Multi-band Filters and Diplexers Design by Multi-Mode and Frequency Transformation

This thesis aims at the improvement in multiband filters and diplexers by proposing two kinds of new design concepts. Based on the SIW (Substrate Integrate Waveguide)implemented on LTCC (Low Temperature Co-fired Ceramics), the first part designs a tri-band filter and elaborating the concept, two diplexers. The second part designs two microstrip quad-band filters.
The LTCC SIW tri-band filter in the first part utilizes the concept of mode orthogonality in the rectangular cavity, thus the coupling coefficients between modes in two cavities can be controlled independently by the location of slots. Furthermore, multiple modes can be excited simultaneously with the proper feeding position. If one of the modes is applied by split-type dual-band response, a tri-band filter with improved band allocation can be realized with minimized number of resonators. Furthermore, two diplexers are designed by exploiting the multi-modal orthogonality. Two channel can share the same input resonator since two modes can be excited simultaneously;
therefore the matching network is not needed in this structure. Also, two orthogonal slots opened in the common output resonator are designed to extract two modes independently and yet with good isolation.
The second part proposes a general idea of frequency transformation for multiband filter design. The poles and zeroes of a single band response are transformed to poles and zeros of a multi-band response according to the cutoff frequency of each band. Two microstrip quad-band filters are presented to verify the concept.