A Fourth Order Constant Absolute Bandwidth Tunable Bandpass Filter with Cross-Coupling

A cross-coupled tunable filter with constant absolute bandwidth (CABW) is proposed in this paper. Coupling matrix is used to calculate the coupling coefficient of the tunable filter. Varactor-loaded resonator with separate electric dominant and magnetic dominant coupling region is employed to realize the cross-coupled topology. A prototype is fabricated and measured. The measurement shows that the filter realizes a -3dB absolute bandwidth of 93 ± 7 MHz in the passband with the tunable range from 1.36 to 1.78 GHz. Two cross-coupled transmission zeros (TZs) beside passband are generated, and thus the frequency selectivity is enhanced.