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A miniaturized square resonator bandpass filter with circular stubs is designed, fabricated, and characterized. Analytical calculations were carried out to determine the critical filter parameters and the design was optimized using a 3D electromagnetic finite-element solver. The measured results were in good agreement with the designed results. The proposed filter exhibits significant improvement in bandwidth compared to the conventional square resonator bandpass filters.

Size reduction is an important issue in developing high performance miniature RF filters in wireless and mobile communication systems. The conventional edge parallelcoupled filter and hairpin filter are constructed in identical resonators cascaded in alternating series orientation which show steeper roll-off on the lower frequency side than on the higher frequency side [1-5]. Research on the modified miniaturized parallel-coupled filters has been reported to improve the filter’s symmetric response and upper stopband rejection characteristics [6-11]. In this paper, a new microstrip resonator filter with circular stubs is investigated for miniaturization and improvement in bandwidth. It is also known that the proposed filter geometries provide good upper stopband rejection characteristics. Analytical calculations and 3D electromagnetic simulations were carried out to determine the filter characteristics. Based on the design, a four-pole Chebyshev microstrip bandpass filter with circular stubs was fabricated and compared with design results.

Miniaturization of filters can be explained as shown in

includes the circular stub mode capacitance of the foldedcoupled lines, and a fringing capacitance between openend circular stub and the adjacent microstrip line. The characteristic impedance, the propagation constant, and the resonator width are represented by and, respectively. The resonant frequencies of the equivalent circuit can be obtained by applying the boundary conditions for a nontrivial solution of and The resonant frequency can be obtained from [15, 16]

where is the center frequency, is the characteristic impedance of the stub, and is the electrical length.

Bandpass filters can be defined by three characteristics: resonator structure, coupling coefficients which are the coupling between resonators, and external quality factor which is the coupling to the terminations [_{12} = 0.1407, K_{23} = 0.1034, K_{34} = 0.1407.

The fundamental resonance frequency depends on the dimensions of the connecting stub length and width, position of the circular stub connecting area, and number of stubs. Extensive 3D electromagnetic wave simulations were carried out using a 3D EM solver [

A miniaturized bandpass filter with circular stubs was fabricated in the form of four identical resonators cascaded in alternating series orientation as shown in

filter provides 160% bandwidth improvement.

A square resonator bandpass filter with circular stubs is designed, fabricated, and characterized for miniaturization and bandwidth improvement. Based on the analytical calculations and 3D electromagnetic simulation, a four-pole Chebyshev microstrip bandpass filter was constructed with the substrate of RT/d6010LM from Rogers Corporation and was fabricated using LPKF Protomat machine. The measurement result was compared with the simulated and modified simulated results. The modified simulated curve was adjusted to the actual sizes of the

fabricated circuit, dielectric constant, and conductor material. The measured results were in good agreement with the designed results. The proposed filter is miniaturized in size compared to the conventional hairpin resonator and square resonator with folded coupled lines. Furthermore, it exhibits significant improvement in bandwidth compared to the conventional square resonator bandpass filters. The miniaturized bandpass filter with circular stubs can be potentially used for compact wireless and mobile communication systems.

The authors would like to thank Dr. Mark Wickert for his advice on circuit fabrication. We would also like to thank Tom Mulcahy and Dr. Jim Wigle for their technical support on LPKF Protomat.