| 摘要 |
A modulator (10) having a continuous-time filter (12) receives an analog input signal (14) and a negative feedback signal (26) from a digital-to-analog converter (18). A desired portion of the input analog signal (14) has a bandwidth centered at a nonzero frequency, FO. The filter has a high gain at the frequency, FS. A digitizer (16) is coupled to the filter (12) and operates at a sampling frequency fS, where the ratio fO/fS is greater than 0.5. With such an arrangement, because the modulator (10) has null frequencies above and below, FS/2, placement of the modulator (10) passband at one of the upper null frequencies enables undersampling, FS<2FO. For example, with the modulator (16) passband at 3FS/4, instead of FS/4, the sampling rate may be reduced by a factor of 3. Thus, for an analog input signal having a frequency FO=225MHz, FS is now only 300 MHz.
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