Wideband Quadrature Σ∆ A/D Conversion for Cognitive Radio – Reconfigurable Design and Digital Mirror-Frequency Suppression

This article proposes a reconfigurable wideband transfer function design for quadrature Σ∆ 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, such as 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 Σ∆ modulator (QΣ∆M), 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 QΣ∆M giving eighth order noise-shaping.