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Sensors/MEMS??

Clock oscillators meet FCC interference standards

Posted: 05 Nov 2008 ?? ?Print Version ?Bookmark and Share

Keywords:MEMS? oscillator? interference?

SiTime Corp. has expanded into the electromagnetic (EM) compliance business with a family of MEMS EM interference products.

The company said its spread-spectrum clock oscillators are used as drop-in substitutes to bring prototypes into compliance with Federal Communications Commission (FCC) rules. The oscillators also meet the compliance standards for Canada, Europe, Japan and Taiwan. The devices aspire to lower cost and hasten development time.

EM compliance standards ensure that electronic equipment does not radiate RF energy that interferes with nearby equipment or cause health hazards. Gauging interference can only be made after a prototype is completely assembled, since final packaging affects the radiation pattern. If a problem is seen, expensive and time-consuming engineering is needed, ranging from redoing board traces to adding shielding.

SiTime's spread-spectrum clock oscillators are expected to reduce the interference footprint of a finished prototype. The clock distributes the radiated EMI energy around a center frequency, like modulating a 100MHz clock over a 2 percent bandwidth between 99MHz and 101MHz. Thus, it lowers its overall output to within standards, without a redesign.

"When you move the rising edge of the clock around a bit, then you are moving the energy of that clock around the center frequency," said Piyush Sevalia, VP marketing, SiTime. "That's the idea behind spread-spectrum clocking when you are distributing the energy over a bandwidth," he added.

The SiT9001 helps designers to drop in a clock oscillator that modulates clock frequency around a target frequency from 1MHz to 200MHz. The oscillator gives differential outputs. Both modules have a triangle wave to modulate the clock frequency by up to 2 percent, used before, at or after the target frequency.

SiT9001 measures 2.5mm x 2mm, which the company claims is the smallest spread-spectrum oscillator available, and can be ordered in a standard surface-mount package for drop-in substitute of existing quartz crystal oscillators. Frequencies range from 1MHz to 200MHz and operate at 1.8V, 2.5V or 3.3V with less than 30ps jitter.

- R. Colin Johnson
EE Times





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