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CMOS harmonic oscillator eliminates quartz crystals

Posted: 08 Apr 2008 ?? ?Print Version ?Bookmark and Share

Keywords:CMOS Harmonic Oscillator? quartz crystals? MEMS?

Mobius Microsystems Inc. has unveiled its CMOS Harmonic Oscillator (CHO) technology that eliminates the need for quartz crystals in many applications. By integrating an oscillator onto an ordinary CMOS chip, the company claims to have removed the last moving part from electronics circuitry.

"We are announcing today the most accurate CMOS oscillator that has ever been built," claimed Tunc Cenger, director of marketing at Mobius. "Our CMOS oscillators are not as accurate as quartz, but our CMOS Harmonic Oscillator adds proprietary compensation circuitry that meets the requirements of a wide variety of timing-chip applications."

Quartz crystals, a piezoelectric material, provide a rock-solid mechanical time reference for traditional electronic oscillators by permitting them to sync with its mechanical vibrations. Using a quartz crystal and a PLL chip, a traditional oscillator can achieve ultra-high accuracy of 25ppm or less. MEMS oscillators follow the same tack, but downsize the quartz-crystal resonator to the size of a microscopic tuning fork. Now, Mobius claims to have eliminated the need for the mechanical reference and PLL altogether, albeit only for applications that can tolerate inaccuracies of 100ppm.

Benefit over MEMS
"Mobius is going one step further than MEMS, since its oscillators have no moving parts at all," said Steve Cullen, contributing analyst with In-Stat. "It's an all-CMOS solution, which is tough to beat, but it's not universally applicable, since it's not as stable as quartz crystals."

Mobius has identified multiple high-volume markets for its CHO technology (where 100ppm is adequate), including providing the timing signals for the serial PCIe peripheral bus; for USB devices; for serial hard disk drives (serial ATA); for flat-panel displays; and for printers. For each of these applications, switching to a CHO chip means that OEMs can reduce their BOM by dropping quartz crystals.

"Mobius has done a good job of identifying high-volume applications that will work well with its CMOS oscillatorwhere a drift of 100ppm is no big deal," said Cullen.

Mobius' first commercial partthe MM8511is pin-compatible replacement with existing spread spectrum PLLs. The MM8511, however, does not require users to hook-up a quartz crystal to it.

"[The] MM8511 replaces both a quartz crystal and its PLL, which demonstrates how CHO can be used to build large-scale CMOS functions," said Cenger.

Timing limitations
Mobius claims that eventually it wants to stake a claim on more territory now owned by quartz crystals, but admits that CHO is not the right solution to every timing chip problem.

"We believe that in the overall landscape of timing chips there will always be a need for quartz crystals, but we are taking CMOS oscillators where they have never been before in terms of performance," said Cenger.

"In the end, there is probably room in the market for all three technologiesquartz crystal, MEMS and CMOS oscillators," said Cullen. "Quartz will probably remain preferred for high-accuracy applications especially ones that require elaborate temperature stabilization, and there are many applications where the smaller size of MEMS chips will work even better. But for those places that can tolerate relatively lower accuracy, Mobius has got a good shot at offering a very low-cost, no-moving-parts solution." Solid-state "no moving parts" inductor-capacitor (LC) circuits have been used to create oscillators since the invention of the transistor. In fact, National Semiconductor Corp., Linear Technology and others already have CMOS oscillator chips on the market, but only for low-accuracy applications that do not require a rock-solid quartz crystal reference.

Unfortunately, the CMOS inductors and capacitors used in these circuits are non-precision, resulting in inaccuracies of 10,000ppm or more. What's more, the circuits are not particularly stable, varying their output frequency in response to changes in temperature, supply voltage and driving current. The traditional solution has been to sync integrated oscillators to a mechanical reference, like a quartz crystal using a PLL circuit.

However, with the invention of the cellphone, radio-frequency circuitry has demanded that CMOS chips improve the native accuracy of inductors and capacitors, while also finding innovative ways to stabilize circuits in the presence of varying temperatures, voltages and currents. As a result, Mobius now claims it is possible to repurpose these advances in RF CMOS to create much more accurate CMOS oscillators.

Ideal platform
"We believe that CMOS is now the ideal platform to build a viable high-performance and cost-effective substitute for quartz crystals," said Cenger. "CMOS has no high-frequency limit, can be put in small, thin packages, is not a foreign non-semiconductor material and is integratable with other devices so you can replace not only the quartz crystal but also its support circuitry."

CHO is enabled by a precision LC resonator that runs at gigahertz frequencies, then is divided down to the megahertz range required by a specific application. The secret of its accuracy, Mobius claims, is the company's patented real-time algorithm that enables it to run in an open loop configuration by compensating for temperature, voltage and current changes that ordinarily affect the frequency of an LC oscillator.

The accuracy of the high-precision inductors and capacitors used on CHO chips is about femto-henries and "farads, respectively, which puts the oscillation frequency of the LC circuit into the ballpark. But a final calibration step compensates for any process variations among devices and sets the final frequency of Mobius's chips.

"After manufacturing and packaging our chips, we program each CHO using a calibration algorithm that references a pristine quartz crystal oscillator," said Cenger. "We trim values inside our chip so that its frequency matches the quartz crystal. After this calibration step, our CHO parts will maintain their frequency indefinitely, unless you decide to change it, which can be accomplished with recalibration."

- R. Colin Johnson
EE Times

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