A 400-MHZ 4th-order CT bandpass /spl Sigma//spl Delta/ modulator with automatic frequency tuning

This paper presents the design and implementation of a high-speed continuous-time (CT) bandpass sigma-delta modulator targeted for an intermediate frequency (IF) receiver with an IF of 400 MHz for GSM/WCDMA application. The loop filters in the modulator is implemented with Gm-C Opamp integrators in biquad configuration. An automatic frequency tuning circuit is employed to reduce the effects of process and temperature variations on the centre frequency. Based on a commercial 0.35-/spl mu/m SiGe BiCMOS process, the 4th-order bandpass modulator is able to obtain maximum SNRs of 77 dB and 62 dB for the signal bands of 200 kHz and 3.84 MHz, respectively, when sampled at 1.6 GHz.