A quadrature bandpass ΣΔ modulator for digital radio

Abstract

In a bandpass /spl Sigma//spl Delta/ modulator, bandpass filtering and feedback around a low-resolution quantizer shape the noise spectrum, facilitating accurate A/D conversion on narrow-band input signals. This is extended to the quadrature, or complex, case if a filter - having complex-valued coefficients and thus not restricted to a symmetrical frequency response - is placed in a /spl Sigma//spl Delta/ loop. The modulator performs a complex A/D conversion on the pair of analog inputs, that are in phase-quadrature, and it has two high-speed bit stream outputs: one represents the real output; the other the imaginary output. When combined, these outputs form a signal that accurately represents the input within a narrow frequency band. This quadrature modulator is useful in architectures for a single-chip digital-radio receiver. The receiver ideally has no image response, and thus the bulky narrow-band image-rejection filter in the RF stage of a single-path receiver can be replaced with a broadband one. The architecture alleviates many problems of the direct-conversion receiver, often considered for single-chip implementations, since the input signal is mixed to a non-zero IF where 1/f noise and DC offsets cause no problem and self-interference is not an issue because oscillator frequency is offset from the carrier frequency. The quadrature modulator IC uses standard fully-differential switched-capacitor circuit techniques and is implemented in 0.8 /spl mu/m CMOS technology.