High Selectivity Coaxial Cavity Filter with Two Independently Tunable Transmission Zeros

In this paper design of a high selectivity coaxial cavity bandpass filter with two independently tunable transmission zeros (TZs) at C-band is presented. Initially a three-pole filter with one inductive cross coupling is analyzed using even and odd mode circuit technique. A single transmission zero at upper passband edge is obtained using an inductive cross coupling and its position is varied by adjusting the value of inductor. Finally, a five-pole filter with two inductive cross couplings is realized to create two TZs at the upper passband edge. Inductive cross coupling is introduced between two non-adjacent resonators using a metal boss. Width and height of the metal boss is adjusted to get the required inductive cross coupling. Further, to vary the inductive cross couplings tuning screws are placed at top cover of cavity filter and length of the screw is adjusted to tune both the transmission zeros at the upper passband edge. Proposed five-pole filter is centred at 3.95 GHz with 500 MHz bandwidth. Experimental results show designed filter offers a sharp rejection of 40 dBc at 3.5 percent away from upper passband edge and also position of TZs can be tuned independently from 4.35 GHz to 4.5 GHz.