A Novel Microwave Phased- and Perturbation-Injection-Locked Sensor With Self-Oscillating Complementary Split-Ring Resonator for Finger and Wrist Pulse Detection

In this article, a novel microwave phased- and perturbation-injection-locked (PPIL) sensor with a self-oscillating complementary split-ring resonator (SO-CSRR) is 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 the wrist artery, which are caused by the periodic capillary or vessel motion under the 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 the 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. The proposed sensor is called a PPIL sensor on account of its sensing mechanism and demodulation technique. In this article, the proposed sensor is employed to detect the finger and wrist pulses. Both the pulse interval and the waveform shape strongly agree with the results obtained using the photoplethysmography sensor. Since the proposed sensor has the benefits of having a simple system architecture, compact circuit size, and high sensitivity, it has great potential to be developed as a wearable sensor for healthcare applications.