Filter design using CAD. II. 2.5-D simulation

Part II of this paper addresses the shortcomings of linear circuit simulators when designing distributed filter structures with center frequencies of around 3 GHz and above. Linear simulators tend to yield results that do not reflect all of the physical parameters that are associated with a practical layout of a device and an electromagnetic solution can often provide more realistic results. Additional effects modeled by an electromagnetic simulator include adjacent line cross coupling (which has most effect on in band response) and propagation of unwanted modes in the dielectric and housing structure (which effects the stop band response). In addition, extra information such as current distributions on conductors can be obtained which yield useful information on current crowding in power applications and radiation loss from high Q structures. Using the previous example of a 2.4 GHz narrowband edge coupled filter as a case study, this paper will take the linear simulation results, practical measurements and compare these with electromagnetic simulations using HP-EEs of Momentum^TM. Results for both the frequency response and current distribution inside the filter are presented. Comment will also be made on “non-standard” structures i.e. those that are not found in model libraries. The role of electromagnetic simulation in this instance is discussed and results for some examples of such structures and their application are presented