Focusing range image in VCO based FMCW radar

This paper presents a simple method for focusing images in VCO based cheap FMCW radar. Image defocusing is caused by non-linear distortions in the voltage controlled oscillator (VCO, very often YIG type). To obtain good quality FMCW radar images, a linear frequency modulated (LMF) signal is required. Even small changes in voltage-frequency VCO characteristics cause range defocusing, and therefore decrease of radar sensitivity and resolution. The method, derived from the polynomial model of non-linear LFM signal phase, is based on a generalized chirp transform of the video (intermediate frequency IF) signal obtained by mixing the transmitted LFM signal with the received (delayed) signal. To avoid complicated laboratory equipment (such as optical delay lines) the whole procedure is performed on live radar signals, using a maximum contrast approach.