A digitally controlled resonant tank oscillator with loop antenna radiating inductor offers high energy efficiency in BFSK transmission, however suffers from frequency drift. Here we present a fast-settling adaptive digital architecture for dual-loop frequency locking of a BFSK transmitter. The method uses interleaved frequency-locked loops (FLL) that allow fast and energy-efficient direct digitally controlled oscillator (DCO) frequency modulation at a data rate exceeding the settling time of the frequency adaptation, while maintaining frequency regulation. The fully digital FLL architecture retains the dual-loop adapted states between transmission bursts, allowing for energy efficient low duty cycle transmissions with minimal or no settling transient at the beginning of a transmission burst. Analysis and simulation results are presented of the architecture operating in the 402-405 MHz MICS band for biomedical applications, indicating locking at 130 kHz frequency resolution at 115 μs settling for 125 kbps BFSK telemetry.