Coupling Coefficient Reconfigurable Wideband Branch-Line Coupler Topology with Harmonic Suppression

The need for a flexible coupler topology to support a wide variety of coupling coefficients and robust suppression of harmonics often results in severely increased insertion loss, system size, and fabrication complexity. In this paper, we propose a two-section branch-line coupler (BLC) topology that simultaneously achieves a wide bandwidth, reconfigurable coupling coefficient, and harmonic suppression. Closed-form equations and a design methodology are provided. To verify the proposed technique, a BLC prototype operating at 3 GHz was designed, fabricated, and measured. The circuit can work under four configurations providing the coupling coefficients of 4, 6, 8, and 10 dB. The prototype achieved a 15-dB harmonic suppression up to 7.5 GHz. The measured fractional bandwidth, defined by 1-dB amplitude imbalance and 5° phase stability, is 30% from 2.55 to 3.45 GHz, the widest of reported reconfigurable couplers.