Frequency Agile 90° Hybrid Coupler Using Barium Strontium Titanate Varactors

This paper presents a tunable quadrature hybrid coupler realized using integrated lumped components. The centre frequency of the hybrid is tuned by controlling the DC bias across barium strontium titanate (BST) varactors. The hybrid was designed to tune from 1710-2170 MHz. Varactors were fabricated by sputtering a 700 nm BST film on a c-plane sapphire substrate and patterning interdigital electrodes on top of the BST film. Promising results were achieved from the first prototype, with a maximum insertion loss of 3.5 dB and minimum return loss of 12.2 dB across the tuning range. Phase quadrature between the coupled ports is within 90plusmn5deg from 1710-2170 MHz.