Microstrip BPFs with Increased Selectivity and Asymmetric Frequency Responses
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ostrip BPFs with Increased Selectivity and Asymmetric Frequency Responses A. V. Zakharov1*, S. A. Rozenko1**, S. N. Litvintsev1***, and L. S. Pinchuk1****
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National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine *ORCID: 0000-0002-1222-1623, e-mail: [email protected] **ORCID: 0000-0002-3525-7127, e-mail: [email protected] ***ORCID: 0000-0002-6171-0036 ****ORCID: 0000-0002-1893-3365 Received August 31, 2019 Revised June 17, 2020 Accepted June 21, 2020
Abstract—Two symmetrical third-order microstrip bandpass filters (BPF) with all mixed coupling coefficients are proposed and analyzed. The first filter is a combline filter with stepped impedance resonators (SIR) closely spaced to each other. This leads to mixed couplings between adjacent resonators. The cross coupling of the end resonators is also mixed K13 = Km13 + Ke13 (MCC). Its magnetic component Km13 is due to a parasitic magnetic coupling between these resonators. To form the electric coupling component Ke13 a thin microstrip line segment was used that connects the resonators through the capacitive gaps. It was found that such a filter has two adjustable transmission zeros (TZ), which can be located both to the right and left of the central frequency f0 of the bandwidth. The second BPF differs from the first in that its central SIR is replaced by a half-wave through-type resonator, which is included in the filter as a two-port circuit. The used half-wave resonator has a U-shape and it works as the admittance inverter, in addition to the resonance phenomenon. This feature leads to a change in the frequency response of the filter. This filter has two adjustable TZs, which are located on both sides of the center frequency f0 of the bandwidth asymmetrically. The direct and inverse problems for the third-order ~ BPF with all mixed couplings are also solved. The solution is based on the conductance matrix [Y ] and its minor M31. The direct problem solution makes it possible to determine the TZs of the filter using the given coupling coefficients. In the inverse problem, the filter’s coupling coefficients are determined by the specified TZs. The samples of two experimental filters and measuring frequency responses are presented. DOI: 10.3103/S0735272720070031
1. INTRODUCTION In microwave technology, there is a practical need for bandpass filters (BPF) with increased selectivity and asymmetric frequency response. In particular, this applies to BPFs that are part of the duplexers [1]. To increase selectivity, transmission zeros (TZ), called attenuation poles, are introduced into the frequency response of the filter. For this purpose, the filter uses cross couplings. The easiest to implement in practice is a third-order BPF with one cross coupling [2, 3]. If simple couplings (electric or magnetic) between resonators are used, this filter has one TZ. In order for the BPF to have an asymmetric frequency response and at the same time to have increased selectivity, it must have at least two adjustable TZs. Moreover, the
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