A Novel Tunable Combline Bandpass Filter Based on External Quality Factor and Internal Coupling Tunings

A combline filter with quasi-elliptic performance with external quality factor and internal coupling tuning is presented in this paper. In this design, a wide center frequency tuning range with a constant 3-dB FBW (fractional bandwidth) and ABW (absolute bandwidth) is obtained. Also, it is demonstrated that the transmission zero located at the lower edge of passband can be adjusted with little effect on the passband characteristic. A prototype of the tunable filter with a tuning range of 1.27GHz to 2.25GHz is fabricated on a substrate with epsilon=10.2 and h=1.27mm. Through tuning internal coupling coefficient and external quality factor, one can make the filter operate over a wide center frequency tuning range, with a constant 3-dB FBW (fractional bandwidth) nearly kept at 54% or ABW (absolute bandwidth) nearly kept at 500 MHz.