Backplane grounding models for controlling common-mode noise and radiation

Common-mode noise unintentionally generated in electronic systems must be controlled to reduce unwanted conducted and radiated emissions and crosstalk. Because common mode noise in a backplane, when coupled into cables attached to the backplane, causes those cables to acts as a monopole antenna, such noise must be minimized. This paper models backplane design techniques to extract common mode noise current from power layers, signal ground layers, and frame ground layers and to drain them to the chassis. A method of gradually lowering the impedance of power layers and signal ground layers with respect to frame ground layers (and frame ground layers with respect to chassis) is explained. This gradual reduction in impedance enables common mode noise in backplane power layers to flow differentially with respect to signal ground layers. Similarly, it allows common mode noise in signal ground layers to drain to frame ground layers and from frame ground layers to chassis. These models were implemented in a large backplane and the resultant measured radiated emissions are presented.