Quadrature channel calibration with a pendulum for accurate displacement measurement with CW Doppler radar

A microwave radar sensor has been proposed for measurement of cardiac and respiratory activity in several applications. However for accurate displacement measurement with quadrature Doppler radar, the quadrature channel imbalance needs to be measured and compensated. Moreover, the imbalance factors do not necessary remain constant, and the imbalance should be calibrated from time to time. The previous imbalance measurement methods require hardware modifications. Thus, they are not easily performed on site at home or in hospital environment, but require a laboratory setting. In general, the requirement of hardware modifications is not practical for commercial radar sensor modules. In this paper, we show that the quadrature imbalance compensation is possible also with a simple set up. A freely hanging sphere is used as a pendulum to generate the calibration signal, and a previously presented ellipse fitting method is utilized to estimate the imbalance factors.