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One-bit sigma-delta modulator with improved signal stability
| Details |
Inventors: White, Stanley A.;
Assignee: Rockwell International Corporation (Seal Beach, CA)
Primary Examiner: Hoff; Marc
Assistant Examiner:
Attorney, Agent or Firm: Hamann; H. Fredrick, Montanye; G. A., Streeter; Tom
The one-bit sigma-delta modulator with improved signal stability disclosed herein includes, in order, a first anti-aliasing filter (20), a chopper (22), a second anti-aliasing filter (30), and a one-bit modulator (32). The chopper (22) and one-bit modulator (32) are driven by a clock generator (38). The chopper (22) is introduced to provide a carrier upon which the signal may ride, thereby eliminating dc stability problems. The first anti-aliasing filter (20) eliminates aliasing from the chopper (22), and the second anti-aliasing filter (30) eliminates aliasing from the one-bit modulator (32). The second anti-aliasing filter (30) may be eliminated if the frequency of the chopper (22) is an exact integer submultiple of the sampling frequency of the one-bit modulator (32). This invention may drive a decimation filter (36), also driven by the clock generator (38), to provide a low cost analog-to-digital converter. |
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DETAILED DESCRIPTION Applicant avoids this problem by placing the signal on a low-frequency carrier.
This placement is accomplished by modulating the signal before the one-bit modulator, and then demodulating the signal within the decimation chain.
The purposes of the device herein described are to: 1.
modulate a baseband signal onto a carrier to avoid offset problems, 2.
perform low-cost (because of the lack of offset problems at the carrier frequency) analog-to-digital conversion; and, 3.
perform digital demodulation (again, no offset problems, but this time because the demodulation and subsequent processing is digital).
The low-cost modulation approach is simple square-wave modulation, or "chopping.
" The means to avoid undesirable aliasing, or foldover distortion, caused by intermodulation products, is the use of the proper filtering in order to avoid images of either signal or noise from folding over onto the signal passband.
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Radio 
