Sub-sampling sigma-delta modulator for baseband processing

A sigma-delta (SD) modulator has been developed for baseband processing in a direct conversion receiver (DCR). A second-order SD modulator with a subsampling mixer, inside the feedback loop, down-converts the incoming RF signals directly to baseband, digitizes them and attenuates noise as well as interferers. A prototype was fabricated in a TSMC 0.25 /spl mu/m process for use in a CDMA2000 transceiver. SNR greater than 53 dB was measured for single-ended baseband inputs and a SNR/sub max/ of 32 dB was measured for differential inputs at 900 MHz, over a 2 MHz bandwidth.