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This paper describes an implementation of a frequency synthesizer for cognitive ratio application. The frequency synthesizer is based on all-digital phase locked loop, and employs adaptive frequency calibration for fast settling. Wide tuning range is achieved by using a ring oscillator with a band-extension technique. The frequency synthesizer is fabricated in a 65-nm CMOS process. It occupies an active area of 0.3 mm2 and has a power consumption less than 22 mW from a 1.2-V supply. Measured frequency tuning range is from 2.7 to 6.1 GHz. The measured phase noise level at 1-MHz offset is -90 dBc/Hz at 5.5-GHz output frequency.