LTCC fold-back bandpass Filter designed with capacitively loaded stubs

This study presents a design of a compact stub-type bandpass filter with capacitively loaded stubs and a fold-back structure. This paper employed the fabrication process of low-temperature co-fired ceramic (LTCC) for filter realization of a multi-layer structure. The proposed filter structure required adding end capacitors to stubs to extend their electrical length, while achieving a length reduction of 30%. This study provided design curves to determine the dimensions of the end capacitor for reaching maximum electrical length extension. In addition, a fold-back configuration was applied to halve the filter size. An experimental filter operating at 5.8 GHz was fabricated and measured to validate the design concept, achieving a highly compact size of 14.3 × 8.2 × 0.76mm³.