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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, 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]

A novel filtering power divider with wide stopband is presented in this letter. It utilizes four coupled quarter- wavelength resonators to obtain dual functions. Discriminating coupling is applied to the input and output feed lines and resonators, which not only to provide suitable coupling strength but also to suppress 3f0 and 5f0 (f0 is the operating frequency). A resistor is connected at the two open-ends of the input feed line to obtain high isolation. Transmission zeros are generated near the passband edges, resulting in high selectivity. For demonstration, a power divider with filtering function as well as wide stopband is implemented. Good agreement between the prediction and measurement validates the proposed method. [ABSTRACT FROM PUBLISHER]

This letter presents a novel compact quad-band bandpass filter (BPF) with controllable operating frequencies and bandwidths. The proposed filter utilizes a novel second-order multi-stub-loaded resonator with four modes. Theoretical analysis indicates that the four modes can be controlled individually, resulting in controllable operating frequencies. Meanwhile, by controlling external quality factors and coupling coefficients, the bandwidths can also be adjusted individually. To validate the proposed idea, an experimental filter is implemented. Good agreement between the predicted and measured results demonstrates the proposed idea. [ABSTRACT FROM PUBLISHER]