Varactor-tuned hairpin bandpass filter with an attenuation pole

In this paper, a varactor-tuned hairpin bandpass filter using a tapped open stub is studied. In this configuration the filter´s center frequency as well as the bandwidth can be controlled. The tapped open stub introduces the attenuation pole which can be located near the passband by adjusting the stub length. Varactors loaded at the end of hairpin resonator and the tapped open stub enable us to control the center frequency with constant bandwidth as well as to have a large tuning bandwidth. Based on the detailed analysis, the varactor-tuned hairpin bandpass filter is designed, in which the tuning bandwidth reaches 15 % of the center frequency at 2 GHz.