Compact WR-3 Filter with Improved Rejection Properties by Double Source-Load Cross-Coupling

This paper describes the realization of a fourth order waveguide filter, which is designed in the WR-3 frequency band (220 GHz - 330 GHz). The filter implements a classical quadruplet topology. A cross-coupling between the first and fourth resonator realizes a pair of transmission zeros (TZs) to improve the near passband rejection properties. A source to load (SL) cross-coupling is added to the topology for the realization of further TZs. In comparison to former investigations, the SL cross-coupling proposed here consists of two individual coupling slots. An SL cross-coupling in waveguide technique can be designed to show a dispersive behaviour, wherefore additional TZs can be introduced to the filter response. A double slot coupling aperture increases the degrees of freedom with respect to placing these additional TZs. As a result, in total seven TZs in the WR-3 frequency band are generated. Six of these TZs can be placed nearly symmetrically above and below the passband. An adapted coupling matrix description as well as a discrete equivalent circuit are proposed as well. Furthermore, parameter studies are carried out and measurement results are compared to simulation.