Fully tunable filter with cross coupling and reconfigurable transmission zero

摘要

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