HIGH-TEMPERATURE SUPERCONDUCTING DELAY LINES

Work on superconducting delay lines started at Lincoln Labo­ratory well before the advent of high-temperature supercon­ductivity, and concentrated mostly on linearly dispersive de­lay lines for analog signal processing. Linearly dispersive delay lines have delay characteristics which vary linearly with frequency over a certain operating bandwidth and can be used to perform pulse compression, a technique to process and detect small signals which may be below the receiver noise floor (1). The pioneering work at Lincoln Laboratory in this area using LTS and, more recently, HTS thin-film tech­nologies has been extensively documented in the literature (1,51).

Table 4. Sample System Requirements That Will Affect the Choice of Cooler and Cryogenic Packaging Approach

Comments

Requirement

Size and weight Cool-down time

Vibration

Power consumption and power supply type Mode of operation Temperature stability and control Unattended lifetime Vacuum lifetime

Stringent in almost all applications

Some applications may require very fast turn-on time (e. g., a few minutes). They would be a driver to­ward higher cooler power and lower HTS device thermal mass For example, a minute amount of mechanical distortion on a circuit caused by vibration from the cooler may generate a phase modulation that degrades the circuit performance E. g., 120 V ac

E. g., continuous, intermittent, short missions and then mostly idle, etc.

While any fine temperature feedback control loop (<±0.01 K) tends to be done using heaters and a tem­perature sensor, some applications may require a certain degree of cooling engine control (<±0.5 K) Some applications (e. g., space) may require a lifetime on the order of 10 years or more All-welded construction; use of getters in a clean, well-conditioned (baked) system

SUPERCONDUCTING FILTERS AND PASSIVE COMPONENTS 727 Table 5. Cooling Requirements That Will Influence the Cooling Power (Heat Lift) Required for a Given Application

Power dissipated in the device

Number of microwave and dc control leads

Surface area

Thermal mass

A filter with a 0.5 dB insertion loss that must pass a 20 W signal will dissipate 2 W of heat that must be re­moved by the cryocooler. Also, semiconductor devices such as low-noise amplifiers, which improve in noise and gain performance when cooled, always dissipate a certain amount of heat which must be taken into consideration

These are the electrical interface between the cryocooled device and the outside world. For example, a filter might require two microwave leads (input and output) and two pairs of dc control lines for the heat sensor and a small heater to keep the temperature constant. These conductors represent a heat loss that the cooler must overcome because they connect the outside ambient temperature with the cold device. While the dc control lines are typically made of thin low-thermal-conductivity, high-resistivity wire (e. g., gauge 32 manganin), the microwave leads must achieve a compromise between insertion and thermal loss Radiation loss is another form of heat loss that the cooler must overcome and therefore must be minimized. The total surface area and their infrared radiation emmisivity are important design parameters. Low-em — misivity radiation shields are typically used between the warm vacuum vessel wall and the cold device For those applications that have a cool-down time requirement, the thermal mass of the device to be cooled is important and will be affected by the microwave packaging material and its shape

Table 6. Some Cryogenic Refrigerator Types Likely to Be Used in HTS Technology

Cooler Type

Heat-Lift Range Available at 80 K

Comments

Split Stirling

0.5-3 W

Available from many manufacturers; used primarily in the tactical military infrared detector in­dustry. Has a cold head separated from a compressor by a metallic transfer line up to 15 cm long

Integral Stirling

0.5-5 W

Also used in infrared detectors; at least one version is being used in an HTS development proto­type. The compressor and cold finger are integrated into one unit

Gifford-McMahon

2->200 W

Widely used in the support of vacuum systems for semiconductor industry; highly reliable and versatile. The compressor and cold head are separate units connected by fluid lines that can be several meters long

Throttle-cycle

~4 W

Reliable and low cost. The compressor and cold head are separate units connected by fluid lines that can be several meters long

Joule-Thomson

0.5-2 W

Generally used as an open-cycle cooling system for short tactical missile IR detector applica­tions

Pulse tube

0.5-2 W

Emerging technology, low cold-head vibration and long lifetime potential

Nondispersive delay lines have a constant delay-versus — frequency characteristic and are typically used as analog memory elements that can store a signal for, say, up to a few hundred nanoseconds while the system is engaged in other processing steps. Work on HTS nondispersive delay lines has also been significant (52-55). Including two recent instanta­neous frequency measurement subsystems based on banks of delay lines (52,55). Clearly, the advantages of superconductiv­ity are that a long length of line can be fabricated in a small volume by defining a long, planar transmission line on a wa­fer. Ref. 54 compares conventional nondispersive delay lines, which require amplifiers between sections of transmission line (e. g., coaxial), with HTS delay lines using projections based on measurements made on relatively short (22 ns) de­lay lines. Key delay-line parameters are delay, bandwidth, in­sertion loss, and third-order intercept point. Conventional de­lay lines that must resort to amplification to boost the signal are limited in dynamic range by the amplifiers.

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