LTCC Filtering Rat-Race Coupler Based on Eight-Line Spatially-Symmetrical Coupled Structure

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In this paper, a compact filtering rat-race coupler implemented in low-temperature co-fired ceramic (LTCC) technology is presented. In order to reduce the dimensions, the schematic and the coupling topology of a planar filtering rat-race coupler are first studied and then simplified to fit the 3-D multilayer LTCC structure. An eight-line spatially symmetrical coupled structure is, therefore, proposed to obtain the required coupling topology between resonators. Thus, the filtering rat-race coupler can be constructed only by using four resonators, which is only half that of the planar one. Consequently, the size could be reduced substantially. For validation, a design example centered at 3.5 GHz is simulated, fabricated, and measured. All the measured results are in good agreement with the full-wave simulation results. Besides, the size of the circuit is only $3.8 \times 4 \times 2.1$ mm3, which demonstrates that by employing the eight-line spatiallysymmetrical coupled structure, the proposed rat-race coupler obtains good performance and compact size.