Implementation and enhancement of Hilbert transform-based calibration in a K band FMCW radar for high-resolution security applications

This paper presents the design and calibration of a short-range, K band (18-26 GHz) frequency modulated continuous wave (FMCW) radar prototype with a theoretical range resolution of 2 cm, making it suitable for security screening applications. Additionally, this work examines the theoretical considerations for expanding the calibration algorithm for potential application in synthetic aperture radar (SAR). Radar based security scanners rely on large signal bandwidth to achieve range (depth) resolution capabilities on the order of a few centimeters; a system capable of 2 cm range resolution – an approximate requirement for a security sensor - must operate with 8 GHz of bandwidth. The radar presented in this paper uses commercially available off-the-shelf components to achieve the required bandwidth while minimizing prototyping cost. However, with 36% fractional bandwidth at 22 GHz, frequency dependent amplitude ripple and non-linear phase responses of the RF front end distort the radar signal, counteracting the benefit gained from wideband operation and degrading the attained resolution. The effects of this signal distortion are mitigated by implementing a Hilbert transform based calibration procedure that involves estimating the complex analytical signal of the measured radar response and then removing the unwanted components using a reference signal. Extending this algorithm for SAR requires recovering relative phase information that is lost during calibration and the procedure is discussed in this paper. At 2.5 m distance, the calibrated radar successfully resolves targets separated by 2.5 cm - a significant improvement from un-calibrated data wherein the same targets were indistinguishable even with 20 cm separation.