Spread spectrum clock-an EMC solution for new generation portable computers

Abstract

Electronic equipment often generates radiated electromagnetic interference (EMI) that affects the operation of other electronic equipment. As more electronic systems operate at higher frequencies and bandwidths, more EMI gets generated. In a portable PC, the data bus and the clock are major EMI sources because they conduct high currents and form large loops. Due to lack of shielding, portable computer systems have come under increasing pressure to comply with strict EMI emission regulations, such as FCC and CISPR. Traditional techniques to reduce EMI attempt to contain or to reduce the amount of generated radiation. Coaxial wires and shielded cables are effective at containing radiation, but are expensive, bulky, and inflexible. Metal chassis with sealed seams are effective for reducing EMI of desktop PCs, but portable PCs are kept light by using plastics. Toxic metal paints are sometimes sprayed on plastic housings for portable PCs to provide shielding. A newer technique to reduce EMI is to vary or modulate the clock frequency in the PC. This technique is known as spread spectrum clock, since the frequency spectrum of the clock is spread over a wider range of frequencies. The paper focuses on this method to reduce the peak EMI emissions of the clock and data signals for EM compatibility.