GaAs monolithic Lange and Wilkinson couplers

An important part of monolithic microwave integrated circuits are the passive distributed circuit elements which divide and combine RF signals. This paper describes the design and performance of a monolithic GaAs X-band three-port Wilkinson coupler [1] and a monolithic GaAs X-band four-port interdigitated Lange coupler [2], [3]. These couplers are useful both as power dividers and power combiners. In the Wilkinson, the input power is split equally between the output ports with zero differential phase shift. For the Lange, there is a phase difference of 90° between the outputs. All ports are well matched and the output ports are highly isolated. The transmission mode chosen for these couplers is microstrip on semi-insulating GaAs with conductor-to-ground plane spacing of 0.1 mm. This low-loss, high permittivity medium is compatible with the present monolithic GaAs FET technology which thus allows combining more complicated monolithic microwave integrated circuits on a single chip. The fabrication process takes advantage of the technology developed for proeessfng GaAs FET´s. For example, connecting the coupling conductors for the Lange coupler requires RF crossovers. To minimize crossover capacitance, an air-bridge interconnection technique is used. Calculated coupling, isolation, and VSWR data for the Wilkinson and Lange couplers are compared with actual measured performance showing good agreement with expected results. Measured loss, minus fixture contributions, shows 0.25 dB for the Wilkinson and 0.75 dB for the Lange.