Investigation of Sideway Coupling Effects of Virtual Ground in Three Types of Coupled Line with Mixed-mode Stimuli

Abstract

Some suggested that four-port single-ended scattering S-parameters (simulated or measured) be converted and used to represent mixed-mode S-parameters; the approach we label here as "SE-matrix converted," or simply "SE-conv." SE-conv is often preferred, since the mixed-mode signal sources and probes are not readily or easily available. To employ the SE-conv formulation, the two lines have to be loosely coupled. This restriction curtails in differential bias in the mixed-mode feeding the considerations of 1) existence of virtual ground, and 2) defect ground that may be present in the system ground. First, when the virtual ground existing between lines is not considered (due to loosely coupling assumption), detailed capacitive referencing (line held at +V to virtual ground, and virtual ground to other line held at –V in differential feed) is thus ignored. Three CPL configurations were employed here to investigate the impacts of close coupling, including the effect of virtual ground and its associated capacitive referencing. The progress is reported in this paper. Secondly, effects of ground defect is directly picked up by SE feeding, while in mixed-mode feeding, the effect is somehow reduced (or resisted) by the virtual ground. The investigation on this issue is in progress, and we will report the results later. Keywords— CPL (Coupled Line), Single-ended and mixed-mode feeds, Differential & common mode stimuli, Coupled line, Stripline, Microstrip, and Co-planar ground, Virtual ground, Defect ground, Scattering parameters, Network analyzer