A Novel Compact Ultra-Wideband Bandpass Filter Using a Microstrip Stepped-Impedance Four-Modes Resonator

A novel compact ultra-wideband (UWB) bandpass filter (BPF) using a microstrip stepped-impedance four-modes resonator is developed in this paper. By controlling the resonant frequencies of the first four modes of the resonator, we get four transmission poles within the passband of the filter. Moreover, by enhancing the coupling between the resonator and the input/output feed lines, we obtain two additional transmission poles. As a consequence, we realize a UWB bandpass filter that has six transmission poles in its passband, although only one resonator is utilized. A design method, based on network analysis and optimization in the z-domain, is established to determine the circuit and geometrical parameters of the filter. The measured filtering characteristics of the filter show good agreement with the theoretical predictions and satisfy well the Federal Communications Commission's indoor limit.