January 7, 2008 -- Dealing with noise and electromagnetic interference (EMI) is an inevitable challenge in any high-speed digital design. Digital signal processor (DSP)-based systems that handle audio-video and communication signals can be particularly vulnerable to these disruptions. The designer should know in advance the potential sources of noise and radiation, and design upfront to minimize these disruptions. Smart planning can save considerable time and rework in the debugging stage, thus saving overall time and cost.

Today's fastest DSPs run at internal clock rates in the gigahertz range, while transmitting and receiving signals at frequencies measuring in hundreds of megahertz. These fast-switching signals can generate considerable noise and radiation which degrade system performance and creates high levels of EMI. It is essential to address noise and radiation problems from the very beginning of the design. Many new designs fail first-time electromagnetic compliance testing for Federal Communication Commission (FCC) certification. Investing a little time in low-noise and low-radiation design methods early in the design can minimize late-stage redesign costs and delays in the product shipment date.

From the start of the design, developers should aim for: robust power sources with low switching noise under dynamic loading conditions; minimum crosstalk between high-speed signal traces; high- and low-frequency decoupling; and good signal integrity with minimum transmission line effects. By working to achieve these goals, developers can avoid the pitfalls of noise and EMI.

By Thanh Tran. (Tran is a senior member of the technical staff at Texas Instruments, Inc.)