A novel reconfigurable bandpass filter with compensable coupling based on microstrip-to-CPW structure

A reconfigurable bandpass filter with compensable coupling and constant absolute bandwidth (ABW) is proposed. The fundamental structure is based on two coplanar waveguide (CPW) resonators and a microstrip-to-CPW transition. Four varactors embedded in the resonators and feed-lines are utilized to adjust center frequency and external coupling, separately. Moreover, this novel feeding structure generates an adjustable transmission zero (TZ) at lower stopband. For demonstration, a second-order filter is implemented with center frequency range of 1.87-2.37 GHz (i.e., 23.6% fractional tuning range) and a constant 3-dB ABW of 410±10 MHz. Specifically, the return loss of passband at each tuning state can achieve greater than 15 dB.