Measurements and simulations for ground-to-ground plane noise DDR RAM daughter cards and motherboards for EMI emissions

Unintentional noise between the ground planes on a daughter card and a motherboard can be a significant source of EMI emissions from multiple board systems. This noise is not predicted from traditional signal integrity tools, nor from most EMC prediction tools unless a full wave analysis is performed. This work focused on the ground-to-ground noise between a motherboard and SDRAM and DDR RAM DIMM modules. Differential voltage measurements were made to determine the amount of noise voltage between the ground-reference of the motherboard and daughter card (DIMM) module. By analysis of the intentional current on the memory bus lines, the voltage between the ground-reference planes can be estimated with good accuracy.