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Design of microstrip stepped elliptic filters without grounded resonators

Keywords:

A.P. Maksimov – Post-graduate Student, Bonch-Bruevich Saint-Petersburg State University of Telecommunications; Engineer, JSC «VNIIRA» E-mail: apmaksimov@gmail.com Faiz Saleh Ali Awgari – Post-graduate Student, Bonch-Bruevich Saint-Petersburg State University of Telecommunications E-mail: fsaleh28@yahoo.com A.R. Kubalova – Ph. D. (Eng.), Senior Lecturer, Bonch-Bruevich Saint-Petersburg State University of Telecommunications E-mail: kubalovaap@mail.ru S.V. Tomashevich – Dr. Sc. (Eng.), Professor, Head of Department, Bonch-Bruevich Saint-Petersburg State University of Telecommunications E-mail: tomashevich.s.v@gmail.com


A new method of design of a compact miscrostrip elliptic bandpass microwave filter is proposed in this paper. Elliptic filter allows to approximate the characteristics of an ideal filter with the smallest number of resonators. In turn, microstrip realization is of interest due to miniaturization of complex microwave devices and in particular for mobile and airborne communications, radiolocation and radio navigation systems. The structure of the filter is based on the half-wave stepped digital elliptic filter by J.D. Rhodes and represents a parallel connection of two planar coupled line arrays associated full length of π/2 on a central frequency of a filter, which are open circuited on both ends. In the Rhodes filter the resonator ends are short-circuited to ground, whereby the structure is strengthened in case of cavity realization. For microstrip realization short-circuiting is undesired. The transformation formulas yielding the structure without short-circuited resonators are presented in this paper. The resulting structure is of a smaller size than the existing microstrip bandpass microwave elliptic filters due to multi-wire line implementation. The quantitative example of a design of a filter topology is presented. Recommendations for choosing a low-pass filter prototype, an electric length of a resonator characteristic impedance step and a substrate material are given. The fullwave 3D electromagnetic simulation of the entire filter structure employing the finite element method is performed, showing the offset of transmission zeros and the widening of bandwidth in relation to a frequency response of a filter with ideal elements. The differences between the ideal element filter frequency response and the response of a microstrip filter are due to the inequality of the even and odd phase velocities in coupled microstrip lines. A method of adjustment of the position of transmission zeros and the bandwidth is proposed by adjusting the gaps between the microstrips and the widths of microstrips. In consequence of the adjustments the stopband rejection deteriorates. In this regard, it is proposed to use the low-pass prototype with the minimum stopband attenuation parameter higher than the required by 25−30 dB.
References:

 

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