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.

A quadrature ΣΔ analog-to-digital converter (ADC) is a promising solution for intermediate frequency digitizing software defined cognitive radio (CR) receivers because of, e.g., multiband capability and power efficiency. However, inherent coefficient mismatches between the in-phase and quadrature rails can severely damage the performance of such receiver by creating mirror-frequency interference (MFI). In this article, a novel frequency-agile and reconfigurable transfer function design, allowing digital post-compensation of the MFI, is proposed. The design is based on a novel closed-form transfer function model for higher-order quadrature ΣΔ modulators (QΣΔMs) under implementation inaccuracies, proposed herein. By doing the compensation in digital domain, it is possible to take into account all error-sources of the receiver chain at once, including, e.g., a quadrature mixer before the ADC. This capability is obtained by preserving the mirror-band signal information and using the noise transfer function of a QΣΔM to remove quantization noise from therein. This is demonstrated in a multiband scenario aimed for CR receivers, where a number of frequency channels can be received and detected in parallel. Practical examples of the transfer function analysis under implementation inaccuracies and the post-compensation performance are given with a two-stage QΣΔM, having stage-orders of four, allowing eighth-order noise shaping.