A subsampling bandpass ΣΔ modulator with lumped and distributed resonators

This paper describes a continuous time bandpass sigma delta modulator able to convert signals above the sampling frequency. The proposed architecture mitigates two of the problems associated with continuous time subsampling modulators. First, alias attenuation in the DACs is compensated by a resonator in the feedback path. In addition, the requirement of resonators whose Q factor depends on the subsampling ratio is relaxed by use of a transmission line in the loop filter. A practical implementation is proposed and simulated. Subsampling a narrow band signal at 2.25 f_s, the proposed modulator achieves 60 dB of SNR with OSR = 128 using resonators with Q = 80.