Microstrip BPFs with Increased Selectivity and Asymmetric Frequency Responses

摘要

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 K _(13)= K _(m13)+ K _(e13)(MCC). Its magnetic component K _(m13)is due to a parasitic magnetic coupling between these resonators. To form the electric coupling component K _(e13)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 f _(0)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 f _(0)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 [Ỹ] and its minor M _(31). 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.