Bandpass filter with flat passband and transmission zeros using parallel-connected resistor loaded hairpin-shaped resonators

A fourth-order flat passband bandpass filter (BPF) is realized using proposed resonators and half-wave open-looped resonators. The fourth-order flat passband BPF is necessary to prepare at least two kinds of resonators that have high and low unloaded quality factors. The proposed resonator is configured based on a half-wave hairpin-shaped resonator with a parallel-connected resistor. It is assigned as a resonator with a low unloaded quality factor. The advantage of the proposed resonator is flexible adjustment of the unloaded quality factor by changing the value and setting position of the resistor. Using equations and electromagnetic simulation, we show that the unloaded quality factor can be controlled by setting the position of the resistor. A half-wave open-looped resonator is assigned as a resonator with a high unloaded quality factor. Furthermore, a fourth-order flat passband BPF with transmission zeros and a flat passband is constructed on a printed circuit board by using the proposed resonators and half-wave open-looped resonators. Flat passband characteristics of a fourth-order BPF is satisfied with the design specifications.