Design, Analysis and Experimental Characterization of a Passive X-Band I/Q Downconversion Subsystem for Maritime Radar Applications

This paper presents the design and experimental validation of a passive I/Q downconversion subsystem for X-band applications, with emphasis on maritime radar operation. The proposed architecture combines passive RF and LO distribution networks with a single-balanced passive mixer, enabling accurate quadrature signal generation without relying on active correction mechanisms. The subsystem maintains controlled amplitude and phase balance across the operating band, directly supporting effective image suppression. Experimental results demonstrate image rejection ratios exceeding 30 dB. The mixer stage exhibits a conversion loss of approximately 6 dB and an input-referred 1-dB compression point of about 14 dBm. The incorporation of an optimized RF matching stub in the mixer improves the impedance conditions at the IF node, leading to enhanced linearity and enabling a favorable trade-off between conversion loss and 1-dB compression point. Performance further improves within the maritime radar allocation, where enhanced balance leads to increased image rejection. These results confirm that high-quality I/Q signal generation can be achieved using a passive and low-complexity architecture, providing a practical and reliable solution for broadband X-band receiver front-ends.