Wideband Quadrature Sigma-Delta A/D Conversion for Cognitive Radio - Reconfigurable Design and Digital Mirror-Frequency Suppression

Abstract

This article proposes a reconfigurable wideband transfer function design for quadrature ΣΔ A/D converters combined with digital post-compensation of frequency-selective mirror-frequency interference (MFI) induced by the inherent mismatches between the in-phase (I) and quadrature (Q) rails of the receiver. When receiving wideband waveforms, e.g., similar to LTE signals, it is crucial for the compensation performance to be able to consider also frequency variations in the image rejection. In addition to compensating the MFI originating from inside the quadrature ΣΔ modulator (QΣΔM), the digital compensation considers also the interference generated at the preceding receiver front-end components, such as an I/Q mixer. The performance of this approach is illustrated with a 20 MHz reception bandwidth, being a practical example of requirements for modern communications waveforms. This concrete example is given with a two-stage QΣΔM giving eighth order noise-shaping.

Design and implementation of quadrature bandpass sigma-delta modulator used in low-IF RF receiver

“…Quadrature band-pass sigma-delta analog to digital converters (ADC), by using a complex filter in the loop, com-bine the complex filtering idea and the real bandpass sigma-delta ADC. The effective signal bandwidth is cut in half which doubles the over-sampling-ratio (OSR) giving ideal SNR improvement [4][5][6][7][8] . Also, it is possible to choose the locations of the zeros of the noise-transfer function (NTF) to minimize errors due to quantization effects, and, finally, the following digital bandpass filter can filter out images from an imperfect preceding image-reject mixer.…”