Theory and Realization of Simple Bandpass Filters with Antiparallel Configuration

This paper introduces a theoretical analysis as well as a design example for bandpass filters (BPF) with a distinctive topology. Based on the analysis of simple two-port symmetrical lossless networks with a parallel structure, a method for obtaining normalized BPF prototypes with desired bandwidths was developed. These prototypes can be scaled to any central frequency and symmetrical real termination in the same way as conventional filters. It is also demonstrated that with a slight modification of the basic BPF prototypes, transmission zeros with controllable frequencies can be introduced in both the lower and the upper stopband region. Such modified prototypes are more convenient for the realization of printed filters than the basic BPF prototypes. The proposed filters have almost identical characteristics in the broad vicinity of the passband region either when composed of ideal lumped elements or of transmission lines (TLs). Due to its simplicity, the proposed concept could be applied for the realization of a printed BPF at a large variety of PCB types, substrates and practical design configurations. A microstrip BPF model is realized for the experimental verification of the presented theory. The measured and theoretical results show excellent agreement, confirming the proposed concept and the exactness of the methodology.