System-clock IC lets you program spectrum spread to reduce EMI

-- EDN, 2/17/2000

What's not programmable these days? With the FS7xx family of clock-generator ICs from International Microcircuits, you can program the spread-spectrum variation of the nominal clock frequency to better suit your EMI needs and constraints (Picture). For PCs, peripherals, embedded systems, and similar applications, the tunable clock sources help you meet regulatory requirements of various countries and authorities.

You can set the variation at 0.6 to 4% of nominal value. Such programmability lets you find the balance point between wider variation for more spread of clock energy (and consequently lower EMI) versus too-aggressive clock variation. This variation may induce system unreliability because of marginal component timing and margin.

The FS7xx family is available for 6- to 82- and 80- to 140-MHz output-clock bands. Within each band, you can get 1, 2, and 4x multipliers so that you can use a lower frequency, less expensive clock crystal if you prefer. For example, you can get 100-MHz output from a 25-MHz crystal with a 4x unit. Prices for these programmable spread-spectrum-clock ICs, which come in eight-pin SOIC packages, begin at $1.13 (1000).

International Microcircuits Inc, www.imicorp.com.

-by Bill Schweber