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Experimental prototype of microstrip BPF

Microstrip bandpass filter with left transmission zero controlled by parasitic cross coupling

Alexander V. Zakharov, Sergii A. Rozenko, Ludmila S. Pinchuk


A third-order microstrip filter is proposed and studied. It is characterized by a left-handed transmission zero fz, caused by the parasitic cross-coupling between non-adjacent resonators. The filter contains a half-wave middle resonator and two quarter-wave resonators located from different sides, near the open ends of the middle resonator. The coupling between all resonators has magnetic character, and the zero of the filter transfer function fz is located to the left of the center frequency of the passband f0. Such filter is described by a modified coupling matrix, where one of the main coupling coefficients is artificially assigned a minus sign. In the proposed filter design, for a given value of the main coupling coefficients, it is possible to provide different values of the cross-coupling coefficient by appropriately selecting the design parameters. This allows adjusting the zero position of the transmission fz for a given bandwidth of the filter, thereby changing the left slope of the amplitude-frequency characteristic. A sequence of steps is proposed for constructing such a filter. The measured and simulated frequency characteristics of the experimental filter are given.


band pass filter; microstrip line; cross-coupling; zero transmission; coupling matrix

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