High-reliability oscillator design for satellite systems poses many challenges to the engineering community. The custom nature of the design efforts as well as the quality requirements tend to lead to large, complex specifications that drive cost, design cycle time, and overall product lead time.
Materials utilized in design and construction are also limited by environmental constraints such as outgassing, radiation, the use of pure tin, and shock/vibration requirements (Fig. 1). In addition to these concerns, all assembly and test processes need to be approved, which can create some additional hurdles that must be overcome at the design stage.
SPECIFICATION CHALLENGES
Customers have their own styles that they use when creating their high-reliability specifications. Design engineers need to thoroughly review these specifications for compliance. The best way to ensure that each and every requirement is being met is with a compliance matrix, which will list every requirement in the specification. Each requirement will have a comment as to how the design will comply with the requirement.
Often times, target specs will be identified in the compliance matrix to document attempted design margins. A thorough review of the compliance will often reveal areas of concern where inadequate margin exists. A significant amount of prototyping, simulation, and design effort occurs in these areas. There are a couple different ways to address these issues.
The first approach for minimizing the complexity and risk of the new design is to reuse as much as possible from a library of designs that have been used in the past and have been thoroughly evaluated and tested. It is relatively easy for single designers to reuse their own designs, but this becomes more complex for larger design teams.
The best way to optimize the reuse of the designs of others is to create common functional blocks that can be easily shared between different designers. These functional design blocks typically consist of the voltage regulations scheme, output buffers, tuning circuits, oven controllers, mode traps, and multipliers. Once these circuits are designed, well understood, and modeled, they should be reused as often as possible to minimize risk and shorten design cycle time.
The ideal solution is to have a customer utilize an existing design in a part that has already been qualified for space use. This significantly reduces design time, material lead time, risk, and cost. This is always much easier said than done. Every customer is going to have unique requirements that will require customization of the oscillator to specifically meet its needs.
The most direct way to convince a customer to utilize existing designs is to have upfront discussions with the engineering communities at the customer and the supplier to make sure the customer is aware of the supplier’s existing proven space designs. Although this may be successful, it generally leads to a custom design that may be difficult for the customer to second source and may not be the lowest-cost alternative.
A better approach to meeting the customer’s requirements while minimizing complexity is to create a standard document that allows the customer to build a specification around standardplatform and already existing designs. This is similar to Vectron’s OS-68338 Hi Rel Clock Specification and DOC200103 Hi Rel TCXO Specification.
Such specifications define the design, assembly, and functional evaluation for a wide variety of packages, supply voltages, stabilities, and output waveforms. These standards allow the customer to choose the component reliability level and the screening level requirements for the oscillator from a standard table that complies with MIL-PRF-55310 and MIL-PRF-38534.
Specifications ensure that none of the performance, quality, reliability, and screening requirements are missed in the design creation. In most cases, the customer does not need to create its own specification since the supplier’s standard document already accounts for the necessary steps to meet all of the highreliability conditions.
The use of standard designs can help qualification by similarity and significantly reduce the qualification efforts. Material costs and lead times will be reduced as well, since the requirements will be met with standard products that can be customized to meet each customer’s specific needs.
Another technique for design standardization, which can improve procurement and manufacturing efficiencies without compromising quality assurance, is to specify crystal oscillators that are currently qualified by the Defense Supply Center Columbus (DSCC) and listed on its Qualified Product List (QPL). DSCC is recognized as the leading qualification authority in the military and space component industry.
For high-reliability clocking requirements, using qualified parts such as Vectron’s MIL-PRF-55310/16S, a hybrid design housed in a standard 14-pin dual-inline package, demonstrates a level of quality, performance, and reliability needed for mission-critical space applications that’s understood throughout the space industry.
COMPONENT ISSUES
Component selection in high-reliability design is much more complex than it is for commercial and military designs. The components have to be able to meet strict outgassing, radiation, reliability, metal composition, and screening requirements. These requirements significantly limit the range of components that are available to the designer. When selecting epoxies, cements, and other adhesives, NASA’s database of approved materials is the best source to ensure selected materials are acceptable and comply with MIL-STD-883, Method 5011 (Fig. 2).
The use of plastic encapsulated microcircuits (PEMs) is not advised for high-reliability space designs. Therefore, all active devices must be available from the manufacturer in die form. Radiation requirements pose some of the most difficult hurdles to the designer. Depending on the type of orbit and the location of the oscillator within the satellite, total ionization dosages (TID) of radiation requirements can range from a few thousand krads to several hundred krads.
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