Combined signal‐interference/split‐type microwave multi‐band bandpass filters

Abstract

AbstractTwo classes of RF/microwave planar quad‐band bandpass filters (BPFs) that simultaneously exploit transversal signal‐interference and multi‐passband‐splitting mechanisms are reported. The first approach makes use of a triple‐band bandpass transversal filtering section (TFS) composed of two in‐parallel stepped‐impedance coupled‐line signal‐propagation paths. By inserting an open‐ended stub at the input and output terminals of this TFS, its middle passband is divided into two sub‐bands to attain an overall quad‐passband filtering response. The second configuration employs a dual‐passband TFS based on a coupled‐line directional coupler with open‐ended stubs at its direct and coupled ports. By loading such TFS with a replica of it at the direct and coupled ports of its coupler, the two passbands are split into four sub‐bands to obtain the quad‐band functionality. The RF operational foundations of these quad‐band BPFs, along with single‐ and multi‐stage design examples featuring sharp‐rejection four‐passband filtering responses with transmission zeros (TZs), are detailed. Moreover, for experimental‐demonstration purposes, two microstrip prototypes are developed and tested. Besides, as a further application, a two‐layer balanced‐circuit version with ultra‐broad common‐mode (CM) suppression of the constituent triple‐passband TFS of the first prototype is built and measured.