Differences between Theoretical and Measured Spectrum in System Employing a Spread-Spectrum Clock for EMI Reduction Purposes

Many digital systems employ a spread spectrum clock technique for an inherent reduction of the Electro-Magnetic Interference. Spread Spectrum clocking consists in a proper modulation of the system clock, thus reshaping the power density spectrum of all synchronized digital signal. The aimof this paper is to show that, depending on the specific setting, very large differences between the theoretically computed power density spectrum and the measured one arise. This is an important issue to take into consideration when a Spread Spectrumsystem is optimized for EMI reduction.