Chest-Worn Self-Injection-Locked Oscillator Tag for Monitoring Heart Sounds

This letter presents a heart sound monitoring system with a chest-worn sensor without contact with the skin. A self-injection-locked oscillator (SILO) and tag antenna are the essential components of the sensor, which operates at 5.8 GHz. The sensor uses the tag antenna to transmit the SILO output signal whose frequency is modulated by the Doppler effect that is caused by the chest movement, to a remote frequency modulation (FM) receiver for extracting the heart sounds. The tag antenna comprises a loop antenna and a concentric loop (CL) in a stack to improve the directivity toward the chest. Experiments were conducted at several auscultation sites to identify the four major heart sounds-the first heart sound (S1), the second heart sound (S2), systole, and diastole-based on a hidden semi-Markov model (HSMM). The results thus obtained correlate well with phonocardiogram (PCG) measurements.