Shiver Me Timers! Using Spread Spectrum Clock Generators

By Dr. Glen Dash, Ampyx LLC

Most methods used for EMI suppression have hardly changed in the last quarter century. The standard tool kit has largely consisted of components that filter and shield, and methods for improving layout. While these tools have become refined over time, nothing much in the kit would have seemed alien to an RF designer of a generation ago.

It was only in the last decade or so that a truly new approach was proposed, the spread spectrum clock generator (SSCG). It reduces EMI by modulating the clock frequency, thereby spreading radiated energy over a frequency range wider than the bandwidth of the standard receiver used to measure emissions. At any one frequency, the detected emission falls.

Spread spectrum techniques are hardly new. The tale of their invention is oft-told, and usually begins with the filing of a 1942 patent by George Antheil and the actress Hedy Lamarr. The patent disclosed a method of frequency hopping used to immunize radio controlled torpedoes from jamming. In the last two decades, the technique has become a fixture in the communications industry because of its advantages in range, immunity, and security.

The use of spread spectrum clock generation to reduce emissions in digital devices was first proposed in a paper presented at the 10th Annual Power Electronics Seminar at the Virginia Power Electronics Center in 1992 (Ref. 1). Author Lin described the modification of a switching power supply circuit so as to modulate its switching frequency, thereby smearing its emissions over a broad swath of the spectrum. In the particular application described, a power supply's nominal switching frequency of 90 kHz was frequency modulated with a sine wave producing a frequency variation off /- 15 kHz at a rate of 400 Hz. Substantial reductions in emissions resulted.

At the 1994 IEEE International Symposium on Electromagnetic Compatibility, authors Keith Hardin, John Fessler, and Donald Bush presented the results of their work to develop practical spread spectrum clock generator for use as a CPU clock in digital devices. The wide modulation frequency proposed by Lin was impractical for use as a clock generator. The authors proposed to vary the frequency of the clock only slightly, deviating a 20 MHz clock by /- 125 kHz, a variation of only .625%. This resulted in a measured attenuation at the third harmonic, 60 MHz, of only 2 dB. However, as the harmonic number increased so did the attenuation. At the 20th harmonic, 400 MHz, the measured attenuation was 10 dB.