Silicon Labs joins battle against quartz

Silicon Laboratories has joined Mobius Microsystems in trying to break the stranglehold that quartz oscillators have on the market for frequency generation in electronic products with clock chips that are made entirely on standard silicon processes.

Like Mobius, Silicon Labs has used a resonating inductor-capacitor (LC) circuit, operating at a frequency of around 4GHz, to provide a stable clock source. The use of an LC circuit makes it possible to build the clock generator on a standard silicon CMOS process in contrast to the micromachined structures that other companies have used to compete with quartz crystals.

Like Mobius, Silicon Labs will program during packaging test at the factory the settings that divide down the source 4GHz signal to an output in the 0.9MHz to 200MHz range. James Wilson, senior marketing manager for wireless products at Silicon Labs, said one major advantage of moving to silicon is the ability to produce arbitrary frequencies. Quartz has to be cut to a precise size and shape to be able to resonate at a particular frequency.

“Sometimes, oscillator vendors have asked customers for a non-recurrent engineering charge for custom frequencies,” Wilson added, noting that a silicon-based oscillator is more robust. “Also, if you decap a quartz oscillator, it is attached to the package like a diving board. That has been an issue for some defence applications because they need to withstand very high acceleration.”

Wilson said the Si500 chips will generally be priced lower than comparable quartz devices: “You could see significant price savings at higher frequencies because quartz manufacturers have had to use [more expensive] third-overtone crystals in that range.”

He explained that the biggest problem in developing the parts was handling the frequency shifts caused by changes in temperature. “The breakthrough came that we came up with was a temperature compensation circuit. The compensation is handled as part of the test program, where the polynomial coefficients needed to adjust the output are stored in on-chip non-volatile memory,” Wilson said.

Silicon Labs has kicked off with devices that generate a single frequency, although the parts can be ordered to provide differential outputs. Mobius started with spread-spectrum clock chips but aims to expaned. Tunc Cenger, director of marketing for Mobius, said: “We are going to build a portfolio of products. The spread-spectrum product was first because we saw an immediate need for that. Going forward we will offer a number of parts.”

Cenger said the company is sampling to a small number of customers clock chips that produce a single frequency rather than a spread-spectrum output, which is generally used to reduce electromagnetic interference peaks in consumer and PC products.

Both companies are sampling products now with the aim of moving to volume production by the end of the year.

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