Multilayer Interdigital Structures for Compact Bandpass Filters Providing High Selectivity and Wideband Rejections

Two compact microwave filters designed using multilayer inter-digital resonators are introduced. The first structure combines vertically and horizontally coupled interdigital resonators for optimizing the surface occupation while realizing high-order filters. A 12-pole filter centered at 18 GHz, with a 2.4 GHz passband, is designed and characterized. The second structure is composed of stepped-impedance interdigital resonators for widening the upper stopband. The filter is centered at 3 GHz with a 0.9 GHz passband, and both simulated and experimental results show an extended stopband up to 21 GHz.