A wideband tunable CMOS channel-select filter for a low-IF wireless receiver

Researchers agree that the active filter for channel-selection limits the dynamic range in a fully integrated wireless receiver, which uses no external components. This paper describes a filter designed for a wideband wireless LAN receiver operating in the 2.4-2.48 GHz ISM band. The receiver converts the desired channel to a low IF to enable on-chip rejection of the image. The analog filter must pass up to a 10 MHz wide single-channel centered after downconversion to IF ranging from 5 to 10 MHz. The classic requirements on all RF IC's apply to this filter, namely how to achieve the desired frequency response with the highest linearity and lowest noise at a given current consumption. For a channel-select filter, linearity is specified as out-of-band 3rd order intercept (IP3), to limit the in-band intermodulation from large interferers lying in the filter stop band. This work addresses the problem in filter architecture and circuit implementation. The achieved dynamic range surpasses all other published filter designs, except those cases where gain is favorably interleaved with filtering.