Space
Figure 2: The oven substrate assembly is mounted on thermal insulated standoffs to minimise heat loss while the output assembly is mounted on the header platform
EMXO to be leak tested using standard helium-based methods mandated by military specifications for electronic components used in space (leak rate of an evacuated package is still an important consideration earthbound).
The oven in the EMXO is proportionally controlled, so its power consumption is inversely proportional to the thermal resistance from the oven to the oscillator
enclosure. That means that the oven draws current to maintain a nearly constant temperature, and during operation the heat flows from the oven to the case through three heat transfer mechanisms: convection, conduction and radiation. Conduction and radiation are influenced by the materials and the construction of the package and – since they remain stable for life – will have an insignifi cant effect on
changing the oven current. Because the rate of heat fl ow 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. This will allow very low leak levels to be detected using simple instruments because if the vacuum degrades as a result of even a very small leak, power consumption will increase signifi cantly.
Microchip Technology has taken advantage of the power consumption/internal pressure relationship by developing a highly accurate process for determining the seal integrity of an EMXO package and allowing it to be qualified for space applications. Analysis carried out on Microchip’s EX- 209/245 shows that the EMXO maintains its stability even when the internal package pressure has increased as high as 1 torr. This means that it would take up to 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. 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 provide an operating life of 15 years. All of the above characteristics make the EMXO an extremely appealing alternative to its better known counterparts for use in spacefl ight applications. EM XOs have already been used in space for more than a decade but their increasing popularity looks set to make them the preferred type of crystal oscillator for such applications.
www.microchip.com
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