Microwave Near-Field Self-Injection-Locked Wrist Pulse Sensor with Mohr-Discriminator Demodulator

A new microwave near-field self-injection-locked (SIL) wrist pulse sensor that uses a Mohr-discriminator demodulator (MDD) is proposed in this letter. The sensing oscillator with a complementary-split-ring-resonator (CSRR)-loaded substrate-integrated waveguide cavity generates a concentrated electric field in the near-field region, used for detecting the periodic wrist skin motion caused by the radial artery. According to the SIL theory, the sensing oscillator will produce a frequency-modulated signal when pulses are detected. This signal is then input into the MDD, acquiring the wrist pulses. Unlike differentiator-based demodulators, this demodulator resolves the non-matched issue, while also improving the frequency deviation to amplitude variation conversion efficiency. The wrist pulse rate measured with the proposed sensor agrees that acquired by a photoplethysmography (PPG) sensor very well. This shows that the proposed wrist pulse sensor has a potential to be developed for healthcare applications.