Implications of Passive Mixer Transparency for Impedance Matching and Noise Figure in Passive Mixer-First Receivers

TLDR

Quadrature passive mixers present the impedance of their baseband port to the RF port and vice versa. This paper analyzes a class of passive mixer‑first, LNA‑less receivers in depth. The transparency property, together with resistive‑feedback differential amplifiers and complex I/Q feedback, enables RF‑port impedance control that can be tuned with baseband resistors, and accounting for higher‑order LO harmonics is essential for accurate impedance matching and low noise figure. The architecture’s noise limits are comparable to standard RF‑LNA‑first receivers, and extending quadrature passive mixers to harmonic‑rejection mixers further improves noise performance and widens the matching range.

Abstract

In this paper, a class of passive mixer-first, LNA-less receivers is analyzed in depth. Quadrature passive mixers are shown to present the impedance of their baseband port to the RF port and vice versa. This transparency property, in combination with resistive feedback differential amplifiers, and “complex” feedback between the I and Q paths, can be used to control the impedance at the RF port. This impedance can be tuned using only baseband components (i.e., resistors). The noise limits of such an architecture are analyzed and simulated, and are shown to be comparable to standard RF-LNA-first receivers. Accounting for the higher harmonics of the LO frequency proves critical in accurately analyzing the behavior of these circuits and their ability to provide an impedance match with low noise figure. Additionally, it is shown that expanding quadrature passive mixers to harmonic rejection mixers allows for even better noise performance and wider matching range.