Abstract: An eighth order active-RC filter for low-IF and zero-IF DVB tuner applications is presented, which is implemented in Butterworth biquad structure. An automatic frequency tuning circuit is introduced to compensate the cut-off frequency variation using a 6-bit switched-capacitor array. Switched-resistor arrays are adopted to cover different cut-off frequencies in low-IF and zero-IF modes. Measurement results show that precise cut-off frequencies at 2.5, 3, 3.5 and 4 MHz in zero-IF mode, 5, 6, 7 and 8 MHz in low-IF mode can be achieved, 60 dB frequency attenuation can be obtained at 20 MHz, and the in-band group delay agrees well with the simulation. Two-tone testing shows the in-band IM3 achieves –52 dB and the out-band IM3 achieves –55 dB with –11 dBm input power. This proposed filter circuit, fabricated in a SMIC 0.18 μm CMOS process, consumes 4 mA current with 1.8 V power supply.

Key words: active-RC filter,  Butterworth,  frequency tuning,  group delay,  noise,  linearity