Center frequency and bandwidth tunable waveguide bandpass filter with transmission zeros

In this paper, a novel center frequency and bandwidth tunable, cross-coupled waveguide resonator filter is presented. The coupling between adjacent resonators can be adjusted using non-resonating coupling resonators. The negative sign for the cross coupling, which is required to generate transmission zeros, is enforced by choosing an appropriate resonant frequency for the cross-coupling resonator. The coupling iris design itself is identical regardless of the sign of the coupling. The design equations for the novel coupling elements are given in this paper. A four pole filter breadboard with two transmission zeros (elliptic filter function) has been built up and measured at various bandwidth and center frequency settings. It operates at Ka-band frequencies and can be tuned to bandwidths from 36 to 72 MHz in the frequency range 19.7-20.2 GHz.