AbstractA fast automatic frequency calibration (AFC) technique is described. For fast AFC operation while eliminating the uncertainty of the initial phase relationship between the reference clock and voltage‐controlled oscillator output, two‐phase reference clocks are employed. The proposed AFC scheme is applied to a 2–6 GHz frequency synthesizer. The frequency synthesizer is implemented in a 0.18 µm complementary metal–oxide–semiconductor process and occupies 0.92 mm2 while the AFC occupies only 0.01 mm2. The AFC operation is completed in less than 1.6 µs. Copyright © 2012 John Wiley & Sons, Ltd.