CRYOGENIC PACKAGING

Key to the insertion of superconducting microwave circuits into electronic systems is the integration of the HTS compo­nents with a cryogenic refrigerator and its associated control electronics. Clearly, for HTS technology to be ultimately suc­cessful, the user must be rendered able to ignore the fact that cryogenics are used at all, by providing long-lifetime cryocool — ers and optimally small cryogenic packages with standard en­velop characteristics and interfaces (e. g., 19 in rack mounts and back-plane blind-mate connectors).

Many important considerations enter into the design of a cryogenic package suitable for a microwave HTS subsystem. Figure 15 is a schematic representation of this package, show­ing its main elements and the various heat inputs that must be considered for an appropriate thermal design. Ref. 41 pro­vides specific details on the cryogenic package for a communi­cations filter subsystem.

The choice of a cryocooler will depend on the system and the cooling requirements. An airborne military application may require the use of a small Stirling-cycle cooler because of volume restrictions. On the other hand, a communications ground station in a remote location that needs to operate un­attended for a long time may require a larger, more reliable refrigerator of the Gifford-McMahon type. Cooling require­ments are imposed by the component or subsystem to be cooled and will determine the amount of cooling power re­quired at the operating temperature. Typical sample system and cooling requirements and some comments as to their sig­nificance are given in Tables 4 and 5, respectively.

Figure 16. Photograph of a HTS filter assembly for commercial wire­less applications (courtesy of Superconductor Technology, Inc.).

Cryocoolers likely to be used in microwave HTS technology will typically have from 1 W to 5 W of cooling capacity. A primary concern systems designers have is the reliability of cryogenic refrigerators, which varies greatly depending on their type and size. Leveraging developments in other fields, such as infrared detectors, the reliability of small, military tactical cryocoolers has steadily increased in recent years, with some manufacturers claiming up to 20,000 h of mean­time to failure (MTTF). On the other hand, larger laboratory or industrial units and specialized coolers for aerospace appli­cations operate for 5 years to 10 years and require minimal servicing. Table 6 lists some of the cryocooler types of inter­est. The intent here is not to be all-inclusive but to provide a basic reference to the type of coolers most likely to be em­ployed in HTS microwave technology. Reference 56 as a good source of the latest developments in cryocooler technology. Figure 16 is a photograph of a commercial HTS filter subsys­tem, showing the cryocooler and associated electronics in their open package.

CONCLUSION

High-temperature superconductor microwave technology of­fers unique advantages derived from the low microwave loss of HTS materials and the inherent low thermal noise in cryo — genically cooled components. The main applications to date are related to increased microwave receiver sensitivity, and this is most likely to have an impact on wireless military and commercial communications systems. The reason is that re­ceiver sensitivity and dynamic range must be preserved in the presence of a large number of spurious signals which, if unfiltered, degrade receiver performance. Generation of clean transmitted signals requires filtering in the transmitter and this, coupled with the need to reject unwanted high-power signals at the receiver, has spurred work on high-power han­dling in HTS filters. Great interest in the United States and abroad exists in the wireless commercial communications market and several companies are testing base-station re­ceiver front-ends consisting of cryogenically-cooled filter-LNA subassemblies.

HTS microwave filters are therefore a promising technol­ogy, especially at frequencies below 3 GHz where the loss in conventional microwave materials force high-performance filters to be very large in order to achieve the required low insertion losses and selectivity. Leveraging developments in infrared imaging detector technology and perhaps new devel­opments of cooled semiconductor components for fast com­puter workstations, cryocooler technology is progressing to the point where long lifetimes and small-size, low-weight cool­ers are now widely available.

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