Frequency Modulation of System Clocks for EMI Reduction

Abstract

The proliferation of electronic products in the home and office is putting increasing pressure on every product to reduce its electromagnetic interference (EMI). At HP’s Integrated Circuit Business Division (ICBD), several different methods have been used to help deal with EMI directly on-chip, among them frequency modulation of the system clock, also called clock dithering, and control of pad output rise and fall times over process, voltage, and temperature (PVT) variations. This latter method is also called adjustable output pad (AOP) control, and sometimes includes programmable adjustment for capacitive loading. This paper focuses on clock dithering as an on-chip technique for EMI reduction. It reviews the basic concept, the work that has been done at several different ICBD design centers and elsewhere, ICBD customer experiences with that work, and lessons drawn from these experiences about design, effectiveness, and customer implementation with ICBD. The paper closes with a brief review of the costs and benefits of implementing dithering and a summary of what customers can expect when working with ICBD. This paper does not aim to be a comprehensive description of dithering circuitry and mathematics, but rather a more narrative description of experiences and rules of thumb. See reference 1 for a more detailed discussion of circuit implementation.