscilloscope below the increased upper bandwidth limit. 
     
     
       33. The method of claim 19, further comprising the step of providing one or more samples to the digital filter in addition to samples corresponding to a waveform to be displayed. 
     
     
       34. The method of claim 33, wherein the additional samples allow the digital filter to substantially settle prior to filtration of the samples corresponding to a waveform to be displayed. 
     
     
       35. The method of claim 19, further comprising the step of providing samples to the digital filter in addition to samples representing a waveform to be displayed, wherein the additional samples are excluded from one or more processing steps after application of the digital filter. 
     
     
       36. The method of claim 35, further comprising the step of excluding these additional samples from further processing after filtering. 
     
     
       37. A digital oscilloscope having an upper bandwidth limit, said oscilloscope comprising:
 a digital filter applied proximate the upper bandwidth limit, said filter having a frequency response in a frequency range proximate the upper bandwidth limit, the frequency response of the digital filter comprising:   a first response portion providing rising gain in a first frequency range,   a second response portion at a second frequency range higher than said first frequency range, said second response portion providing substantially constant gain, and   a third response portion at a third frequency range higher than said second frequency range, said third response portion providing decreasing gain.   
     
     
       38. The oscilloscope of claim 37, wherein the magnitude response or phase response of the digital filter is the substantial inverse of an unboosted frequency response of the oscilloscope proximate the upper bandwidth limit. 
     
     
       39. The oscilloscope of claim 37, wherein the digital filter is a finite impulse response filter. 
     
     
       40. The oscilloscope of claim 37, wherein the digital filter is part of a composite filter having additional frequency response portions. 
     
     
       41. The oscilloscope of claim 37, wherein the digital filter further comprises a fourth response portion at a fourth frequency range higher than said third frequency range, said fourth response portion providing substantially constant magnitude response. 
     
     
       42. The oscilloscope of claim 41, wherein the substantially constant magnitude response is attenuating. 
     
     
       43. The oscilloscope of claim 37, wherein the digital filter provides substantially no gain or attenuation below the frequency range of the first response portion. 
     
     
       44. The oscilloscope of claim 37, wherein a magnitude response of the digital filter is the substantial inverse of an unboosted frequency response of the oscilloscope proximate the upper bandwidth limit. 
     
     
       45. The oscilloscope of claim 37, wherein the gain in the first response portion rises substantially log-linearly. 
     
     
       46. The oscilloscope of claim 37, wherein the second response portion includes a first region in which the gain rises and a second region in which the gain falls. 
     
     
       47. The oscilloscope of claim 37, wherein the third response portion includes a first region that provides gain and a second region that provides attenuation. 
     
     
       48. The oscilloscope of claim 37, wherein digital filter does not substantially degrade the step response of the oscilloscope. 
     
     
       49. The oscilloscope of claim 37, wherein the digital filter attenuates out-of-band noise. 
     
     
       50. The oscilloscope of claim 37, wherein the digital filter substantially flattens the frequency response of the oscilloscope below the upper bandwidth limit. 
     
     
       51. The oscilloscope of claim 37, further comprising an acquisition system to provide one or more samples to the digital filter in addition to samples corresponding to a waveform to be displayed. 
     
     
       52. The oscilloscope of claim 51, wherein the additional samples allow the digital filter to substantially settle prior to filtration of the samples corresponding to a waveform to be displayed. 
     
     
       53. The oscilloscope of claim 51, wherein the additional samples are excluded from one or more processing steps after application of the digital filter.