Model driven design of high performance microwave filters

Microwave filters are steadily evolving towards the use of more complex transmission line structures as building blocks. This is needed to cope with the highly demanding specifications of modern telecommunication equipment. 

The design procedures that are used to realize these filters however have not evolved at the same pace. They still rely on the use of lossless, analytical models that are borrowed from standard line structures to approximate more complex, lossy structures. The lack of accuracy that results from this crude approximation is then filled using massive numerical optimization based on EM simulations. This process is both time-consuming and blocks the intuitive insight in the operation of the device. The loss of physical insight makes it harder for the designer to make proper design choices and can lead to designs that are too sensitive.

To circumvent these disadvantages, we propose to introduce model driven design. This is done by using 2 different approaches:

1. Introducing an accurate equivalent circuit model into the design procedure

An accurate equivalent circuit model is extracted for an elementary section of the considered microstripstructure. The discrete parameters of the model are related to the geometrical parameters of the structure. We will maintain the analytical models from the classical design procedure and extend them to make the procedure sufficiently accurate, thereby eliminating the need for optimisation.

2. Introducing metamodeling into the design procedure

The geometrical parameters of the complete microstripstructure are linked to the filter characteristics or the filter specification parameters through metamodels. The metamodels are then used in the optimization process instead of the computationally expensive EM simulations, resulting in an accurate and efficient optimization directly on the structure of interest

Defective ground structure

As a test-case example a defective ground structure filter (DGS) is used (see Figure 1), which can easily be sliced in elementary sections which is need for the first approach. An accurate equivalent circuit model was already successfully extracted for such an elementary section. The next step is to include this circuit into the design procedure. A simple DGS has been designed by using the metamodeling technique to show its feasibility. Using a metamodeling-based design a designer needs more than 5000 times less CPU time compared to an EM-optimization-based design.

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