Harmonic Transponder Sensor Based on Independent Impedance Control for Motion Sensing

This paper presents the design and demonstration of a novel harmonic transponder sensor (HTS) that can measure the position and motion of a target in noisy and reflective indoor environments. In conventional HTS implementation, an impedance mismatch at the input or output can occur because of the impedance variation of the input or output antenna caused by the sensor location or surrounding environment. The impedance mismatch of the input (or output) also affects the output (or input), thereby degrading the conversion gain (CG) of the HTS. An independent impedance control matching network (IICMN) is proposed to deal with the input and output impedances of the HTS separately, regardless of source and load impedance changes. The proposed HTS with an IICMN has a high CG with wide input power and frequency ranges. The proposed HTS was implemented with a size of 44.3-mm x 29.4-mm, and an experiment was conducted to demonstrate its CG. It achieved a peak CG of -14.3-dB at 3.1-GHz. The measured CG remained above -20-dB within a power range of -25-5-dBm at 3.1-GHz and a frequency range of 3.05-3.2-GHz at an input power of -13.7-dBm. The nonlinear responses of the HTS with the dualband antenna were observed based on the distance and output power. The measurement results showed that the second harmonic signal could be detected at a distance of 2.3-m with an equivalent isotropic radiated power of 31-dBm. The feasibility of the antenna-integrated HTS in estimating spinal cord and nerve injuries was demonstrated using a knee-jerk reflex test. When the distance between the transmitter's antenna and the antenna-integrated HTS was 0.5-m and the transmitted effective isotopically radiated power was 28.8-dBm, and a received power variation from -80.8 to -59.7-dBm was observed according to the leg movement.