A frequency calibration method for calibrating an output frequency of a voltage-controlled oscillator is provided. The voltage-controlled oscillator includes a first capacitor bank, a second capacitor bank, and a third capacitor bank. The first capacitor bank and the third capacitor bank are initially disabled and the second capacitor bank is initially enabled. The method includes, when the initial output frequency is lower than a reference frequency, adjusting the capacitance of the second capacitor bank until the calibrated output frequency is greater than the reference frequency, and when the initial output frequency is greater than the reference frequency, enabling the first capacitor bank and gradually increasing the capacitance of the first capacitor bank until the calibrated output frequency is lower than the reference frequency.