A Tracking Quantizer for Continuous Time Quadrature Bandpass Sigma-Delta Modulators

A quadrature bandpass sigma-delta modulator with a tracking quantizer consisting of only two comparators for each signal path is presented. It is designed for a low IF narrow band receiver front end. Two comparators follow the input signal with a resistive D/A converter which supplies a variable and stable reference voltage. The frequency limitations introduced by this quantizer concept are discussed and solutions are presented. Additionally, the fragile loop stability of a tracking quantizer based modulator is enhanced by a fast digital control based on selection of precomputed A/D converter sampling values. For a highly linear feedback a non-return-to-zero current steering D/A converter is controlled by a low complexity dynamic element matching algorithm. The modulator has been designed and layouted in a 130 nm RF CMOS technology occupying an area of 0.87 mm
²
. System level simulations show 86 dB SNR at 1 MHz bandwidth with 96 MHz clock frequency. Furthermore, a low power consumption of 3.8 mW at 1.2 V supply voltage is achieved.