Bandpass Filters with 2N+1 Transmission Zeros at Real Frequencies

Abstract

This paper discusses N-order bandpass filter (BPF) with all mixed couplings between adjacent resonators, and between end resonators and loads. It has (2N + 1) transmission zeros (TZ) at real frequencies. To the existing (N + 1) TZs generated by mixed couplings, another N TZs are added, which generated by resonators with special properties. Each such resonator is a λ/4 resonator, in which the connection point is located relatively close to the short-circuited end. In this case, the input admittance of the resonator Y(jω) has a pole ω_p, which is located to the right and at a small distance from the main resonant frequency ω₀. This pole ω_p of the Y(jω) function leads to the appearance of a right-handed TZ in a BPF characteristic. If we use N resonators with different connection points, then N additional TZs will appear. The design of that BPF is considered. The measured and simulated frequency responses of the second-order BPF with 5 TZs are presented.