Tolerance analysis of bypass-, cross- and direct-coupled rectangular waveguide band-pass filters

Tolerance analyses of bypass-, cross- and direct-coupled rectangular waveguide filters are presented. It is found that, contrary to common belief, standard direct-coupled waveguide filters are not less sensitive with respect to their passband behaviour than filters with cross or bypass couplings. Moreover, they are considerably larger than bypass-coupled TM₁₁₀-mode cavity designs, which always present the shortest solution and allow a simple implementation of transmission zeros. The general approach of the tolerance analysis is verified through measurements of a large number of fabricated direct-coupled seven-pole inductive-iris filters. It is concluded that both the path and the radius of the manufacturing end-mill cutter influence the return loss and passband spread of the fabricated filter components.