A SAR-based Fast Automatic Frequency Tuning Circuit in a 3-th Order Active-RC Complex Filter

This paper presents a 3-th order active-RC complex filter with fast automatic frequency tuning circuit. The bandwidth and center frequency of a complex filter are mainly related to the absolute time constant value of resistors and capacitors. Due to the process variation, the value may vary even over ±40%. A frequency tuning circuit for active-RC complex filter can avoid the affect of process variation and provide ideal frequency response. This paper proposed an on-chip fast frequency tuning circuit with successive approximation register (SAR) logic. Compared with the conventional counter-based logic, the proposed circuit can exhibit high tuning speed. This 3-th order active-RC complex filter with 5-bit fast automatic tuning circuit is designed in SMIC 55nm process.