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A novel method of estimating gain of a digitally-controlled oscillator (DCO) for wireless RF applications is proposed and demonstrated. By executing the calculation algorithm just-in-time at the beginning of every packet, the DCO gain can be conveniently tracked and compensated. Precise and timely knowledge of the DCO gain makes possible various predictive operations of a frequency synthesizer such as the presented digital direct frequency modulation using a hybrid of predictive/closed modulation method. This enables the employment of sophisticated and fully-digital frequency synthesizers capable of compensating for analog nonidealities. The demonstrator test chip has been fabricated in a digital 0.13-μm CMOS process. The presented ideas have been incorporated in a fully-compliant Bluetooth transceiver.