Coupling Matrix Description of WR-3 Waveguide Filter with Multiple Transmission Zeros Created by Source to Load Cross-Coupling

In this paper, a fourth order source to load (SL) cross-coupled WR-3 band (220–325 GHz) waveguide filter is presented. Due to the dispersive characteristic of the SL cross-coupling, two extra transmission zeros (TZs) are obtained, which are not predicted by the common coupling matrix theory. One of these TZs can be positioned freely on the frequency axis while the position of the second one depends on the position of the first one, allowing the realization of asymmetric filter responses. It is shown that the SL coupling proposed in this filter set-up reveals a quadratic frequency dependency. Based on this observation, a coupling matrix description which agrees well with an appropriate discrete equivalent circuit as well as the simulation results can be found. A prototype is manufactured with a high precision CN C milling machine as proof of concept.