New triple-mode ring resonator bandpass filter with source-load coupling

New triple-mode resonator bandpass filters are implemented by three coupled rings. The three rings are connected to a ground via through three short high-impedance sections. These short sections are equivalent to inductors for providing inductive coupling among the resonant modes. The closed-loop rings can also be implemented by stepped-impedance equivalence for achieving size reduction and wide upper stopband. In addition, with the aid of source-load coupling, two extra transmission zeros can be created on both sides of the passband to improve the frequency selectivity of the filter. Two filters are fabricated and measured to validate the proposed idea. The measured results show good agreement with the simulation.