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This paper describes a reconfigurable analog baseband (ABB) for a software-defined radio (SDR). A wide variety of filter characteristics needed for SDR can be obtained by a reconfigurable filter based on a newly developed duty-cycle controlled discrete-time transconductor. The ABB, implemented in a 90 nm CMOS process, provides second- and fourth-order Butterworth, Chebyshev, and elliptic responses with bandwidths from 400 kHz to 30 MHz. The chip draws only 12 mA and achieves a P1dB of +7 dBm with a 1.0 V supply. The input-referred integrated in-band noise is 0.31 mVrms and the die area is as small as 0.57 mm2.