GaAs MMIC tunable directional coupler

This paper presents a tunable GaAs MMIC directional coupler. Design equations for a hybrid coupler with ideal tunable capacitances are derived to determine the range of capacitances for the desired coupling tuning range. The 2GHz coupler is implemented in a 0.5 µm GaAs process with Schottky varactor diodes, and occupies an area of 2.2mm×1.4 mm. Over a bias range of 0V to −1.5V the coupling coefficient ranges from 6.6 to 60 dB, or 53.4 dB of dynamic range at 2 GHz. The input return loss for the device is better than 12 dB for all bias voltages and is better than 20 dB for coupling coefficients from 23 to 60 dB with isolation better than 17 dB.