US Patent 7893779 Modulated supply spread spectrum

A system reduces a received RF signal from EMI generated by a digital electronic system that includes a clock. In the present invention the clock frequency, that generates signals and strobes data out, is purposely changed or modulated, by, illustratively, driving the power node of the clock. The typical filter circuit between the clock power node and the power supply is used to advantage in that the filter impedance allows a buffer to more easily drive the clock power node since the low impedance of the power supply is isolated by the filter circuit. The changing of the clock frequency spreads the EMI RF harmonics over a spectrum so that any harmonics received by an RF receiver will be short lived and therefore of small magnitude.