Near-Field Coherent Sensing of Vibration With Harmonic Analysis and Balance Signal Injection

The near-field coherent sensing (NCS) system can detect weak cyclic motion inside living bodies, such as human wrist pulses and the heartbeat of small animals. To couple significant energy to the motion source, a GHz RF carrier was employed, where the vibration amplitude was often much smaller than the wavelength. To optimize the signal quality, we presented a small-signal model with nonlinearity analysis by Jacobi-Anger expansion. The quiescent-point determination and signal composition were then discussed to guide the hardware and algorithm design. The model was validated by electromagnetic simulation in CST Studio and a simple robotic platform. A novel algorithm of adaptive control by signal injection was applied to boost both the signal magnitude and data converter utilization. The signal-to-quantization-noise ratio (SQNR) is improved by 35 dB and signal-to-noise ratio (SNR) by more than 10 dB. Sensing of vocal cord vibration was demonstrated as a potential application, where a harmonic radio frequency identification (RFID) system was chosen for its low noise floor to achieve high-fidelity recording of music scales and procedure words.