A Simple Microstrip Bandpass Filter with Analytical Design Theory and Sharp Skirt Selectivity

Abstract

A simple coupled-line circuit configuration is proposed to design microstrip bandpass filter with sharp skirt selectivity in this paper. This novel circuit consists of two-section coupled lines with a common connected point. An equivalent odd-mode and even-mode analysis is applied to analyze transmission and reflection property, and the effective analytical scattering parameters' expressions are achieved. Design curves for this novel bandpass filter are given while six typical numerical examples are presented. The presented simulation results show that the reflection zeros in pass band and transmission zeros in stop band can be tuned by changing the even- and odd-mode characteristic impedances of these coupled lines. Finally, two microstrip bandpass filters with different transmission and reflection zeros are fabricated, measured and compared. The experimental results show excellent bandpass filtering property with sharpened rejection skirts and low insertion loss.