Quasi-elliptic dual-band filter design using stepped-impedance resonators and coupling topologies for narrow-to-wide-band applications

Design guidelines are presented for new planar quasi-elliptic dual-band filters in applications requiring high performance, compact size, ease of fabrication and integration with remaining circuitry. Novel coupling topologies between folded stepped-impedance resonators (SIRs) are utilised to generate two narrow or two wide passbands at desired centre frequencies with multiple attenuation poles between passbands. Source-load coupling is employed to place additional attenuation poles within the lower and upper stopband regions. The main feature of these dual-band filters is their approximately symmetrical response, including high selectivity of each passband and controllable fractional bandwidths for either narrowband or wideband coupling schemes, respectively. In addition, the simplicity of the design approach and the use of folded SIR structures result in compact size. Several microstrip dual-band filters in various cascaded SIR configurations with narrow or wide passbands are designed and measured. The measured results are in very good agreement with those obtained from different electromagnetic simulation packages such as Ansoft Designer and IE3D.