MEMS attack interference with spread-spectrum scheme

PORTLAND, Ore. — MEMS oscillator maker SiTime Corp. said Monday (Nov. 3) it is expanding into the electromagnetic compliance business with a family of MEMS electromagnetic interference products.

SiTime (Sunnyvale, Calif.) said its spread-spectrum clock oscillators are designed as drop-in replacements to bring prototypes into compliance with Federal Communications Commission rules. They also meet most compliance standards for Canada, Europe, Japan and Taiwan. The devices aim to reduce cost and development time.

Electromagnetic compliance standards ensure that electronic equipment does not radiate RF energy that nterferes with nearby equipment or cause health hazards. Measuring interference can only be done after a prototype is completely assembled, since final packaging influences the radiation pattern. If a problem is detected, expensive and time-consuming engineering is required, ranging from redoing board traces to adding shielding.

SiTime's spread-spectrum clock oscillators are designed to reduce the interference footprint of a finished prototype. The clock works by distributing the radiated EMI energy around a center frequency—say, modulating a 100-MHz clock over a 2 percent bandwidth between 99 MHz and 101 MHz—thus lowering its overall output to within standards, without a redesign.

“When you move the rising edge of the clock around a little bit, then you are moving the energy of that clock around the center frequency,” said Piyush Sevalia, vice president of marketing at SiTime. “That's the idea behind spread-spectrum clocking—you are distributing the energy over a bandwidth.”

The SiT9001, which allows designers to drop in a clock oscillator that modulates clock frequency around a target frequency from 1 MHz to 200 MHz. The SiT9002 offers differential outputs. Both modules use a triangle wave to modulate the clock frequency by up to 2 percent, centered before, at or after the target frequency.

SiT9001 measures 2.5 mm x 2.0 mm, which the company claims is the smallest spread-spectrum oscillator available, and can be ordered in a standard surface-mount package for drop-in replacement of existing quartz crystal oscillators. Frequencies range from 1 MHz to 200 MHz and operate at 1.8V, 2.5V or 3.3V with less than 30 picoseconds jitter.

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