Efficient all-digital frequency synthesizer based on multi-step look-ahead sigma-delta modulation

Abstract

An all-digital frequency synthesizer system architecture with single-bit digital output generating frequency signals while shaping the quantization noise outside of the useful frequency range is presented. The system uses two identical low-pass single-bit Multi-Step Look-Ahead Sigma-Delta modulators (MSLA SDMs) in a quadrature (I-Q) configuration driven by two multi-bit digital orthogonal sinusoidal signals generated by a direct digital synthesizer (DDS). MSLA SDMs operate in the baseband and their outputs are interleaved and frequency up-converted to generate the desirable high-frequency output signal. The system architecture and the hardware complexity of it are parametrized on the number of look ahead steps, the Over Sampling Ratio (OSR) and the filter's order of the MSLA SDMs as well as on the balance between OSR, desirable frequency range span and Signal to Noise and Distortion Ratio (SNDR).