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An energy-efficient 920 MHz FSK transceiver for wireless body sensor network (BSN) applications is implemented in 0.18 μm CMOS technology with 0.7 V supply. A transceiver architecture based on injection-locked frequency divider (ILFD) is proposed for the low energy consumption. In the receiver, the ILFD in the signal path converts the received FSK signal to amplitude-modulated signal which is applied to the next envelope detector. In the transmitter, the ILFD is used as digitally-controlled oscillator (DCO) which directly modulates the FSK signal with digital data. The DCO replaces the frequency synthesizer to eliminate the crystal oscillator (XO), which leads to reduce power consumption and cost. The transceiver can detect whether injection locking occurs or not, and calibrates the frequency drift of DCO over temperature variation thanks to ILFD based architecture. The receiver and transmitter consume 420 μW and 700 μW , respectively, at - 10 dBm output power with a data rate of 5 Mb/s, corresponding to energy consumption of 84 pJ per received bit and 140 pJ per transmitted bit.