Synthesis scheme for wideband filters consisting of three-coupled-lines including the cross-coupling between non-adjacent lines

To accurately design wideband filters composed of three-coupled-lines, an improved equivalent circuit is derived by taking the cross-coupling between non-adjacent lines into consideration. Since the cross-coupling between non-adjacent lines is also dependent on the coupling between adjacent lines, conventional synthesis techniques cannot be used directly. Instead, an improved synthesis scheme based on successive iterations is proposed to synthesise the filters. As examples, two 2-stage microstrip three-coupled-line filters [bandwidth: 2.67-5.33 GHz (fractional bandwidth: 67%)] with a reversed connection sequence are synthesised, simulated, and experimentally realised to validate the proposed theory. The frequency responses of the designed filters using the authors' new model and synthesis theory compare well with those by simulation and experiment, whereas an error of about 17% is found in bandwidth if the authors use the conventional model that neglecting the cross-coupling between non-adjacent lines. In addition, the asymmetric property of the three-line unit is revealed.