Large Improvement in Image Rejection of Double-Quadrature Dual-Conversion Low-IF Architectures

The reasons for a degradation of image-rejection performance in double-quadrature-double-quadrature and single-quadrature-double-quadrature dual-conversion low-IF downconverters are fully discussed in this paper. Polyphase filters (PPFs) are inserted at proper positions to minimize the effects of device/signal mismatches, and thus improve the image rejection without calibration. Both a 0.35-μm SiGe heterojunction bipolar transistor 5.2-GHz double-quadrature-double-quadrature downconverter with an RF PPF and a 0.18-μm CMOS 2.2/4.8-GHz single-quadrature-double-quadrature downconverter with a switched-band low-noise amplifier (LNA) and a narrowband inter-stage PPF are demonstrated. Compared with our previous work, the 5.2-GHz downconverter achieves a 15-dB improvement in image-rejection ratio (IRR) of the first image signal (IRR₁) even without a pre-selection filter or LNA. Additionally, the dual-band downconverter has a 25-dB improvement in IRR of the second image signal (IRR₂) , which nearly reaches the theoretical limit of a four-stage PPF covering 20-40 MHz.