Doppler Radar Motion Sensor With CMOS Digital DC-Tuning VGA and Inverter-Based Sigma-Delta Modulator

In most of the traditional miniature Doppler radar systems, ac coupling is adopted between the radio frequency front-end and baseband amplifier to block the dc offset due to clutter reflection. However, ac coupling causes distortion to low frequency signals and loses the dc information, which represents slowly changing or stationary moment of the target. A dc-coupled radar sensor system using a variable gain amplifier (VGA) with software-configured digital tuning architecture is presented in this paper to realize dc tuning and dc offset calibration. Furthermore, to process the output signals from the VGA, a novel low-voltage low-power inverter-based second-order sigma-delta modulator has been developed. The frequency of mechanical motion can be detected from the spectrum of the presented modulator output digital bit stream without a decimation filter, which can significantly reduce the cost and the power consumption. Furthermore, it can potentially detect the motion amplitude using the relative strength of the harmonics in the spectrum of the output digital bit steam. Experiments show that the presented digital dc-tuning VGA can completely relieve the radar sensor from circuit saturation and the inverter-based sigma-delta modulator can perform under low voltage and low power to accurately detect the movement frequency of the target without a decimation filter. Both the presented VGA and modulator show great potential for integrating a Doppler radar sensor system into a low-power high-accuracy single chip in the future.