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Keysight App Note :: De-Embedding and Embedding S-Parameter Networks Using a Vector Network Analyzer.
Keysight Technologies, Inc. offers a wide range of portable and benchtop vector network analyzers for characterizing components from 5 Hz to 120 GHz. These instruments are available with a wide range of options to simplify testing in the field, laboratory, and production environments.
Traditionally RF and microwave components have been designed in packages with coaxial interfaces. Complex systems can be easily manufactured by connecting a series of these separate coaxial devices. Measuring the performance of these components and systems is easily performed with standard test equipment that uses similar coaxial interfaces.
However, modern systems demand a higher level of component integration, lower power consumption, and reduced manufacturing cost. RF components are rapidly shifting away from designs that use expensive coaxial interfaces, and are moving toward designs that use printed circuit board and surface mount technologies (SMT). The traditional coaxial interface may even be eliminated from the final product. This leaves the designer with the problem of measuring the performance of these RF and microwave components with test equipment that requires coaxial interfaces. The solution is to use a test fixture that interfaces the coaxial and non-coaxial transmission lines.
The large variety of printed circuit transmission lines makes it difficult to create test equipment that can easily interface to all the different types and dimensions of microstrip and coplanar transmission lines. The test equipment requires an interface to the selected transmission media through a test fixture. Accurate characterization of the surface mount device under test (DUT) requires the test fixture characteristics to be removed from the measured results. The test equipment typically used for characterizing the RF and microwave component is the vector network analyzer (VNA) which uses standard 50 or 75 ohm coaxial interfaces at the test ports. The test equipment is calibrated at the coaxial interface defined as the “measurement plane,” and the required measurements are at the point where the surface-mount device attaches to the printed circuit board, or the “device plane”. When the VNA is calibrated at the coaxial interface using any standard calibration kit, the DUT measureents include the test fixture effects.
Over the years, many different approaches have been developed for removing the effects of the test fixture from the measurement, which fall into two fundamental categories: direct measurement and de-embedding. Direct measurement requires specialized calibration standards that are inserted into the test fixture and measured. The accuracy of the device measurement relies on the quality of these physical standards . De-embedding uses a model of the test fixture and mathematically removes the fixture characteristics from the overall measurement. This fixture “de-embedding” procedure can produce very accurate results for the non-coaxial DUT, without complex non-coaxial calibration standards.
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