A Complex Image Rejection Circuit With Sign Detection Only

In direct-conversion receivers, radio frequency (RF) signals are down-converted to low or zero intermediate frequency (IF) using complex in-phase and quadrature (I/Q) mixers with no prior image filtering. Due to I/Q path gain and phase errors, image leaks into the signal band during the down-conversion process. A generic image rejection algorithm is proposed to reject image in the baseband using a zero-forcing sign-sign adaptive feedback concept. The orthonormal property of complex I/Q channels is exploited to update their gain and phase errors by detecting only four signs, and image is corrected with four multiplications and two additions. The proposed image rejection algorithm can be implemented in a digital, analog, or hybrid form. A complex baseband sample and hold (S/H) with a digital error detector, which is a hybrid example, achieves an image rejection of 65 dB while sampling at 40 MS/s. The prototype chip fabricated in 0.18-mum CMOS occupies 800times450 mum², and consumes 23 mW at 1.8 V