Digital mirror-frequency interference compensation for multiband quadrature sigma-delta ADC based cognitive radio receivers

This article proposes a novel frequency-agile transfer function design for quadrature ΣΔ modulators (QΣΔMs), allowing digital post-compensation of mirror-frequency interference generated by the mismatches between the receiver I and Q branches. In addition to mitigating the interference created inside the QΣΔM, doing the compensation in digital domain allows taking into account the implementation inaccuracies of the preceding receiver front-end components, such as a quadrature mixer. This is made possible by using the signal and noise transfer functions to maintain the mirror-band signal information in the digital domain with sufficient signal-to-noise ratio. The performance of the transfer function design and digital compensation are illustrated in a multi-band scenario aimed for cognitive radio reception. Practical examples are given with eighth order noise shaping, allowed by a two-stage QΣΔM, having stage-orders of four.