A noise model for CMOS single-balanced mixers is proposed by considering the noise currents in the MOSFETs, and it is shown that simple analytical equations can be derived for the spectral density of the output drain noise current in the case of thermal noise. The model agrees well with measurements made on the variation of the noise figure of the mixer with the bias current and local oscillator power. The theory is useful for simple and accurate modeling of the thermal noise in CMOS mixer design. (C) 2004 Elsevier Ltd. All rights reserved.