Issues with electromagnetic interference (EMI) are usually overcome through the use of EMI filters, ferrite beads, or chokes. Alternatively, the designer may opt to add a power layer and groundplane to the board or add more metal shielding, a special coating, and RF gaskets. Because EMI stems from system clocks, however, it can be most efficiently and economically reduced via the use of spread-spectrum-clock (SSC) oscillators. In a seven-page application note titled, “Low EMI Spread Spectrum Clock Oscillators,” Jason Yen of Mercury United Electronics, Inc. explains how engineers can more effectively comply with EMI regulations.
By minimizing the interference generated by embedded clock oscillators at the source, Yen explains that it is possible to reduce the requirements for post-EMI-generation suppression, such as EMI filters, chokes, and ferrite beads. The key is spread-spectrum technology (SST), which spreads source energy over a broader bandwidth and controlled frequency range (for example, center frequency of ±1%) with a controlled modulation rate. With this approach, the total source energy remains the same. The peak energy, however, has been spread out to nearby frequencies.
Spread-spectrum clock oscillator (SSCs) takes advantage of the SST to provide low-EMI frequency sources. In discussing the benefits of these oscillators—and spread-spectrum techniques in general—the note compares center and down spread. It also defines the modulation carrier frequency. The document notes that higher-order harmonic frequencies achieve higher EMI reduction. In addition, the greater modulation percentage more greatly reduces EMI emissions. In addition to all of the harmonics, the fundamental frequency will benefit from reduced EMI reduction with the use of an SST.
Mercury United Electronics, Inc., 9299 9th St., Rancho Cucamonga, CA 91730, 886-2-2406-2779, www.mercury-crystal.com.
