Identifying EM Radiation from a Printed-Circuit Board Driven by Differential-Signaling

Abstract

Recently, differential-signaling (DS) techniques such as low-voltage differential signaling (LVDS) have come into wide use in digital electronics devices to suppress electromagnetic interference (EMI). In this paper, we attempted to newly identify the frequency responses of the EM radiation from a PCB driven by LVDS, using three EMI-antenna models. EM radiation is modeled and analyzed as EMI antennas, which depend on the configuration of the PCB. The first EMI antenna is a loop type due to the signal current flowing on the paired lines (EMI antenna I). The second EMI antenna is comprised of the ground plane and cable for a dipole type antenna due to a common-mode current flowing along the PCB with cable (EMI antenna II), and the third EMI antenna is comprised of the trace on the ground plane for the loop-type antenna due to the signal current (EMI antenna III). It is demonstrated that the larger EMI antenna, which consists of the ground plane, is the dominant horizontal-component radiation factor at the lower frequencies. The proposed model can explain the characteristics of EM radiation from a PCB driven by differential signaling and also identify the primary radiation factor. The antenna model provides enough flexibility for different geometrical parameters and increases our ability to provide insight and design guidelines.