Tools for Combline Filter Modelling

摘要

In order to design filters effectively, it is necessary to be able to predictfrequency characteristics of a certain kind of filter structure suitably fast. Considering combline filters, instead of full 3-D field analysis program such as HFSS, a fast circuit simulator program is often a good choice for a modelling tool, especially in the early stages of new filter design. Modelling by a circuit simulator program works quite well due to the fact, a combline filter structure behaves like a multiconductor transmission line (MTL) or a set of MTLs cascaded, when assuming the frequency low enough. In higher frequency band, this MTL method doesn't work anymore. Using a circuit simulator means of course that we have to know the circuit parameters for certain physical structures. In case of MTLs, we have to be able to compute the MTL per-unit-length parameter values, the values of capacitances and inductances between the different conductors. Also, it may occur that one is willing to model the line discontinuities or losses. Then, fringing field capacitances and line attenuation factors can be used, respectively. In general, using a circuit simulator is fast and handy. Some helding tools are needed, however, to compute the circuit parameter values by numerical field analysis.