Abstract: A fast and optimal adaptive frequency calibration circuit is proposed for wideband frequency synthesizer of satellite navigation receiver applications. The adaptive frequency calibration circuit can quickly find ideal target frequency band of the multi-band VCO by the division ratio of the multimode divider of the frequency synthesizer, The feedback calibration is performed to search the actual upper and lower target frequency band of the target frequency, and compared with the deviation, selected the nearest target frequency band. The adaptive frequency calibration circuit is designed using mixed signal circuit to reduce the step size of search by analog circuit and to improve the accuracy of the calibration algorithm and reduce the number of search steps by digital circuit. Simulation results based on 0.18 μm CMOS process show that compared with the traditional frequency calibration circuit, the proposed method can achieve shorter locking time and select the optimal frequency band, which is beneficial to the loop stability and phase noise optimization of frequency synthesizer.