Direct Synthesis and Design of a General Sequentially Coupled Wideband Bandpass Filter With $N$ -Transmission Zeros

This paper presents a general sequentially coupled wideband bandpass filter and its direct synthesis and design theory. The filter can achieve the same number of controllable transmission zeros (TZs) as that of resonators above the passband. Each resonator consists of a pair of shunt connected capacitively loaded stub and a grounded inductive stub. Resonators are connected through a short section of transmission line with designated electric length. The short connecting lines allow the filter a wide spurious free stopband. For given specifications and prescribed TZs, the elementary lumped element circuit model can be directly synthesized. Three basic lumped element circuit building blocks are approximated by legitimate distributed element circuits with which the wideband distributed element circuit can be systematically obtained. To demonstrate the performance of the proposed filter configuration and the effectiveness of the developed theory, a fourth and an eighth order of filter prototypes were designed and measured, showing the promising potential for practical applications.