A Passive-Mixer-First Acoustic-Filtering Superheterodyne RF Front-End

Abstract

This article presents an agile passive-mixer-first superheterodyne RF front-end that utilizes a gigahertz acoustic filter as its intermediate-frequency (IF) load - essentially a mixer-first acoustic-filtering RF front-end. The passive mixer frequency-translates a sharp but fixed-frequency acoustic-filtering response to a much higher and mixer-clock-defined tunable frequency while preserves input matching, high linearity, and introduces minimal loss. In contrast to low- or zero-IF passive-mixer-first receivers which often use active filters at baseband, using an acoustic filter as the IF load is fraught with fundamental challenges. We introduce ON-chip LC tanks, as an impedance shaper, to suppress the acoustic filter impedance components at harmonic frequencies and an all-passive recombination network at IF to share one acoustic filter among multiple paths. Also, we extend the existing analysis of mixer-first front-ends from assuming a load impedance with a low-pass frequency response to having a generic load impedance. Our analysis unveils impedance aliasing in a generic mixer-first front-end, motivating the need for an ON-chip impedance shaper. A front-end prototype using a 65-nm CMOS switched- LC passive mixer followed by an off-the-shelf 1.6-GHz surface-acoustic-wave (SAW) filter is designed. In measurement, the RF front-end operates across 2.5-to-4.5 GHz achieving 5.5-dB noise figure and +29.4-dBm IIP3 at 1 × bandwidth offset.