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This brief presents a frequency tracking loop (FTL) to realize a crystalless wireless sensor node (WSN) for wireless body area network (WBAN). By tracking a remote wireless RF reference for system clock calibration, the proposed FTL allows WSNs to tolerate a large-frequency error from on-chip CMOS oscillators. Moreover, to achieve energy-efficient transmissions in crystalless, a sufficiently accurate convergence clock is required to enable burst overmegabits-per-second system throughput with minimized operation duty cycle. For the dedicated purpose, a comparison-based binary-search tracking scheme, which ensures accurate and robust convergence against noisy wireless channel, is further developed to manage the operation of FTL. The intermediate frequency back-end part of FTL is implemented in 90-nm CMOS process. Measurement results show that the FTL extends an initial tolerance of system clock error to ±3% and achieves a final quartz-crystal comparable ±50-ppm accuracy. This enables 4.85-Mb/s wireless links and improves 79% energy efficiency by RF operation-time reduction, giving a power-saved and miniaturized WSN device for WBAN applications.