New studies on microstrip ring resonators for compact dual-mode dual- and triple-band bandpass filters

This paper deals with the proposal, design and implementation of a class of compact dual- and triple-band bandpass filters with two transmission poles in each passband using a single microstrip ring resonator. Two methods are first presented to design two types of dual-mode dual-band bandpass filters using a single resonator. One is to synchronously excite the two pairs of the first and third-order degenerate resonant modes of a ring resonator; the other is to use the first two pairs of degenerate resonant modes of a ring resonator. To achieve these targets, the port-separation angle along a ring is chosen as 45°/135°, respectively. Inspired by these two methods, the first three pairs of degenerate modes of a ring resonator are further explored to design a compact dual-mode triple-band bandpass filter on a single resonator. The operating principle and design procedure for all filters are described clearly based on their equivalent transmission-line model. Several prototype filters are finally designed, fabricated and measured. The measured results show the two visible poles in each passband, thus evidently proving our proposed design theory.