Abstract: A complex filter is designed for a low-IF GPS receiver after analyzing the requirement of image signal rejection.This complex filter is based on OTA-C biquad structure and realized through linear translation.Full differential OTA with source degeneration is used to expand input linear range.A ring VCO based digital tuning loop is proposed to minimize frequency variation.The circuit is implemented in a 0.18μm CMOS process.Measured results show that the band width is 3.1MHz with 50dB out-of-band attenuation at 5MHz offset.Frequency tuning error is less than ±1.5%.Measured image rejection ratio is more than 35dB in the whole pass band.The filter and tuning circuit dissipate 0.82mA and 0.23mA under 1.8V power voltage, respectively.