Microstrip Dual/Quad-Band Filters With Coupled Lines and Quasi-Lumped Impedance Inverters Based on Parallel-Path Transmission

Microstrip quad-band bandpass filters with controllable passband center frequencies are presented and intensively investigated in this study. The newly proposed quad-band filter principally comprises two dual-band filters. Specifically, the two distinct types of dual-band filters with different dual-band generating mechanisms are studied and then combined to provide the quad-band responses. By incorporating parallel coupled lines as input sections and interstage impedance inverters built with quasi-lumped inductors, the type-I filter is constructed with thorough network analysis. As for the type-II filter, with novel quad-pole-splitting techniques analyzed by mixed-mode S-parameters, dual passbands of close band proximity could be realized. Eventually, based on the parallel-path transmission theory along with the two dual-band filters, a novel quad-band filter is thus implemented. The measurement and simulation results show good agreement.