Xu-Guang Wang

A Tunable Combline Bandpass Filter Loaded With Series Resonator

This paper proposes a varactor-tuned microstrip combline bandpass filter (BPF) that is loaded with lumped series resonators instead of the short-circuited end of combline. This configuration has the advantage of enhancing the tunability of the suggested BPF by varactor diodes in series resonators, which leads to a wide tuning range even with a small capacitance ratio (Cratio) of the varactor. Therefore, a low controlled bias voltage varactor-tuned BPF can be achieved. In addition, due to the controllable slope parameter of the proposed resonator, the coupling between resonators can be controlled. As a result, the proposed tunable BPF can keep nearly constant bandwidth over the wide tuning range. The insertion loss resulting from the varactors is compensated using an active capacitance circuit with negative resistance. Theoretical analysis and design approach are described. Experimental results of a demonstrative BPF show a 500-MHz tuning range (1.7-2.2 GHz) with Cratio of only 2.6 (1-5-V bias voltage) while maintaining nearly constant absolute bandwidth and low insertion loss. The measured performance of the fabricated filter shows good agreement with the simulated one.

Compact Quad-Mode Bandpass Filter Using Modified Coaxial Cavity Resonator With Improved

This paper presents a novel quadruple-mode coaxial cavity resonator and its application to bandpass filters with compact size and improved quality factor (Q-factor). Unlike the conventional coaxial cavity resonator, the proposed resonator is made up of four inner conductive posts within a single cavity, which provides new quadruple resonant modes to realize bandpass filter. No metallic walls inside the cavity are required, and thus the utilization efficiency of the cavity space is improved. As a result, the unloaded Q can be approximately 15% higher in comparison to the conventional coaxial resonator, or 30%-35% volume saving can be achieved while maintaining the similar Q-factor value with the conventional designs. In addition, due to the multiple cross-coupling occurring within the cavity, including the source-to-load direct coupling, four flexible transmission zeros can be created to realize different filtering functions. The complete design methodology of filters based on the proposed resonator is discussed, starting with the resonator characteristics and the coupling scheme. Simulations as well as experimental results of four- and eight-pole filters are presented to validate this attractive design concept. Good agreement between measured and computed results is obtained.

Q

This letter presents a reconfigurable active bandpass filter based on a controllable slope parameter. The proposed filter has reconfigurable bandwidths and center frequencies, and both can be continuously changed using varactor diodes as the tuning elements. The wide tuning range of the bandwidth is achieved by adjusting the slope parameter of the resonators. This letter describes the design and testing of a two-pole reconfigurable filter with tuning ranges in the center frequency of 2.0-2.6 GHz and 3 dB bandwidth of 127-598 MHz at the center frequency of 2.15 GHz.

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In this letter, we present a simple design method for dual-band bandpass filters (BPFs) with two independently controllable bandwidths. The proposed design is based on hybrid resonators which have both series and shunt resonances. For compact size, an interdigital BPF type is utilized. The simple design procedures are established based on an equivalent lumped circuit of the proposed dual-band BPF. Moreover, the proposed BPF has good skirt characteristics through transmission zeros created by the series resonance in the hybrid resonator and the cross-coupling. The third-order dual-band BPF is designed at center frequencies of 1.85 and 2.85 GHz with fractional bandwidths of 11% and 7.3%.

Design of a Reconfigurable Active Bandpass Filter Based on a Controllable Slope Parameter

This paper presents a tunable bandpass filter (BPF) for cognitive radio applications. The proposed two-pole BPF is implemented using lumped elements and the frequency tuning is realized by PIN diodes. Due to the L-C parallel circuit J-inverter, constant absolute bandwidth is achieved. 4 PIN diodes in each resonator and the capacitor bank are used to cover the desired frequency tuning range from 480MHz to 687MHz with 20MHz bandwidth. The circuit is theoretically analyzed, and the complete design process is described. The fabricated filter shows wide tuning range of 42.5% with insertion loss varying from 2.7dB to 3.8dB and good linearity. The 1-dB bandwidth of the filter is kept 20±1MHz.

Design of Dual-Band Interdigital Bandpass Filters Using Both Series and Shunt Resonators

This paper presents a modified microstrip Wilkinson power divider with improved stopband characteristics. By using embedded stepped-impedance low-pass filters and a cross-stub, not only wide stopband performance but also a controllable transmission zero for specified nth harmonic suppression can be effectively obtained. In addition, the explicit design equations of the proposed power divider are derived. Several power divider examples centered at 1.5 GHz are designed, fabricated and measured for demonstration. Experimental results show a good agreement with the simulated ones, which validates the proposed design.

Design of tunable bandpass filter using PIN diode with constant absolute bandwidth

This paper discusses the resonant characteristics, such as the resonant frequencies and Q factors, as well as tuning methods of the resonant frequency of the triple-mode dielectric resonators, for their possible application to compact bandpass filters for mobile communication systems. The resonators include spheres and cylindrical rods that can be applied to compact bandpass filters. These high Q ceramic resonators can replace the conventional coaxial type resonators, which are large sized and also have relatively low Q values. Both simulated and measured results show that the Q values of the sphere and rod reach 27,000 and 24,000, respectively. Decoupling of degenerate modes is also briefly discussed.

Design of Wilkinson power divider with embedded low-pass filter and cross-stub for improved stop-band characteristics

This paper presents a dual-band power divider using shunt open-stubs with enhanced attenuation characteristics. The improved transmission responses are achieved by transmission zeros created from shunt open-stubs, and the center frequencies of two passbands are also controlled by the shunt open-stubs. Moreover, the proposed power divider has control of bandwidths in two passbands. The bandwidths of two passbands are adjusted by the characteristic impedance and the electrical length of the admittance inverter between shunt open-stubs. To demonstrate the proposed method, a dualband power divider is designed at center frequencies of 3.2GHz with the fractional bandwidth (FBW) of 10% and 4.8GHz with the FBW of 7%, respectively.

Resonant Characteristics of Triple-Mode Dielectric Resonators

In this paper, a four-pole microstrip bandpass filter (BPF) with excellent stopband characteristics is proposed. The proposed filter consists of two quarter-wavelength resonators and a stub-loaded stepped-impedance resonator (SIR). Higher order harmonics of the proposed filter are suppressed by using two kinds of resonators which have the same fundamental resonant frequency, but different harmonic responses. Moreover, a cross coupled-line between two quarter-wavelength resonators creates four transmission zeros which improve the selectivity as well as the stopband performance of the proposed filter. The filter is designed with the center frequency of 2.1 GHz, and the stopband with attenuations higher than 20 dB extents up to approximate 10.8 GHz, which is 5.14 times the center frequency of the filter. Experimental results show good agreement with the simulated ones, which validated the proposed design.

Design of dual-band power divider using shunt open-stubs with enhanced attenuation characteristics

This paper presents a dual-band bandpass filter (BPF) using stepped-impedance resonators (SIRs) with enhanced stopband characteristics. The proposed filter can easily suppress its spurious responses by controlling the complex coupled line which consists of the electric-coupled parts of the two SIRs and feed lines. In order to improve the attenuation characteristics between the two passbands, source-load cross coupling is also introduced. Two-pole dual-band BPF with fractional bandwidths of 3.4% and 2.0% at the center frequencies of 1.8 GHz and 2.6 GHz is designed, simulated and measured. Transmission zeros near the passbands are generated and consequently enhanced skirt characteristics are achieved. The spurious responses are suppressed up to five times of the first passband center frequency.