Quadrature Demodulation with DC Cancellation for a Doppler Radar Motion Detector

Continuous wave direct-conversion microwave Doppler radar can be used to detect cardiopulmonary activity at a distance. Demodulation sensitivity to a target’s distance in a single channel system can be solved by using a quadrature direct-conversion receiver. A particular challenge in this technique is accurate demodulation in the presence of unwanted dc offset. A center tracking compensation method has been proposed to overcome this issue. In this paper, we present an estimation method for center tracking, and introduce a linear demodulation method. While non-linear (arctangent) demodulation is required for larger displacement or higher frequency systems, linear demodulation also provides accurate results for small displacement at lower frequencies at lower computational cost. In addition, we propose to use a real-time dc cancellation method, prior to center tracking compensation, to fully exploit the dynamic range of the pre-amplifiers and the resolution of the ADC, with minimum time delay and distortion. Experimental results demonstrate that linear and non-linear demodulation with the dc canceller result in accurate heart rate measurement with a standard deviation of less than 2 beats per minute. Large motion of the target, such as walking, was also successfully measured using non-linear demodulation.