A sixth-order continuous-time bandpass sigma-delta modulator for digital radio IF

This paper presents a sixth-order continuous-time bandpass sigma-delta modulator (SDM) for analog-to-digital conversion of intermediate-frequency signals. An important aspect in the design of this SDM is the stability analysis using the describing function method. The key to the analysis is the extension of the linear gain model for the sampled quantizer with a phase uncertainty. The single-loop, one-bit SDM is tuned at 10.7 MHz, is sampled at 40 MHz, and achieves 67-dB signal-to-(noise+distortion) ratio in 200 kHz and 80 dB in 9 kHz. The third order intermodulation is at -82 dBc for a -13-dBFS input level. The 0.5-μm CMOS chip occupies 0.9×0.4 mm² and consumes 60 mW at 3.3 V (digital) and 5.0 V (analog). The sample frequency is variable and can be set from 30 to 80 MHz