A Microstrip Bandpass Filter with an Electronically Reconfigurable Transmission Zero

This paper presents a type of two-pole microstrip bandpass filter with one reconfigurable transmission zero. The couplings between the resonators and the non-resonant nodes are used to generate the transmission zero. The desired tunability of the transmission zero is realized by changing the center frequencies of the two resonators. In the practical designs, varactors are employed to shift the center frequencies of the resonant node. To verify the working principles, filters implementing the proposed topology are fabricated and measured. Good results have been obtained