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Solution to address stability problems

Posted: 30 Jan 2009 ?? ?Print Version ?Bookmark and Share

Keywords:resonator MEMS? stability temperature problem? crystals quartz?

MEMS use silicon processing techniques to craft resonators on CMOS chips, replacing quartz crystals for time bases. But silicon is not as stable a material as quartz, requiring chipmakers to add active temperature compensation circuitry that increases power consumption.

Silicon Clocks Inc. claims to have solved the stability problem with a passive approach that compensates for temperature with materials that consume no power but achieve results rivaling quartz crystals.

Quartz crystals typically provide frequency stability from 0.25ppmC to 1ppmC compared with 15ppmC to 30ppmC for typical MEMS resonators. The new temperature compensation scheme, called CMEMS-ZeroThermal, delivered a 30-fold improvement in frequency stability of 0.5ppmC to 1ppmC, the company claims.

"Silicon Clocks has come up with a very clever solution to temperature compensation for MEMS resonators that enable them to rival quartz crystals without adding any active circuitry," said market analyst Mark Sherwood of Consulting Services & Associates LLC.

The conventional method of compensating for temperature changes in MEMS oscillators is to add an active fractional-N synthesizer to achieve finer frequency resolution within a feedback loop. Silicon Clocks' approach instead adds a proprietary material around the resonator that puts stress on it as temperature changes.

The added stress, as the temperature rises, compensates for the usual change in resonant frequency, according to the company, thereby steadying the frequency of the oscillator over at least a 100C temperature range.

"Others have tried passive approaches, such as adding a different material to the silicon resonator itself. We tried this too but failed to get it to work very well," said Didier Lacroix, Silicon Clocks' CEO. "Our current method instead adds material around the periphery of the resonator."

Silicon Clocks said it is attempting to patent its new approach, which it plans to license to CMOS chipmakers. Licensees would be able to add the MEMS oscillator to their CMOS wafers, hence the CMEMS moniker.

- R. Colin Johnson
EE Times

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