Chest-Worn Heart Rate Variability Monitor With a Self-Injection-Locked Oscillator Tag

This article presents a chest-worn device for monitoring the heart rate variability (HRV) of a person in motion without direct contact with the skin. The device operates at 5.8 GHz and mainly consists of a self-injection-locked oscillator (SILO) and a tag antenna. The device transmits the output signal of an SILO that is frequency-modulated (FM) by the Doppler effect of the physiological movement of the chest via the tag antenna to a remote FM receiver. Accordingly, the heartbeat is detected and the HRV can be estimated. The tag antenna comprises a circularly polarized ring slot antenna and a split-ring resonator (SRR) array in stack to enhance sensing directivity while reducing the moving clutter in the joint sensing and transmission process. For validation, an HRV monitoring system was developed, based on the proposed device, to detect the cardiac intervals of a wearer who performs various activities, including walking and running. The results thus obtained agree closely with electrocardiogram (ECG) measurements.