Local Oscillator Phase Shifter for IQ Mixer in A Synthetic Aperture Radar Application Based on Quadrature Hybrid Branch-line Coupler

Synthetic Aperture Radar (SAR) is a powerful instrument in microwave remote sensing applications because of its capability for all-weather time operation and producing high-resolution images. The IQ mixer is a vital component of the RF (radio frequency) SAR subsystem, which increases the bandwidth of the transmitted signal and improves the resolution of the SAR system. The IQ mixer is composed of a quadrature LO phase shifter, two matched mixers, and a power combiner-splitter. This paper proposes a design for a quadrature LO phase shifter based on a microstrip quadrature hybrid branch-line coupler. The coupler provides different good phase outputs around 90◦. The output and input transmission lines have been placed on the outer side of the coupler to offer a flat gain over a range bandwidth (150 MHz), positively influencing the output response. The fabricated coupler satisfies all the specification requirements and can be used as an LO phase shifter.