Electronic Design

Continuously Programmable Spread-Spectrum Clock Minimizes EMI Problems

As computer and digital systems migrate toward higher frequencies, reducing undesired electromagnetic emissions to meet FCC standards becomes a challenge. Consequently, the pressure on clock generator chips and other ICs is tremendous. One company has developed a line of spread-spectrum clock ICs that are continuously programmable.

The FS7XX family from International Microcircuits Inc. (IMI) lets engineers continuously program the amount of variation or modulation of the system clock for minimal EMI radiation. By using programmable spread-spectrum clock ICs, engineers also can obtain up to a 20-dB reduction in system-wide EMI.

Traditionally, spread-spectrum clock generator chips offer fixed modulation periods. As a result, they cannot be tuned for optimum performance. Unlike those with fixed modulation periods, the FS7XX family offers variation in clock frequency from 0.6% to 4%. This is accomplished by selecting a proper loop-filter value. As the frequency-variation range gets wider, the clock energy is spread over side bands, and the radiated EMI energy levels drop. The continuously programmable feature ensures that the system is tuned for optimal performance. In other words, it radiates minimum energy without affecting system performance. Also, in reality, small variations don't degrade system operation because most logic states change at or during the clock edges.

To keep crystal costs low, the FS7XX members offer 1X, 2X, and 4X clock multipliers. Consequently, engineers can use a 4X multiplier with an inexpensive 25-MHz crystal to achieve 100-MHz clock. While the operating frequency range for FS78X devices is 6 to 82 MHz, the FS79X members cover 80 to 140 MHz. Based on 0.6-µm CMOS, the typical power dissipation is 37 mW for 3.3-V FS78X units, and 73 mW for the FS79X version.

All devices come in 8-pin SOICs. The FS78X also comes in 8-pin TSSOPs. In 1000s, pricing starts at $1.13.

International Microcircuits Inc., 525 Los Coches St., Milpitas, CA 95035; (408) 263-6300; www.imicorp.com.

See associated figure.

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