A single-block fixture enables the rapid and accurate electrical characterization of planar microwave electronic components, by use of the through-reflect-line (TRL) technique, in air or in a vacuum at any temperature from ambient down to cryogenic temperatures. Heretofore, the TRL technique has involved the use of a split-block fixture in a procedure that unavoidably and undesirably includes breaking vacuum and/or thermal cycling back to room temperature. The design of the present single-block fixture makes it possible to complete the testing of a microwave component without interrupting a vacuum and/or cryogenic condition.

Figure 1. The Predecessor to the Single-Block Fixture is a split-block fixture that is used in three different configurations.

A planar microwave device under test (DUT) is typically characterized by use of an automatic network analyzer (ANA). To be able to characterize the DUT as a discrete, isolated device, one must be able to compensate mathematically for the electromagnetic characteristics of the test fixture, coaxial-to-microstrip transitions, coaxial cables, and other components used to couple test signals between the ANA and the DUT. For this purpose, one must perform calibration measurements to characterize the ensemble of all components of the test system up to a set of reference planes where the DUT is to be installed in the test fixture.

Figure 2. The Single-Block TRL Fixture holds the DUT and the calibration standards in the vacuum/ cryogenic chamber. Connections between and external ANA and one of the devices on the fixture are made through the appropriate input/output pair of coaxial cables.

For calibration measurements, one inserts a calibration standard - a device with known electromagnetic characteristics - in place of the DUT. In the split-block version of the TRL technique, the split block is configured in three different ways to obtain different calibration standards (see Figure 1). The split-block version of the TRL technique works well at room temperature. However, this version is too cumbersome for vacuum and cryogenic testing, in that the block must be rewired and reconfigured between calibration steps. The rewiring and reconfiguration must be performed under ambient conditions and they take time, adding to the cost of testing. During the rewiring and reconfiguration time, ANA reference levels can drift, with consequent increases in measurement errors. In contrast, there is no need to reconfigure or rewire the present single-block fixture; therefore, calibration can be completed more easily and quickly at any temperature, and measurements can be more repeatable.

The single-block fixture (see Figure 2) holds the DUT plus through, reflect, and delay (line) calibration standards in their prescribed test setups and is placed in a vacuum/cryogenic chamber. The fixture includes a 0.010-in. (254-μm)-thick alumina substrate with etched gold strips serving as transmission-line conductors. The input and output transmission lines are patterned for a characteristic impedance of 50 Ω .

Connections between external circuitry and these input/output lines are made via coaxial connectors and coaxial-to-microstrip transitions. Coaxial cables run from these connectors to points outside the vacuum/cryogenic chamber. There are no moving parts and there is no switching of electrical connections in the vacuum/cryogenic chamber. Instead, access to the DUT or to one of the calibration standards is gained by connecting, outside the vacuum/cryogenic chamber, to the appropriate coaxial cable.

This work was done by F. A. Miranda and B. T. Ebihara of Lewis Research Center and A. S. Creason of Ohio Northern University, M. Mejia of University of Pennsylvania, and S. S. Toncich of Bird Electronics. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com under the Electronic Components and Circuits category.

Inquiries concerning rights for the commercial use of this invention should be addressed to

NASA Lewis Research Center
Commercial Technology Office
Attn: Tech Brief Patent Status
Mail Stop 7 - 3
21000 Brookpark Road
Cleveland
Ohio 44135.

Refer to LEW-16567.


NASA Tech Briefs Magazine

This article first appeared in the August, 1998 issue of NASA Tech Briefs Magazine.

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