Electromagnetic Wave Group - Advanced Microwave and RF-SOP Technology

 LTCC is a matured packaging technology for the electronic industry in Taiwan. Basically, it is a multi-layered structure, capable of high-resolution signal traces and three-dimensional stack-ups. In our research, it has been used to construct the laminated waveguides [1], also called substrate integrated waveguide (SIW), for microwave and millimeter-wave applications. Taking advantages of its salient features, including multi-layered capability, layout flexibility, low loss, low cost, and high integration, this research has exploited the various superior performance made possible to the passive components.

The multi-layered nature of LTCC technology enables three-dimensional arrangements of filter cavities, thereby developing the miniaturization techniques for band-pass filters design. Not only cavities are vertically stacked to reduce the footprint area [2] but also the folded cavities are proposed to further deduce the filter size [3]. The most challenging ones accomplished by these researches are to propose several novel coupling mechanisms for realizing both electrical and magnetic couplings with strong or low coupling strengths. Desired filter responses and specifications can be accomplished by properly choosing the proposed coupling structures. Successful demonstrations have been presented for both Chebyshev and quasi-elliptic filters, demonstrating up to 12X size reduction by vertically stacked quadruple folded resonators [4].

Being in essence a metallic waveguide which exhibits well known modal pattern and polarization, the SIW implemented by LTCC can be smartly employed to design other passive components with special performance features. One is the design of dual-band filters by taking advantage of the existence of multiple cavity modes [5]. The major design concept is adequately choosing geometric shape of SIW resonators to control the frequency bands, and positions of open slots and feeding probes to realize the desired coupling coefficients and external quality factors at both bands simultaneously. Recently, such idea has been extended to the successful design of tri-band filters, demonstrating minimum number of resonators and improved band allocation [6]. By LTCC, the SIW resonators are vertically stacked, and the filter size can be miniaturized.

Another one is the design of planar laminated waveguide magic-T with broad band performance [7]. By LTCC, two orthogonal slots are used to excite even- and odd-symmetric field patterns, respectively, resulting in a good isolation between each other. The great technical originality is the setup of equivalent circuit model with semi-analytical estimation of its input admittance, which facilitates the structure design for wideband performance. Recently, the Chebyshev filter response is imbedded with the magic-T by vertically stacked SIW resonators with highly symmetric coupling structures [8]. Taking advantage of the field distribution patterns of the TE102 and TE201 modes, the in-phase and out-of-phase responses of the magic-T function are accomplished, while the desired filter response is realized with proper coupling strengths. With the degenerate, but orthogonal cavity modes, a fewer number of cavities are required and the circuit size is further reduced.

Due to our efforts, it has become evident that the laminated SIW structure based on multi-layered LTCC packaging technology has become a promising solution for the MMW passive components, which can exhibit great advantages of mass production, high resolution, low cost, high quality factor, low interference, and special features.

 In modern wireless and mobile communication systems, filters usually play important and essential roles. Multi-band communication systems have the advantages of high stability, reliability, and integrity. Integrating multi-function into a single receiver is a popular trend. Hence, the multi-band filters become essential components in developing multi-band systems.

For a typical transceiver design, high isolation and compact size diplexers are often required to separate the transmitted and received signals through a single antenna. Novel diplexers designed with common resonator sections have been proposed [1], by exploiting the variable frequency response of the stepped impedance resonator (SIR). Size reduction is achieved by introducing a few common resonator sections in the circuit, while still keeping good isolations. Furthermore, the controllable multi-resonance property of SIR has been exploited to design multiband bandpass filters [2]. The design concept is to add some extra coupled resonators to increase the degrees of freedom in extracting coupling coefficients; therefore, the filter is capable of realizing the specifications of coupling coefficients at all passbands. Wide stopband feature can be realized by tailoring the resonating frequencies to achieve multi-order spurious mode suppression [3].

Another design method for tri-band filters with improved band allocation is also proposed [4]. For the bands assigned to adjacent frequency regions, transmission zeros are introduced to split one of the single bands into two and the third one by SIR. As a result, tri-bands filters with record-high mode separation ratio are given, and verified by experiments. On the other hand, for systems that cover wide frequency bands, e.g. the UWB system, a dual-wideband filter design methodology is also proposed [5]. Implemented by SIR with frequency mapping approach, systematic synthesis method is developed and two filters are tested to validate this design.

Recently, the net-type resonator originally proposed by our group for compact filter design [6] has been analyzed theoretically for its dual- and tri-mode electrical behaviors. Constructed by a short-ended and several open-ended transmission-line sections, it has the advantages of small size and more flexible resonant frequency allocation. Three experimental examples, including a dual-mode bandpass filter, a dual-passband filter, and a triplexer, have been designed and fabricated with microstrip technology [7]. Also, a compact size and high isolation microstrip quadruplexer based on four folded tri-mode net-type resonators has been presented [8]. It shows the advantages of a small circuit size, no additional five-pole matching junction, and good isolation better than 40 dB for each channel.

All the examples fully utilize SIR to realize various filters with special features required by multi-function systems.