Why spread spectrum clocking of computing devices is not cheating

Abstract

Spread spectrum clocking (SSC), also known as clock dithering, is a widely accepted and utilized method of reducing electromagnetic emissions to meet regulatory requirements. By intentionally spreading the energy of the clock harmonics into a band several megahertz wide, the amplitude of the radiated signals can be decreased by factors as high as 20 dB. Unfortunately some members of the radio and telecommunications industries (as well as the EMC community) view this technique as a "cheat". It has been claimed that the reduction in emissions gained by the use of SSC does not translate to a lower probability of interference, and that accordingly, devices that use clock dithering should adhere to stricter regulations. This paper addresses those concerns, and explains why they are not valid.