Mixed-mode S-parameter design of ultra-wideband coupled-line baluns

The design and mixed-mode S-parameter characterization of a 2.5-10.5 GHz bandwidth balun are presented in this paper. It consists of a two-stage Wilkinson divider for power splitting followed by two coupled line sections for plusmn 90deg phase shifting. To increase the even-mode and decrease the odd-mode impedances patterned ground shielding is used. The compact balun structure occupies an area of less than 5 mm² and has been fabricated on a conventional double-layer printed circuit board to validate simulation results by measurements. The manufactured balun has a input- and output return loss better than -10 dB over the entire frequency range. The measured insertion loss is les1 dB. Good agreement between simulation and measurement were achieved.