A new quadrature mixer based on the CMOS Gilbert mixer has been proposed. Analytical formulae have been derived for the conversion gain and noise of the proposed mixer. Simulations show a 10 dB improvement in image rejection versus a pair of Gilbert mixers, for low LO amplitudes as are increasingly required by low supply voltages. The proposed quadrature mixer exhibits the same linearity and conversion gain as a pair of simple Gilbert mixers, for the same power and input capacitance. The quadrature mixer can be designed to exhibit a noise advantage over a pair of Gilbert mixers, through proper relative sizing of the mixer core and transconductor transistors. A test chip containing the quadrature mixer and a Gilbert mixer has been fabricated, and the performances of the mixers are being measured.