Time out! CMOS ousts quartz crystals

No moving parts
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. (Santa Clara, Calif.), Linear Technology (Milpitas, Calif.) 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 more than 10,000 parts per million 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 cell phone, 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.

"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 on the order of 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."