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A third-order channel selection filter for multi-mode direct-conversion receivers is presented. The filter is designed with a Butterworth prototype and with the target wireless applications of Bluetooth, cdma2000, wideband CDMA, and IEEE 802.11a/b/g/n wireless LANs. Linear-region MOS transistors are used to perform voltage-to-current conversion. The wide tuning range is achieved by the current multipliers and linear voltage-to-current converters. Implemented in the TSMC 0.18 mum CMOS process, the measurement results show that the filter can operate successfully over a cutoff frequency range of 500 kHz to 20 MHz, and is compliant with the requirements of different wireless applications. The power consumption is 4.1 mW to 11.1 mW for minimum and maximum cutoff frequencies respectively from a 1.2 V supply voltage. The circuit performance compares favorably with previously reported works.