A new microwave near-field self-injection-locked wrist pulse sensor that utilizes a Mohr-discriminator demodulator is proposed in this letter. The sensing oscillator with a complementary-split-ring-resonator-loaded substrate-integrated waveguide (SIW-CSRR) 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 self-injection-locked theory, the sensing oscillator will produce a frequency-modulated signal when pulses are detected. This signal is then input into the Mohr-discriminator demodulator, 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 sensor very well. This shows that the proposed wrist pulse sensor has a potential to be developed for healthcare applications.
Except for using Mohr-discriminator demodulator, the amplitude-based demodulator, which is composed of a CSRR notch filter for the frequency deviation to amplitude variation conversion and an envelope detector, is also used to acquire the wrist pulse signals.
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