Bandwidth-Compensation Method for Miniaturized Parallel Coupled-Line Filters

This paper proposes a bandwidth-compensation method for miniaturized filters based on short-ended parallel coupled lines. Capacitive loading of such coupled lines is a relatively simple means of reducing the line lengths. In this study, a method is developed that predicts exactly the degree of reduction in the fractional bandwidth due to miniaturization. Using this method, the fractional bandwidth of the prototype coupled line filter can be adjusted, enabling miniaturized filters to maintain the targeted fractional bandwidth. The proposed bandwidth-compensation method applies for any type of filters with coupled lines realized with various transmission lines, with uniform or nonuniform line lengths. Experimental results are also presented that verify the validity of the method.