Agilent Technologies 4294A Specifications download pdf (Page 113)

APPENDIX A

The concept of a test fixture’s additional error

1. System configuration for impedance measurement

Very often, the system configured for impedance measurements utilizes the following components

(See figure below as well).

1. Impedance measurement instrument

2. Cables and adapter interfaces

3. Test fixture

System configuration for impedance measurement

The Impedance measurement instrument’s characteristic measurement accuracy is defined at the

measurement port of the instrument. This means that the accuracy at the measurement port has

traceability and the measurement values are guaranteed.

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 with the intent to maintain high accuracy of the measurement instrument

while extending the measurement port. Most of the time, the measurement accuracy of the instru-

ment 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

fixtures are provided to adapt to various shapes and sizes. It is either connected directly to the mea-

surement port of the instrument or to the port of the extension cable or conversion adapter as

described earlier. Its structure determines the applicable frequency range and signal level. Hence, it

is necessary to use the appropriate one for the corresponding measurement conditions. In addition,

each test fixture has its own characteristic additional error, which is listed in its operational manu-

al.

2. Measurement system accuracy

The equation for the accuracy of a measurement system is shown below:

(Measurement accuracy) = (Instrument’s accuracy) + (Test fixture’s additional error)

The measurement instrument’s accuracy is determined by an equation with terms, which are dependent

on frequency, measured impedance and signal level. By substituting the respective measurement condi-

tions 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 operational man-

ual.

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Agilent Technologies 4294A Specifications Page 113