A Wearable Self-Injection-Locked Sensor With Active Integrated Antenna and Differentiator-Based Envelope Detector for Vital-Sign Detection From Chest Wall and Wrist

This paper presents a new wearable self-injection-locked sensor with a self-oscillating active integrated antenna (AIA) and a differentiator-based envelope detector for detecting vital signs from the chest wall and the wrist. The AIA is designed to radiate the electromagnetic signal and sense the scattered-back signal, which is phase modulated by the physiological movement of the human chest and wrist. The received self-injection signal from the subject under test locks the AIA and simultaneously introduces a variation in the output magnitude (amplitude modulation) and a shift in the oscillation frequency (frequency modulation). In other words, the output signal of the AIA is a frequency-modulated carrier with an amplitude-varying envelope. To acquire the vital signs from the modulated signal, an envelope detector is integrated with a microwave differentiator to form a differentiator-based envelope detector, which demodulates signal. The heartbeat and wrist pulse rates measured by the proposed sensor agree well with the results acquired by a finger pulse oximeter. Since the proposed demodulator has a simple architecture, it has a potential to be integrated with the AIA in a multilayered printed-circuit board for the realization of a compact wearable self-injection-locked radar sensor.