A 60-dB image rejection filter using Δ-Σ modulation and frequency shifting

Abstract

We propose a novel image rejection scheme for a low-IF (low intermediate frequency) receiver. A Δ-Σ modulator converts I/Q signals to digital values, and then they are digitally processed. The Δ-Σ modulator is a second-order complex band-pass type; the proposed architecture is suitable for various multi-channel communications and/or cognitive radio. As the first step in the digital signal processing, a spectrum is shifted so that the desired signal band is centered at 0 Hz. Next, by LPFs (low-pass filters), an image signal and the quantization noise of the Δ-Σ modulator are removed. These LPFs also function as a decimation filter; thus a dedicated decimation filter is not needed, and an extra area and power for it are saved. The test chip occupies 0.75 mm2 in a 180-nm mixed-signal process. The power is 6.0 mW at 1.8 V. The IRR (image rejection ratio) achieves 60 dB.