by Hoklay Pak, Microchip Technology, Inc.
For electronic systems, space presents challenges not found in terrestrial environments, ranging from daily temperature swings from exceptionally cold to scorching hot, ionizing radiation that can destroy components, as well as the shock and vibration experience in getting there. For crystal oscillators, this requires the ability to generate an extremely stable reference frequency that maintains its precision while drawing the least amount of power in the smallest possible package, and do this for up to 15 years.
The go-to device for achieving this has long been the Oven Controlled Crystal Oscillator (OCXO), but the Evacuated Miniature Crystal Oscillator (EMXO) delivers equal or even better performance and ruggedness in half the size, with less power consumption, and other advantages. So, it should be no surprise that the EMXO continues to increase in popularity.
As systems of all types are becoming increasingly digital, the quartz crystal that generates this frequency may seem archaic. However, this precision piezoelectric component remains novel because it must be machined to extraordinarily tight tolerances so it can vibrate at a specific frequency. It must also maintain extremely high stability because of its inherently high value Q. That said, crystals are very sensitive to even small changes in temperature that cause their frequency to vary, and while these variations can be tolerated in more mundane applications, in others they cannot.
The temperature-compensated crystal oscillator (TCXO) was designed to mitigate this problem by adding a temperature-sensitive reactance circuit in its oscillation loop. Even with this, the resulting level of improvement may be insufficient for more demanding applications. A tenfold improvement in stability can be achieved by placing the crystal in a small oven, creating an OCXO but the oven employed in a typical OCXO is a relatively power hungry device that makes it larger and heavier. This increased power consumption can be a significant problem in spaceflight and other applications where minimizing size and weight are essential.
The EMXO was created to deliver the same level of performance as an OCXO, but in a smaller, lighter hermetically sealed package while significantly reducing power consumption—key factors in spaceflight applications. Microchip Technology’s EX-219 is a good example of the latest EMXO design, with performance characteristics shown in Table 1.
The concept of the EMXO has been around for many years and it’s taken that long to bring it to fruition, but the results were well worth the effort. For example, while an OCXO uses a low thermal conductivity insulation to minimize power consumption, an EMXO uses a vacuum as the insulation method. This produces a contamination-free vacuum level of 10-6 torr and reduces insulation weight to virtually nothing. There is no contamination from weld splashes, dust or vapor, and this extremely high level of vacuum decreases very little over time. The contamination-free environment also facilitates the use of an open crystal blank rather than a larger packaged type, further reducing both size and weight.
This means that the internal mass of an EMXO can be made smaller than that of a typical OCXO, so there is less volume for the oven to heat, as well as lower power consumption. Since the EMXO is evacuated and has much less thermal mass than an OCXO, its warm-up time is also much faster. And as the crystal blank is integrated within the hybrid package, size reduction is improved further still, allowing the EMXO to be realized in a package less than half the size of a typical OCXO.
The EMXO circuit consists of a heated substrate (oven) and an output substrate. A stress-compensated, doubly-rotated crystal (SC/IT-cut) is used to obtain good phase noise, slower aging rate, and lower g-sensitivity. The crystal has a four-point mounting structure for ruggedness and low g-sensitivity. Synthetic swept quartz is used to achieve higher radiation tolerance. The thermally insulated structure maintains a nearly constant temperature over its operating temperature range. The output substrate, which does not need to be as thermally isolated, is mounted directly on the case (Figure 1).
The Challenge of Testing EMXO Leak Rate
The EMXO has many advantages including its leak rate, which is so low (1×10-12 atm·cc/s helium) that it is less than what the equipment intended to measure it can achieve. Government space agencies require packaged devices to satisfy leakage requirements determined by fine leak testing. But as the EXMO’s leak rate is lower than what commercial instruments can measure, it is impossible for an EMXO to be leak tested using the standard helium-based methods mandated by military specifications for electronic components used in space. It should be noted that even though the leak rate of an evacuated package is not significant while in space, it is an important consideration while still earthbound.
Hermetic packages are typically sealed using resistance welding or seam welding, and are usually backfilled with a mixture of a noble gas and helium as a tracer, at a pressure of about 1 atmosphere. This allows the ability to detect a leak rate between 1×10-10 and 1×10-9 atm·cc/s using commercial instruments having a resolution of 1×10-8 atm·cc/s.
Helium bombing is a common technique used to measure leak rates of evacuated enclosures such as the EMXO. During this test, a small amount of helium is injected into the sealed package prior to leak testing. However, a drawback of the bombing process is that helium can diffuse and permeate into the metal and glass of the package. During the fine leak detection process, this helium may be released from the metal or glass, resulting in a pessimistic leak rate. This process is known as desorption and can induce an apparent leak rate of 1×10-9 atm·cc/s helium.
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 is, 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 as they remain stable for life will have an insignificant effect on changing the oven current. 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. This allows 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 significantly increase.
Microchip takes advantage of the power consumption/internal pressure relationship by having developed a very accurate process for determining the seal integrity of an EMXO package and allowing it to be qualified for space applications. Analyses have been performed on the EX-209/245 to validate this process.1 The results show that measurements using oven current to measure seal integrity can screen parts with a leak rate of 1×10-6, 1×10-7 and 1×10-8 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 up to 70 years for the EMXO’s internal package pressure with a leak rate of 1×10-12 atm·cc/s helium to reach a low vacuum of 0.1 torr (Figure 2). If a 1×10-11 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.
The EMXO may be less well known than its counterparts for use in space, but it offers significant improvements that together make it an appealing alternative. Although it uses a vacuum as its main differentiator, construction techniques such as cold welding and hybrid construction also significantly contribute to the result. For these reasons, the EMXO has been used in many spaceflight applications for over a decade, and as it gains recognition, its adoption is sure to increase.
1. “Using Oven Current Instead of Fine Leak Detector to Screen Seal Integrity of EMXO Cold-Weld Evacuated Package,” Hoklay Pak, Microchip Technology, March 2021.