Get to Know the Evacuated Miniature Crystal Oscillator

A stable frequency reference is one component that few systems can do without, and the crystal oscillator has been serving this purpose admirably in one form or another for many years. The most stable types among crystal oscillators are Oven Controlled Crystal Oscillators (OCXOs), so they’re commonly used where the highest level of stability is required. But this leaves out another important precision oscillator, the Evacuated Miniature Crystal Oscillator (EMXO). If you have never heard of it, let me enlighten you – it’s an impressive device.

The EMXO was developed by Vectron International, which is part of Microchip Technology, with the goal of delivering a similar level of performance as an OCXO but in a smaller, lighter, hermetically-sealed package, and with lower power consumption. Like an OCXO, the EMXO uses an oven to maintain frequency stability as crystals inherently drift in frequency with changes in temperature.

To achieve its advantages, the EMXO designers made significant changes to the way ovenized oscillators are fabricated. First, unlike the OCXO that uses low-thermal-conductivity insulation within the package, the EMXO uses a high-level vacuum. This results in an environment free of weld splashes, dust, or vapor and reduces insulation weight to almost nothing. It also enables the use of an open crystal blank rather than a larger packaged one that further reduces weight.

The crystal is a stress-compensated, doubly-rotated synthetic swept crystal (SC/IT-cut) type to obtain excellent phase noise performance, a slower aging rate, lower g-sensitivity, and higher radiation tolerance. The crystal blank is integrated within a hybrid package that also contributes to size reduction. As the internal mass of an EMXO is smaller than that of a typical OCXO it takes up less space, which means there is less volume for the oven to heat. This, in turn, significantly reduces power consumption and decreases warm-up time. The result is an oscillator package less than half the size of a typical OCXO.

The aging rate of a crystal is a key metric of an oscillator’s performance over time and is most pronounced in the 30 days of operation. Packaging leak rate is one of the major contributors to long-term aging. To mitigate this, the EMXO enclosure is sealed using cold welding that creates a metallurgical bond between the metal surfaces without adding heat during the sealing process. After many repetitions of testing, the designers found that its leak rate was extremely low, so low in fact that it was (and still is) outside the range of the fine leak testing equipment intended to measure it.

On the face of it, this would be a good problem to have. However, all government space agencies require packaged devices to satisfy leakage requirements determined by fine leak testing, making it virtually impossible for an EMXO to be leak tested using these methods. That is, commercial instruments have a resolution of 1x10^-8 atm·cc/s, but the EMXO’s leak rate is much lower at 1x10^‑12 atm·cc/s helium. So, since its leak rate is very low, the designers came up with a way to verify the rate by focusing on power consumption.

The oven in an EMXO is proportionally controlled, so its power consumption is inversely proportional to the thermal resistance between the oven and the oscillator enclosure. As a result, the oven draws current to maintain a nearly constant internal temperature, with heat flowing from the oven to the case through three heat transfer mechanisms: convection, conduction and radiation. Conduction and radiation are the lesser problems because they are influenced by the materials and the construction of the package that remain stable for life and have an insignificant effect on oven current.

However, as the rate of heat flow through convection in the EMXO is affected by changes in internal package pressure, a leaky unit with higher internal pressure will inherently draw more current. As such, very low leak levels can be detected using readily available instruments because if the vacuum degrades because of even a very small leak, power consumption will significantly increase. Microchip took advantage of the relationship between power consumption and internal pressure to develop a very accurate and simple process for determining the seal integrity of an EMXO package that allows leak performance to be qualified for space applications. 

The results show that measurements using oven current to measure seal integrity can screen parts with a leak rate of 1x10^-6, 1x10^-7 and 1x10^-8 atm·cc/s helium in minutes, several hours, and a few days after seal, respectively. The EMXO maintains its stability even when the internal package pressure has increased as high as 1 torr, so it would take 70 years for the EMXO’s internal package pressure with a leak rate of 1x10^-12 atm·cc/s helium to reach a low vacuum of 0.1 torr. And if a 1x10^-11 atm·cc/sec helium leak rate and 0.5 torr internal package pressure are chosen for a conservative margin of safety, the latest EMXOs can achieve an operating life of 15 years.

So, while the EMXO may not be a household word, it offers significant improvements that together make it an extremely appealing alternative to OCXOs in space applications, and has been widely used by NASA and commercial and military satellite launch companies. Microchip’s latest model is the EX-219, and you can learn about it Microchip Oscillators portfolio.