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In this paper, a modulation technique, viz., differential frequency shift keying (DFSK), has been proposed for ultralow power wireless sensor node applications, which has been shown to be the most energy-efficient transmission technique for short-distance wireless communication systems, such as wireless body area network. The proposed technique is a variant of the binary frequency shift keying and is based on the principle that even though the frequency values of a radio frequency (RF) tone-pair of an oscillator may vary under various environmental conditions, the difference between the frequencies remains largely invariant. The proposed DFSK technique significantly relaxes the frequency stability requirement in the transmitter RF front-end at the cost of a little increased transmission power, which, in turn, significantly simplifies the circuit complexity as well as reduces the overall circuit power consumption within the transmitter. Theoretical analyses of the bit error rate, phase noise, interferer effects, and energy consumption for DFSK have been provided, supplemented by the simulation results using detailed MATLAB modeling for the medical implant communications service frequency band.