A 2-MHz BW 64.5-dB SNDR 0.45-1.05 GHz Direct IF/RF Digitization Subsampling Bandpass DSM Utilizing a Capacitive-Stacking N-Path Filter

Abstract

This brief presents a novel subsampling bandpass Delta-Sigma modulator (BPDSM) based on transconductor (Gm) - N-path filters (NPF) for direct radio-frequency (RF) / intermediate-frequency (IF) digitization. The proposed architecture introduces two key innovations. First, a subsampling technique is applied that places the input center frequency at $(3/4) f_{S}$ (versus conventional $f_{S}/4$ operation), where $f_{S}$ is the sampling frequency. Second, a capacitive-stacking NPF that provides 6-dB passive gain is utilized for: (1) suppressing the amplifier’s thermal noise when referred to the modulator’s input; (2) improving the quantization noise shaping by eliminating redundant notches at even multiples of $f_{S}/4$ in the modulator’s noise transfer function. Fabricated in 65-nm CMOS, the modulator prototype occupies 0.11 mm2 and achieves 0.45 - 1.05 GHz tunability, the highest upper frequency reported for Gm-NPF BPDSMs to the best of the authors’ knowledge. When clocked at 800-MHz, it demonstrates a 64.5-dB peak SNDR over 2-MHz bandwidth centered at 601-MHz, consuming 0.37 mW from a 1.2-V supply. The design records state-of-the-art figure of merit (FoM) values of 161.8 dB Schreier’s FoM and 67.4 fJ/conversion-step Walden’s FoM for BPDSMs operating above 450 MHz.