Fixtures

Fixtures are needed to interface non-coaxial devices to coaxial test instruments. It may also be necessary to transform the characteristic impedance from standard 50 ohm instruments to a non-standard impedance and to apply bias if an active device is being measured.

For accurate measurements, the ilxture must introduce minimum change to the test signal, not destroy the test device, and provide a repeatable connection to the device.

Hewlett-Packard offers several fixtures for TO cans, stripline, and microstrip devices. Refer to Chapter 11, “Compatible Peripherals. n

If You Want to Design Your Own Fixture

Ideally, a Gxture should provide a transparent connection between the test instrument and the test device. This means it should have no loss or electrical length and a flat frequency response, to prevent distortion of the actual signal. A perfect match to both the instrument and the test device eliminates reflected test signals. The signal should be effectively coupled into the test device, rather than leaking around the device and resulting in crosstalk from input to output. Repeatable connections are necessary to ensure consistent data.

RealisticaIly, it is impossible to build an ideal fixture, especially at high frequencies. However, it is possible to optimize the performance of the test llxture relative to the performance of the test device. If the fixture’s effects on the test signal are relatively small compared to the device’s parameters, then the llxture’s effects can be assumed to be negligible.

For example, if the fixture’s loss is much less than the acceptable measurement uncertainty at the test frequency, then it can be ignored.

6-170 Application and Operation Concepts

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HP 8753E manual Fixtures, If You Want to Design Your Own Fixture