Design of Bandpass Filters with High Mode 5λ/4 Oscillations and Equidistant Transmission Zeros Pairs

The report presents a design sequence of a third-order bandpass filter (BPF) based on parallel resonators and J-inverters. This filter is based on synthesized transmission line (TL) resonators with special properties. These resonators provide the filter with six transmission zeros (TZ) resulting in increased BPF selectivity. At the same time, the cross or mixed couplings are not used in this filter. Moreover, these TZs are three equidistant pairs. Each TL resonator provides an adjustable pair of equidistant TZs. The TL resonators are stepped impedance resonators (SIR) with SA/4 oscillation, whose length is 5 times longer than the quarter-wave resonator. This makes it possible to use these resonators at very high frequencies, including the millimeter range. The design sequence discussed, including established patterns, can be used for N-order BPFs with 2N adjustable TZs. The prototype of a third-order BPF with six TZs is considered.