A Compact Lumped-Component Coupler with Tunable Coupling Ratios and Reconfigurable Responses

A novel ultra-compact microwave coupler with tunable coupling ratios and reconfigurable responses is presented in this paper. The proposed device is designed based on lumped components instead of transmission lines (as used in most of conventional tunable couplers). To achieve the desired tunable performance with compact size, the minimum number of tuning elements (i.e. only two varactors) are used. First, the coupling ratio between two output ports is tuned by applying suitable bias voltages. With a fixed phase difference between two output ports, this novel compact tunable coupler can achieve a very wide coupling ratio tuning range. Second, the proposed coupler can be reconfigured to function as a crossover when the controlling capacitance is tuned to a small value. Closed-form design equations are derived for the proposed coupler. An experimental prototype is simulated and fabricated at 1 GHz. The measurement results agree well with the design theory and numerical simulations.