CMOS mixers and polyphase filters for large image rejection

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-rejec t topologies and their limitations, and it is shown how this can replace co mplex trigonometric equations commonly found in the literature. Practical p roblems in design and layout that limit the performance of image-reject upc onversion and downconversion mixers are identified, and solutions are prese nted or limits explained. This understanding is put to work in a low-IF CMO S wideband, low-IF downconversion circuit, which repeatedly rejects the ima ge by 60 dB over the wide band of 3.5 to 20 MHz without trimming or calibra tion.