Microstrip Quazi-Elliptic Bandpass Filter With Two Pairs of Antiparallel Mixed-Coupled SIRs

An analysis of the second-order mixed-coupled bandpass filter (BPF) of a general form is carried out. The resonators in this BPF are presented by a resonant frequency f_{0} and a susceptance slope parameter b . The position of the transmission zero (TZ) generated by the mixed coupling is determined. A correspondence is established between the mixed coupling representation as the sum of magnetic K_{\mathrm{ m}} and electric K_{\mathrm{ e}} components and the linear representation form m (\Omega) = m_{0} {-} a \,\,\Omega . It is shown that the slope of the frequency dependence m(\Omega ) is negative. The conditions for equidistant arrangement of two TZs in BPF with mixed couplings between adjacent resonators are formulated. A fourth-order microstrip quazi-elliptic BPF with two pairs of antiparallel mixed-coupled stepped-impedance resonators (SIRs) is proposed. It reduces the parasitic interaction between nonadjacent resonators and makes it possible to obtain the required frequency responses more accurately. The results of the measurements and simulation are presented.