Nonconstrained Wrist Pulse Detection System Based on RF Resonator and PLL

In this paper, a nonconstrained wrist pulse sensor based on radio-frequency proximity coupling at 2.4 GHz is proposed. The proposed system consists of a voltage-controlled oscillator (VCO) combined with a planar resonator and a phase-locked loop (PLL). The resonator senses the distance variation from the skin occurring because of the wrist pulse and plays a role as a feedback element of the VCO. The oscillation frequency of the VCO varies due to the impedance variation of the resonator, and the PLL operates as a converter of the frequency variation to the DC output voltage variation. The measurement result verified that the proposed system can detect the heart rate in nonconctact and nonconstrained condition with 2 mm gap from the skin.