Varactor-tuned hairpin bandpass filter with enhanced stopband performance

In this paper, a varactor-tuned hairpin bandpass filter (BPF) using tapped open stub is proposed so that the passband bandwidth can be maintained nearly constant within the tuning range. The tapped open stub not only works as a K-inverter but also introduces an attenuation pole that can be located at upper or lower side of the passband. Varactors loaded at the end of hairpin resonator and the tapped open stub enable the proposed hairpin BPF to control the center frequency with constant bandwidth. Design equations are derived so that the constant bandwidth requirements can be satisfied with reasonable equivalent circuit parameter value. Hairpin tunable BPF with one tapped open stub is designed and tested at 2 GHz. Also, two open stub´s tunable BPF of elliptic-type performance is experimented at 1 GHz.