A contribution to the computer-aided design and realization of microstrip filters

In the design of microstrip filters it is important to obtain filters with characteristics which correspond without adjustment to the desired filter properties. Quasi-static procedures currently used provide fairly accurate results when the dispersion of the lines is not too high. The remaining task for the engineer is related to the appropriate realization of line impedances in microstrip technology. A description is presented of approaches which make it possible to calculate with good accuracy the required geometrical data from the line characteristics. In calculations regarding a single microstrip line, it is possible to neglect the dispersion of the fundamental mode as far as the variation over the filter bandwidth is concerned. However, this dispersion has to be considered to obtain the desired center frequency and the line impedances. Results of any degree of accuracy can be obtained by applying Galerkin's method to the current densities on the strip. The considered procedure makes it possible to calculate the effective permittivity and the impedance of the line as a function of geometry and frequency. The synthesis requires the solution of the inverse problem. Results related to the design of low-pass, band-pass, and band-stop filters have been obtained with the aid of a computer program implementing the described method.