Fully Tunable Filter With Cross Coupling And Reconfigurable Transmission Zero

In this article, a novel coupling structure is proposed. The surface voltage and current distribution on this structure can be tuned by changing the ratio of the two capacitances so that the central frequency and coupling coefficient can be tuned. The coupling structure can also be tuned to change the sign of coupling coefficient, which is utilized to control the transmission zero of cross coupled tunable bandpass filter. A second order bandpass filter with tunable central frequency and bandwidth is designed and measured. The measurement shows that the central frequency tuning range is 0.768 to 1.512 GHz and the fractional bandwidth tuning range is 0.052 to 0.18 at 1.14 GHz. It shows that the filter can also be tuned with constant fractional bandwidth or constant absolute bandwidth. A tunable cascaded triple bandpass filter with tunable central frequency and bandwidth is designed and measured, as well. The cross-coupled transmission zero can be configured to locate at either high side or low side of the passband by changing the coupling type of the filter topology.