RF Emission Analysis of Differential Busses

Modern interconnect standards put increasingly stringent requirements on signal integrity and EMI. Application of differential signalling with certain restrictions on signal levels, termination, timing and unbalance can solve the signal integrity issues and it is often assumed that it will eliminate EMI issues as well. The RF emission of differential busses is affected by driver IC properties, signal specifications and 3D layout parameters. This paper presents a methodology to combine models at the IC level with 3D EM models. Examples calculated for the PCI Express bus demonstrate that both signal properties and geometry parameters are important and that achieving low EMI is not as trivial as often suggested in the standards and/or the product data sheets.