A Wireless-Frequency-Locked-Loop-Based Vital Sign Sensor With Quadrature Tracking and Phase-Noise Reduction Capability

Most conventional continuous wave (CW) vital sign sensors suffer from the phase noise of the voltage-controlled oscillator (VCO). This work develops a novel wireless-frequency-locked-loop (FLL)-based sensor to overcome this issue. This technology uses the propagation delay of the sensor signal to make the entire sensing circuit and the subject under test (SUT) a wireless FLL. The wireless FLL uses a quadrature-tracking technique to track the phase of the propagation signal and the phase variation that is caused by the vibration of the SUT. This technique thus enables the wireless FLL not only to reduce the phase noise of the VCO but also to sense the vital signs of the SUT. Moreover, the phase-noise reduction of the wireless FLL increases with the sensing distance. The signal-to-noise ratio (SNR) of the proposed wireless-FLL-based sensor in vital sign sensing is therefore much higher than that of a conventional CW sensor. The SNR performance of the wireless-FLL-based sensor is further optimized using a highly sensitive VCO. Experimental results demonstrate that the SNR of the wireless-FLL-based sensor is around 16 dB better than that of the conventional CW sensor at a detection distance of 4 m. The SNR enhancement increased by approximately 6.6 dB when the detection distance is doubled. With this SNR enhancement, the heart-rate measurement accuracy of the developed 2.4 GHz wireless-FLL-based sensor is 90.5 % at a detection distance of 8 m.