| 摘要 |
28 A continuous-time tunable Gm-C architecture for a .DELTA..SIGMA. modulator 10 includes a tunable resonator 18 and a low bit, high sample rate quantizer 22 connected in a feedback loop 27. The resonator shapes the quantization noise spectrum so that the bulk of the quantization noise occurs outside the signal spectrum. A tunable Gm cell 30 tunes the resonator's resonant frequency to the signal spectrum's carrier frequency to maximize the modulator's SNR. The tunable Gm cell includes a fixed Gm cell 80 having transconductance Gf, a current divider 82, and a recombination circuit 83 that together effectively multiply Gf by a factor .alpha., where -1 <= .alpha. <= 1, without effecting the cell's common mode current Icm. A positive current source (21a, 21b) supplies Icm, while maintaining a common mode resistance of R/2 and a theoretically infinite differential mode resistance. Thus, the resonator's resonant frequency can be varied from DC to approximately 1 Ghz while maintaining a stable common mode operating point and improving the modulator's quality factor.
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