External Q computation by fuzzy inference method; Application to microstrip filter design

Coupling factor of a resonator to an external circuit is described by the external Q which is an important parameter for designing microwave filters. Analytical computation of external Q needs exact determination of the field distribution around the coupling port. But the complexities of I/O ports in microwave/millimeter wave filters make this procedure a very complicated process. On the other hand lack of analytical solution for field distribution in some kinds of resonators such as square open loop resonators cause to increase the complexity of external Q computation. In this paper a very fast and accurate fuzzy inference method is implemented to overcome this drawback. The fuzzy modeling method is explained and a four pole Chebychev filter is designed as an example to show the accuracy and efficiency of the introduced method. The simulated response of the designed filter is compared with an ideal four pole Chebychev one. The results are in a very good agreement.