Spurious Self-Suppression Method: Application to TM Cavity Filters

This article presents a new approach for improving spurious free bands in filters. The main idea is to control the coupling coefficients of the spurious resonances, in order to force them to cancel each other. This approach is completely different from the classical one where all couplings are minimized in order to excite spurious resonances as less as possible. In fact, in this approach, all spurious couplings are maximized except the central coupling that is instead minimized. This method is first explained in terms of equivalent circuits. Then, in order to show its applicability to real microwave filters, it is applied to the TM cavity filters and used to remove both spurious resonances due to higher order cavity modes and to TE $_{10}$ mode of coupling irises. Design examples of fourth-and sixth-order TM cavity filters are presented. With respect to the classical TM cavity filter, where the first spurious appears around 11.4 GHz, the TM filters designed by exploiting the self-suppression method present spurious free bands up to 20.2 GHz (20 GHz for sixth-order filter), improving the filter response up to 2.2 times the center frequency of 9.2 GHz. Another advantage is that, in contrast to the classical spurious suppression method that does not allow responses with TZs in TM filters, the self-suppression method allows for a number of TZs equal to N-2 (N being the filter order) and it also allows a certain control of their position.