Undersampling RF-to-digital CT ΣΔ modulator with tunable notch frequency and simplified raised-cosine FIR feedback DAC

This paper presents a continuous-time fourth-order band-pass ΣΔ Modulator for digitizing radio-frequency signals in software-defined-radio mobile systems. The modulator architecture is made up of two resonators and a 16-level quantizer in the feedforward path and a raised-cosine finite-impulsive-response feedback DAC. The latter is implemented with a reduced number of filter coefficients as compared to previous approaches, which allows to increase the notch frequency programmability from 0.0375/_s to 0.25/_s, while keeping stability and robustness to circuit-element tolerances. These features are combined with undersampling techniques to achieve an efficient and robust digitization of 0.455-to-5GHz signals with scalable 8-to-15bit effective resolution within 0.2-to-30MHz signal bandwidth, with a reconfigurable 1-to-4GHz sampling frequency¹.