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A new DC coupled continuous-wave (CW) radar sensor using a fine-tuning adaptive feedback loop technique to calibrate the DC offset is presented. This technique not only makes the radar sensor work with sufficient high gain at the DC-coupled baseband stage, but also allows it to detect movement with stationary moment. Experiments demonstrate that the proposed DC coupled radar sensor can completely relieve the baseband stages from saturation and retain all the desired DC information, while the conventional AC coupled radar sensor leads to significant signal distortion when a movement has stationary moment.
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Introduction: The miniature Doppler radar sensor has become a hot research topic and has been used for applications such as physiological measurement [1], microwave interferometry [2] and structural health monitoring [3]. With either direct- or indirect-conversion architectures, the radar suffers from DC offset at the RF front-end output, which may saturate the following stages of baseband amplifiers. In radar sensor applications, the DC offset is mainly caused by the reflections from stationary objects around the target, which is difficult to deal with since it depends on different test environments, and circuit imper- fections such as the self-mixing of LO and interferers [4]. To overcome this demerit, AC coupling has been commonly used in such radar sensors. However, AC coupling causes significant signal distortion when the target motion has a very low frequency or a DC component, owing to the highpass characteristics of the coupling capacitor. This is a problem in Doppler radar motion sensing when a target has stationary moment. To deal with it, researchers have proposed several approaches to employ DC coupling in radar sensors. In [5] a method was proposed for calibrating the DC offset while preserving the DC information. However, extra effort is demanded before real time measurement. In [6], low DC offset was achieved using mixers with high LO-RF isolation. However, this method only alleviates the DC offset from circuit imperfection, and the remaining DC offset due to reflection from stationary objects still limits the dynamic range of the baseband amplifiers. In this Letter, we propose a DC coupled CW radar sensor using a fine-tuning adaptive feedback loop technique. The radar output is first fed into a laptop, which then extracts the...