Combined approach to design of microwave interdigital filters

The paper presents a simplified analytical method of calculating interdigital filter circuit parameters based on a narrow-band representation of a coupled line circuit. It allows fast implementation of the analytical calculations of even- and odd-mode characteristic impedances of coupled lines forming an interdigital filter for subsequent CAD calculation. The main advantage of this method is the possibility of choosing various original filter parameters according to the designer's desire. We present also a calculation example and results of an experimental filter fabrication.