Design of a new type micro-tri-mode filter

In this paper, a new type micro-tri-mode filter with simple structure is presented. On the basis of the square ring dual-mode filter, it can introduce the third mode by loading the open stubs in the center of the square ring dual-mode filter. Adjusting the size of the open stubs can reduce the resonant frequency of the third mode, so it can form a micro-tri-mode filter. Meanwhile, through the introduction of capacitive coupling between the source and the load, so that this filter has three transmission zeros and can be adjusted by the direct coupling between the source and the load to change location of transmission zeros. The results of simulation are very good. The passband is 2.85 ∼ 3.15 GHz. The insertion loss is greater than −2dB and the return loss is less than −12dB, and there are two transmission zeros at 2.45 GHz and 3.45 GHz.