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Ultra-wideband bandpass filter based on a multimode stripline resonator with extremely wide stopband

DOI 10.18127/j20700784-201812-06


Ya.F. Bal’va – Ph.D. (Phys.-Math.), Research Scientist, Kirensky Institute of Physics , Siberian Branch, Russian Academy of Sciences (Krasnoyarsk)
A.A. Leksikov – Ph.D. (Eng.), Research Scientist, Kirensky Institute of Physics , Siberian Branch, Russian Academy of Sciences (Krasnoyarsk)
E.O. Grushevsky – Post-graduate Student, Kirensky Institute of Physics , Siberian Branch, Russian Academy of Sciences (Krasnoyarsk)
M.O. Savishnikov – Post-graduate Student, Siberian Federal University (Krasnoyarsk)
V.S. Denisenko – Post-graduate Student, Siberian Federal University (Krasnoyarsk)

Frequency-selective devices of microwave frequencies and, in particular, band-pass filters are among the most important elements of communication systems, radar, radio navigation, various measuring and special radio equipment. A common disadvantage of the known approaches to designing ultra-wideband (UWB) filters is a relatively narrow high-frequency stopband, which is maximum extends to a frequency around 4f0 at a suppression level less than 40 dB. Another serious limitation of the overwhelming majority of known UWB filter designs is an absence of a closed shield housing, which significantly degrades electromagnetic compatibility of filters and limits its field of application.
In this paper, we propose a novel structure of a miniature multimode stripline resonator, which can be implemented to an ultra-wideband filters design with characteristics substantially better than those of known analogs. A cross-shaped conductor located on top side of a dielectric substrate, which is short-circuited to the screen from one end, and three short-circuited conductors from another, form the resonator. The strip conductors are stepped-impedance, which substantially reduces the dimensions of the resonator. The presented frequency response of a UWB filter based on the resonator under investigation proves that a passband of the filter is formed by the three eigenfrequencies f01, f02 and f03 of the three-mode resonator. Investigations of the effect of design parameters of the proposed resonator on the frequency-selective properties of filters based on it are presented. The relative bandwidth of the filters based on the resonator can be varied within considerable limits that is important the practical point of view a filters design. It was found that the distance LG from the ground to the point of connection of the conductors to the cross mostly effects on the passband width. The dependence of the relative bandwidth of the filter Δf/f0 versus LG is given. It can be seen that with an increase in LG, there is an almost linear increase in Δf/f0 in the range from ~ 20% to 120%.
A fifth-order filter was designed and fabricated with a central frequency of the passband f0 ≈ 1 GHz and its relative width Δf/f0 = 101%. The filter was made on 0,5-mm alumina substrate. The substrate was suspended on 4,5 mm from up and bottom screens. The minimum insertion loss in the bandwidth of the manufactured filter was only 0,15 dB. The existence of an attenuation pole close to the high-frequency slope of the bandpass significantly increases the selectivity of the structure. The measured stopband at a level of -55 dB extends up to frequency 8f0. The calculated and measured frequency response of the transmission and reflection coefficient of the manufactured filter is presented.

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