Design and Analysis of Enhanced Mixer-First Receivers Achieving 40-dB/decade RF Selectivity

A “second-order” passive mixer-first receiver is proposed to improve channel selectivity, linearity, and noise figure (NF) in the presence of out-of-band blockers, by presenting an impedance that rolls off at 40 dB/decade as the load to an N-path filter. The synthesis of this impedance is described in a step-by-step manner starting from the required impedance transfer function to its actual circuit realization. Various tradeoffs and limitations of the architecture are described in detail, and layout-related techniques are also provided. Two integrated circuit prototypes were fabricated in 28-nm bulk CMOS as proof of concept for this circuit, including a low-power version. The receiver, capable of broadband operation from 0.2 to 2 GHz, achieves an out-of-band IIP3 of +33 dBm and a blocker P1dB of +12 dBm. Additionally, it achieves an NF of 4.4 dB with less than 2-dB degradation in NF for a 0-dBm blocker.