Specifications

A-1
Appendix A. The Concept of a Test Fixture’s Additional Error
A.1 System configuration for impedance measurement
Frequently the system configured for impedance measurements uses the following components
(see Figure A-1.)
1. Impedance measurement instrument
2. Cables and adapter interfaces
3. Test fixture
Figure A-1. System configuration for impedance measurement
The impedance measurement instrument’s accuracy is defined at the measurement port of the
instrument. This means that the accuracy of the measurement values at the measurement port is
guaranteed and has calibration traceability.
In an actual measurement, there can be an extension of the measurement port with a cable or an
adapter conversion to match the test fixture’s terminal configuration. For this reason, cables and
conversion adapters are provided for connectivity with the measurement port. These cables (and
adapters) are designed to maintain high accuracy of the measurement instrument while extending
the measurement port. Most of the time, the measurement accuracy of the instrument and the cable
(or adapter) are specified together as a whole.
A test fixture is an accessory used to connect the DUT to the measurement instrument. Many test fix-
tures are provided to adapt to various shapes and sizes of DUTs. A test fixture is either connected
directly to the measurement port of the instrument, or to the port of the extension cable or conver-
sion adapter, as described earlier. The test fixture’s structure determines the applicable frequency
and impedance ranges. Hence, it is necessary to use the appropriate test fixture for the desired mea-
surement conditions. In addition, each test fixture has its own inherent characteristic error, which is
detailed in its operation manual.
A.2. Measurement system accuracy
The equation for the accuracy of a measurement system is:
(Measurement accuracy) = (Instrument’s accuracy) + (Test fixture’s error)
The measurement instrument’s accuracy is determined by an equation with terms that are depen-
dent on frequency, measured impedance, signal level, and measurement time mode. By substituting
the respective measurement conditions into the equation, the measurement accuracy is calculated. If
a cable or a conversion adapter is used, then the specified measurement accuracy is the accuracy of
the measurement instrument with the cable or adapter. This combined measurement accuracy is
shown in the instrument’s operation manual.