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Passive mixers have become an essential component of SAW-less communication receivers in the last few years, this has encouraged many researchers to study their impedance characteristics. When observed from the input radio frequency (RF) port of the passive mixer, the impedance exhibits bandpass filtering characteristics centered around the frequency of the local oscillator (LO) signal. The theory of baseband impedance up-conversion is widely accepted in the analysis of passive mixer circuits as well as in describing the origin of this bandpass impedance characteristics. In this paper, we revisit this theory and show that it is not always accurate since it does not account for switching transients. We begin our work by showing that the most commonly used single-balanced mixer is not the right choice for a half-circuit used in analyzing the input impedance of a fully-balanced passive mixer. We propose an alternative half-circuit, and proceed with a novel analysis that accounts for switching transients. This switching-aware analysis allows capturing the switched-capacitor behavior of the passive mixer. We then derive closed-form equations for the mixer impedance using purely-capacitive loads as well as resistive-capacitive (RC) loads. We also discuss the impact of finite LO rise and fall times on the mixer impedance. Simulation results of quadrature passive mixers using ideal and MOSFET switches show very close match with the proposed theoretical analysis. [ABSTRACT FROM PUBLISHER]