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Oven-controlled crystal oscillators (OCXOs) are used when frequency vs. temperature requirements are too stringent to be met by a basic crystal oscillator (XO) or temperature-compensated crystal oscillator (TCXO). With an OCXO, the temperature of the crystal and critical circuits is kept constant as the temperature outside the oscillator varies. Controlling the temperature inside the oscillator with an oven maintains this constant temperature. In an OCXO, the changes in the ambient temperature are sensed and then fed back to an oven control that continually maintains a constant optimum temperature inside the oscillator enclosure. An OCXO can improve the crystal's inherent stability by more than 5000 times.

However, the oven control system is not perfect. The open loop gain is not infinite, and there are internal temperature gradients inside the oven (oscillator) and, the circuitry outside the oven shell is subject to ambient temperature changes that can “pull” the frequency.

The improved temperature stability performance of a conventional OCXO over an XO or TCXO comes at a steep price. OCXO power consumption, for instance, is greater by a factor of more than 200. Size is also a consideration. In an ordinary OCXO, a crystal is enclosed in a metal case, which is then placed inside an oven shell together with temperature-sensitive circuitry, and then surrounded by thermal insulation. All this, plus any additional circuitry are then placed in a metal housing making for a bulky package, which becomes difficult to miniaturize.

To overcome these obstacles, evacuated miniature oven-controlled crystal oscillator (EMXO) was specifically developed to achieve OCXO performance while significantly lowering power consumption and reducing package size (Figure 1). As demonstrated by EX-380 and EX-620 series oscillators, the smaller size and low power consumption makes the EMXO suitable for portable and battery-powered applications. As a result, the EMXO has been successfully designed into portable test equipment as well as military man-pack radios. The newly released space-qualified EX-245 is aimed at satellite applications where power and size are critical.

These characteristics go hand in hand since reducing package size makes it easier to improve power consumption. First, the volume of the package was made as small as possible to reduce the volume that the oven needs to heat. Second, as demanded, the most effective insulation was used. This is exemplified in the EX-620 series. Here, the package was designed to half DIP dimensions of 0.52" × 0.52" × 0.3," and the oscillator uses a vacuum as the insulation medium — a dramatic improvement over conventional polyfoam or fiber-based insulation material. In addition, it eliminates the use of large packaged crystals, which up until now had to be used to achieve good aging. Instead, a way to use an open crystal blank was found and made practical. Furthermore, the designers have succeeded in resolving outgassing and contamination issues, which could degrade performance. To provide the needed thermal insulation, it required manufacturing the oscillator with a high internal vacuum level and low internal outgassing. A high level of cleanliness was needed to prevent contamination of the open (unencased) crystal blank and to ensure exceptional long-term crystal aging.

A key design feature of this package used the concept of integrating the precision crystal in blank form in combination with hybrid microelectronics circuitry. In doing this, obtaining good aging performance was paramount. Hence, a cold-welded package was chosen rather than a more traditional resistance-welded package. Cold-weld sealing provided a true metallurgical bond between ductile metal surfaces without added heat from the sealing process. Under the high tonnage pressure introduced through the indentation of the welding die, a plasticity flow of material takes place on the mating surfaces. The end result is a hermetically sealed enclosure without contamination from weld splashes, dust and vapors. And, most important, cold-weld sealed enclosures achieve a high level of vacuum integrity.

Mechanically, the hybrid circuit and crystal assembly is suspended directly over a highly insulating structure to minimize heat energy loss through conduction. In addition, the entire assembly is thermally insulated to the enclosure by vacuum at a pressure level of 10-6 torr. Based on the steady-state thermal conduction calculation, this package design resulted in a thermal resistance of >300 °C/W.

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