Planar Bandpass Filters Based on Resonators Generating Transmission Zeros [Feature]

This review discusses a relatively new approach that differs from the previous two approaches using cross couplings and mixed couplings to form transmission zeros (TZ) in bandpass filters (BPF) with parallel-type resonators and admittance inverters. The TZs improve frequency responses of BPF and make them more diverse. The new approach allows for a significant increase in the TZ number in an N-order BPF from (N+1) to (3N+1). It is based on the idea that each resonator adds one or two TZs to the transfer function of BPF, which are located at real frequencies. These TZs are due to one or two anti-resonant frequencies of the resonator, which are adjustable and close to the operating resonance frequency. Planar resonators implement a unique arrangement of resonant and anti-resonant frequencies. The resonator topologies contain uniform or stepped-impedance transmission line segments. The lumped elements can be connected to the input of these resonators. This article presents seven resonators with one anti-resonance frequency and 11 ones with two anti-resonance frequencies close to the operating resonance frequency. BPF examples with few resonators with many TZs and various frequency responses are considered. Thus, the second order microstrip BPF with combline topology has 7 TZs. Using the previous two approaches, we will have at least six resonators to implement 7 TZs. The properties of BPFs on resonators generating TZs are systematized. Prospects for further approach development in generating TZs by the resonators are indicated.