A Study on Correlation Between the PCB Layout and EMI from Chassis

Investigations of radiated emission from a conductive chassis connected to the GND of a PCB with ICs showed higher emissions from a PCB mounted to a chassis than PCB without chassis. The results also showed that the location of the ground connection between the GND plane on the PCB and the chassis can affect emission levels by up to 9 dB. To determine the path of the noise current from the PCB to the chassis, we measured the current at their junction with a thin-current-probe. The position dependence of the junction current frequency spectra correlated with that of radiated emission spectra. Moreover, EMI levels correlated with distance from the screw to the nearest bypass-capacitor.