An abrupt change in microstrip linewidth, commonly called a microstrip impedance step, is a discontinuity that appears frequently in MMICs, such as, stepped impedance transformers and matching networks for example. Therefore, the accurate description of these step-discontinuties is important for the computer-aided design of conventional and monolithic MICs. In this work we have studied the set of modes that appears in a boxed microstrip in order to apply modal analysis in the spectral domain to the study of simple and cascaded step discontinuities. In order to check its precision, our method has been applied to a nine section low-pass filter, made up of a cascade of microstrip step discontinuities. Several filters have been designed, built and tested at different frequencies. We have compared these designs with Touchstone and shown that at low frequencies both methods give similar results, but when the frequency is higher, the Touchstone results are farther from the experimental results than the design obtained with the modal analysis method
