Low-Reflection Signal-Interference Single- and Multipassband Filters With Shunted Lossy Stubs

Abstract

Signal-interference single- and multiband bandpass-type transversal filtering sections (TFSs) with input- and two-port low-reflection (LR) behavior are reported. These LR TFSs consist of the following three parts: 1) a reflective-type TFS shaped by two in-parallel unequal length transmission line segments; 2) a lossy open-ended stub shunted at the input or both at the input and output nodes of the building reflective-type TFS for the input- and two-port-LR TFS cases, respectively; and 3) input/output power-matching transmission line sections. In this manner, the stopband RF-signal energy that is not transmitted by the reflective-type TFS is dissipated by the resistor(s) of the lossy open-ended stub(s) instead of being reflected back to the source. This allows LR capabilities to be obtained in the overall modified TFSs. The operational principles of the proposed input- and two-port-LR TFSs, along with theoretical design examples and their comparison, are presented. Moreover, multi-TFS-stage LR bandpass filters (BPFs) are shown. They are based on the in-series cascade of several LR TFSs to enlarge the bandwidth and power rejection levels of the stopbands while keeping their LR nature. For experimental validation purposes, microstrip prototypes of a three-TFS-stage BPF with broadened input-LR stopbands and a quintuple-band input-LR TFS are developed and characterized.