Spurious suppression through the control of their couplings: application to TM cavity filters

In this paper, a new approach for improving filter spurious free band is proposed. The main idea is to control the coupling coefficients of the spurious resonances in order to force them to cancel each other. The required coupling levels can be estimated by using an equivalent circuit. It is observed that, in contrast with what is commonly done, it is not convenient decreasing all couplings to the spurious resonances. The only coupling to be minimized is the central one, all other should instead be maximized. This procedure can be used for suppressing spurious resonances generated by higher order modes. This method has been here applied to TM cavity filters. As shown in the paper, it is in general applicable to even order filter only. In this paper, a two-pole TM cavity filter centered at 9 GHz with a 3 dB FBW of 1.45% has been designed and manufactured. By applying this technique, the spurious free band has been increased from 12.4 GHz of the initial design to 19.7 GHz. Measured results show the feasibility of the proposed approach.