A novel microwave perturbation-injection-locked (PIL) and phased- and perturbation-injection-locked (PPIL) sensors with self-oscillating complementary split-ring resonator (SO-CSRR) are proposed for finger and wrist pulse detection. This SO-CSRR is designed to generate a concentrated electric field in the near-field region, and then senses the skin surface variations of the fingertip or wrist artery that are caused by the periodic capillary or vessel motion under systolic and diastolic pressure states. When the fingertip is placed on the CSRR, the periodic variation of the skin leads to a periodic resonant frequency deviation based on perturbation theory. This frequency deviation accompanies the corresponding phase shift. By injecting this phase-shifted signal into the SO-CSRR, it generates a frequency deviation at its output. This is the frequency-modulated signal based on the injection-locked theory. Moreover, as the SO-CSRR is locked by the phase-locked loop, the vital-sign information contained in the frequency-modulated signal can be demodulated at the tuning-voltage terminal. This frequency-modulated signal can be also demodulated by amplitude-based demodulator, which is composed of a microwave differentiator and an envelope detector. The proposed sensors are employed to detect finger and wrist pulses. Both the pulse interval and waveform shape strongly agree with the results obtained using the photoplethysmography sensor. Since the proposed sensors has the benefits of having a simple system architecture, compact circuit size, and high sensitivity, they have great potential to be developed as wearable sensors for healthcare applications.
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