Monitoring Displacement by a Quadrature Self-Injection-Locked Radar With Measurement- and Differential-Based Offset Calibration Methods

This paper presents a system for remotely monitoring the displacement of a moving target. The quadrature selfinjection-locked (QSIL) radar uses an additional phase shifter to the traditional SIL radar to repeatedly switch the phase delay of injection signals between 0° and 90°. Hence, it can be utilized to determine the displacement of the moving target without the distortion due to large injection angles. Moreover, this paper proposes the measurement- and differential-based methods to calibrate dc and phase offsets before arctangent demodulation is performed to extract the displacement information. In the experiments, a 2.4-GHz prototype was implemented to detect the moving metal plate with various peak-to-peak displacements from 0.25 to 5 cm, which are 0.02× to 0.4× the operating wavelength, with a root-mean-square error of less than 1.5%. Also, the prototype was verified to detect the cardiopulmonary activities of a seated subject.