A fast f_c automatic tuning circuit suitable for WCDMA systems is proposed. The circuit employs master-slave architecture using digitally controlled G_m-C filter for avoiding long transient response. The tuning feedback loop contains a 2-bit up-down counter ADC for fast tuning operation. Furthermore, to avoid degradation of f_c tuning accuracy due to reference feedthrough, an analog loop filter with notch located near reference frequency is used. The fast f_c automatic tuning circuit is fabricated in a SiGe BiCMOS process. The tuning time within 200 μs is achieved for 35 chips from 2 lots and the standard deviation of 25.5 kHz is obtained for the average f_c of 2.12 MHz.
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