Balanced and Unbalanced PCB Layer Stack-up for Controlling Radiated Emissions

Canceling mutually opposing fields of a closed circuit can control differential-mode current radiation. When high frequency components of digital pulses fail to form a closed circuit, they often radiate or become common-mode noise. In spite of prudent EMC design, high frequency common-mode current is inadvertently produced, especially in large PCBs. This paper describes circumstances in which high frequency common-mode current production is difficult to control in power and ground layers of PCBs. Methods to solve radiated emission problems using a combination stack-up consisting of balanced and unbalanced power and signal ground layers are explained. Both high and low frequency characteristics of PCB layer stack-up are reviewed and the effectiveness of interlayer inductance and capacitance in minimizing common-mode generation and radiation is analyzed. Radiated emissions measured on experimental PCB stack-ups for balanced and unbalanced power and signal ground layers are presented and the results discussed.