The sensitivity of RF CMOS receivers using a directconversion or a low-IF architecture is strongly affected by flickernoise. This paper gives theoretical guidelines to predict the flickernoise in Gilbert-cell mixers. The conversion gain, the equivalentinput and output noise, and the effect of the pole at the single in-ternal RF node are discussed. For the first time, results which arevalid in all modes of operation are given. Such complete results arerequired for some ultra low-power and low-voltage applications,since the transistors might be operated in moderate or even weakinversion region. The theoretical gains are found to remain within a2-dB margin with respect to the measurements of a UHF downcon-verter built in a 0.5μm process, for a large range of bias conditionsand local oscillator swing.