Three-branch channelized N-path bandpass filter for a Zigbee application

In this paper, an original silicon-integrated RF differential channelized active bandpass filter is presented. The circuit is implemented by using a three-branch bandpass N-path switched-capacitor filter architecture. A selective pre-filter (FM) is implemented at the input of the receiver that allows an attenuation higher than 10 dB for the adjacent channel interferer located 23-MHz away from the center frequency of the considered channel. Two other parallel-type branches of N-path bandpass filters (FA1 and FA2) with downward and upward frequency shifting are used. The outputs of the pre-filter and the two other bandpass filters are subtracted to create two close-to-passband transmission zeros that enable to achieve high selectivity in the overall filtering transfer function. The channelized bandpass filter can be frequency tuned from 0.2 GHz to 1 GHz. The gain, NF, and channel bandwidth of the filter at 500 MHz are around 20 dB, 4.7 dB, and 5 MHz, respectively. The circuit has been designed with 0.25 µm BiCMOS process of NXP technology using Cadence SpectreRF circuit simulator.