Removal of Specific Harmonics by Rise Time Control

It is well known that increasing the rise/fall time of a digital signal reduces the high-frequency spectral content, thereby enhancing the chances of meeting regulatory electromagnetic interference (EMI) requirements. Unlike the EMI regulations, most radio-frequency interference (RFI) issues in mobile devices are band limited. Given the growing concern regarding timing constraints of high-speed serial links, increasing the rise time cannot sufficiently suppress a specific harmonic that causes RFI. In this study, we demonstrate that a certain harmonic can be practically removed by making (not necessarily increasing) the rise time a multiple of the reciprocal of that harmonic frequency.