CMOS mixers and polyphase filters for large image rejection

Abstract

This paper presents an in-depth treatment of mixers and polyphase filters, and how they are used in rejecting the image in transmitters and receivers. A powerful phasor-based analysis is used to explain all common image-reject topologies and their limitations, and it is shown how this can replace complex trigonometric equations commonly found in the literature. Practical problems in design and layout that limit the performance of image-reject upconversion and downconversion mixers are identified, and solutions are presented or limits explained. This understanding is put to work in a low-IF CMOS wideband, low-IF downconversion circuit, which repeatedly rejects the image by 60 dB over the wide band of 3.5 to 20 MHz without trimming or calibration.