Miniaturized dual-mode ring resonator bandpass filter with a wide passband

Dual-mode ring resonator filter is designed to have a relatively wide passband with a miniaturized circuit area. The miniaturization is achieved by folding the ring trace to a spiral configuration. The two degenerate modes are split up by an asymmetric inductive perturbation. As a result, the circuit area is only 29% of that of a conventional ring. The strong I/O coupling required by the wide passband property is implemented by adding an impedance transformer for matching the external quality factor (Q_ext) with the circuit bandwidth. Measured results show good agreement with the simulation.