"A Novel Capacitive Cross - Coupling for Enhancement of Microwave Cavity Filter"

Abstract

Normally, the microwave cavity filter is cast and gilded, which causes a fixed transmission zero point. Thus, the specifications of the filter cannot be turned during its operation. A novel enhancement capacitive cross-coupling structure is proposed to fabricate a microwave cavity filter with a turnable transmission zero point. This structure is implemented by adding a new tap screw to the center of the supporting Teflon piece of cross-coupling dumbbell resulted in an adjustable capacitor. The capacitive cross-coupling factor is achieved a greater value leading to an improved slope of the cavity filter. The simulation results show that cross-coupling bandwidth can be varied in the range from 35.4 MHz to 40 MHz by changing the screw length from 0 mm to 7 mm, respectively. To verify the efficiency of the novel design, this structure is used to fabricate a microwave cavity filter in eNodeB for Band 3 - Long Term Evolution (LTE) application. The filter is built up of 10 coaxial resonator cavities with two new capacitive cross-couplings. The simulation results of the filter fully meet specification requirements including a band of 1805 to 1880 MHz, return loss below -12.7 dB, passband ripple of 0.1 dB, and insertion loss below 0.2dB. For a bandwidth below 20 MHz, the signal rejection reduces 1 dB by changing the screw length from 0 mm to 6 mm. In addition to Band 3, this structure can be developed for different frequency bands.