Electronic Design

Oscillators Face The Final Frontier

Space applications like satellites present unique sets of challenges that designers need to consider if they want to ensure performance and reliability.

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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In addition to TID, survival and immunity to latch-up must be considered for single-event upset (SEU) and single-event effects (SEE). There are several databases publicly available with radiation data to help the designer with the proper component selection. Depending on the radiation requirements, the components available may be significantly reduced.

Even when the proper components are selected and utilized, many customers still require lot testing of active devices to ensure the requirements will be met. The use of approved radiation test facilities is necessary for testing at the component level and, in some cases, at the oscillator level.

The effects of radiation in space will affect the performance of the oscillator over its life. Not only do the components need to survive the effects of radiation, but its impact on the parametric values of the components must be well understood as well. Several analyses typically need to be performed on all new designs, such as worst-case circuit analysis (WCCA), end-of-life calculations (EOL), and failure-mode effects analysis (FMEA).

The parametric shifts from radiation need to be utilized in these analyses. Utilization of the functional block design approach mentioned above can significantly reduce the design effort by reusing previously evaluated and proven portions of the circuit.

ASSEMBLY AND TEST PROCESSES
The workmanship and quality standards for space products are the most stringent of any industry. These requirements need to be considered at the design stage, not viewed as a requirement for operations to handle. Certain reliability levels, for instance, limit the amount of rework that can be performed on a flight device. Therefore, adequate design margins need to be considered to ensure compliance so requirements can be met within these constraints.

For example, several factors in the design can affect the phase noise performance of the oscillator. For PCB-based (printedcircuit board) designs utilizing PEMs, component replacements to optimize the phase noise performance are very typical. This is not an option with space-level designs since you can very quickly exceed the limit for the number of component replacements, which will result in the unit being scrapped.

Since the devices will be operating in a vacuum, some of the acceptance and screening tests need to be performed in a vacuum in production. The frequency and temperature stability of the oscillator can significantly change when going from ambient pressure to a vacuum. These changes need to be well understood at the design stage so the proper margins can be designed in and the proper ambient pressure targets can be set in production to ensure the specifications will be met in a vacuum (Fig. 3).

The design should also be capable of being manufactured to previously approved processes. In many instances, the assembly and test processes have to be audited and approved by the customer. If design features are going to require new assembly processes and/or test processes, the customer should be involved early in the process validation to prevent delays down the road. Evaluation tools such as real-time x-ray or destructive physical analysis should be used to evaluate and approve new processes.

SUMMARY
The challenges of designing oscillators for space are much more complex than commercial or even military markets. These challenges are escalated due to component restrictions, complex specifications, and assembly concerns. The best way to address these issues is to reuse as much content from previously qualified designs to help minimize risk, as well as shorten design cycles and manufacturing lead times.

DAVID BAIL, director of product marketing, obtained his master’s degree in electrical engineering from Syracuse University. He also obtained a bachelor’s degree in engineering physics from the University of Maine and an MBA from Temple University.

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