Design of microwave filters for portable electronics is complicated by conflicting requirements to be met simultaneously such as high selectivity, low insertion loss and compact size. Substrate integrated waveguide (SIW) technology allows designing low-profile high-Q resonators and low-loss bandpass filters based thereof. However, SIW filters are not well-suited for telecommunication applications because of remarkably large size in plane. The size of a SIW cavity can be dramatically reduced by a capacitive loading. Capacitively loaded cavities (CLCs) operating in the TM110 mode can be as small as 1/8 of the guided wavelength and even smaller, i.e. comparable in size with lumped-element resonators. Although the unloaded Q-factor decreases proportionally to cavity size, miniaturized CLCs can exhibit much higher Q-factor than that of lumped-element resonators. This paves the way for designing small-size and low-loss filters for wireless communications and different applications. Miniaturized capacitively loaded SIW cavities are favorably implemented by means of the low temperature co-fired ceramics (LTCC) technology. The goal of the paper is to demonstrate manifold possibilities and flexibility offered by the LTCC technology to the design of advanced microwave filters on CLCs. Different design and manufacturing aspects are considered. Various design examples of high-performance LTCC resonators and filters for single- and dual-band wireless applications are presented. The designed resonators and filters were manufactured using the commercial DuPont Green Tape 951 LTCC system. The LTCC filters on miniaturized CLCs are shown advantageous with regard to small size, low loss, and absence of spurious response over a wide frequency range.
One dimensional capacitive loading in a frequency selective surface for low profile antenna applications