Optimization of 3D multilayer RF components using the design of experiments (DOE) technique

The successful use of the design of experiments (DOE) approach in an optimization feasibility study for two microwave balanced to unbalanced transformers (baluns) is presented. The medium of interest is the multi-layer low temperature cofired ceramic (LTCC) and two different topologies, one microstrip and one stripline, are investigated. The design goals are perfectly balanced outputs from 2-3 GHz and a resonant frequency of exactly 2.4 GHz. It is demonstrated, using only eight simulations, that perfectly balanced outputs are not possible under given conditions in the case of the microstrip balun. Nevertheless, the stripline balun can be optimized due to its almost symmetrical structure, and both simulations and measurement results verify the conclusions. The DOE method is very simple to implement and gives a clear understanding of the system behavior at the beginning of the design process, reducing the amount of work required for achieving the design goals by orders of magnitude compared to the widely used trial-and-error approach.