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This paper presents a method for designing low-loss varactor-based tunable bandpass filters (BPFs) with constant absolute bandwidth (CABW) and wide tuning range based on coaxial resonators. The whole structure consists of a metallic cavity and a piece of printed circuit board (PCB). Varactors and their biasing components are mounted on the PCB, which are connected to the coaxial resonators to enable frequency tuning. The coupling control between coaxial resonators and the nonsynchronous tuning of the resonators are utilized to realize the CABW over a wide tuning range. Moreover, transmission zeros are generated near the passband, resulting in high skirt selectivity. For demonstration, a second-order tunable coaxial BPF with a wide tuning range from 698 to 960 MHz (31.5%) is designed and fabricated. Due to the use of high- $Q$ -factor coaxial resonators, the proposed tunable filter realizes low insertion losses ranging from 0.82 to 2.03 dB under a narrowband specification. The 1-dB bandwidth is measured as 35.3 ± 2.7 MHz, showing excellent CABW responses. Moreover, a third-order tunable coaxial BPF is also designed and implemented with enhanced skirt selectivity and out-of-band rejection. Comparison with other reported tunable BPFs is given, where the proposed designs show excellent performance of wide tuning range, CABW, high selectivity, and high tuning speed. [ABSTRACT FROM AUTHOR]

In this paper, we propose a method for designing a dual-channel balanced filter and a dual-channel balun filter based on the quad-mode dielectric resonator (DR) for the first time. By sharing one common quad-mode DR, two balanced filters or two balun filters are integrated as one single-cavity configuration, featuring compact size and high integration. A cylindrical DR with two short ends is investigated to construct the quad-mode DR. By properly arranging input and output feeding probes, two modes of the DR are only excited by the feeding probes of one channel and the other two modes are excited by that of the other channel. Accordingly, signals cannot be transmitted between the two channels, resulting in high isolation. Moreover, the required out-of-phase characteristics of the balanced and balun filters can be obtained by the inherent electromagnetic field properties of the DR without adding additional circuits, featuring a simple structure. For demonstration, a dual-channel balanced filter and a dual-channel balun filter are designed and fabricated, showing excellent balanced or balun filter performance of each channel as well as high isolation between the two channels. As compared to the other reported DR balanced and balun filters, the proposed designs exhibit a significant size reduction, which are attractive in wireless systems. [ABSTRACT FROM AUTHOR]

In this paper, a low-temperature co-fired ceramic (LTCC) diplexer is presented with compact size and high isolation. By using the common stub-loaded resonator shared by two channel filters, combining and matching networks are eliminated and only three resonators are used to design the two second-order channel filters, resulting in simple structure and miniaturized size. A novel transmission path is utilized to not only realize the higher channel filter but also prevent the signal at the lower frequency band. And a specific coupling region is selected to simultaneously realize the coupling strength required by lower channel filter and a transmission zero at higher-band frequency. In this way, each channel filter can generate a transmission zero at the passband frequency of the other channel filter, resulting in high isolation. Moreover, the bandwidths and center frequencies of the two channel filters can be individually controlled due to the design flexibility of the multilayer LTCC structure. For demonstration, a diplexer operating at 3.5 and 5.5 GHz is fabricated. The core circuit has the compact size of 1.9 mm $\times1.7$ mm \times2$ mm or 0.054~\lambda _{g} \times 0.048 \lambda _{g} \times 0.057~\lambda _{g} . Within the 3-dB passbands of the lower and higher channel filters, the measured isolation levels are better than 40 and 45 dB, respectively. [ABSTRACT FROM PUBLISHER]