Specifications

3-17
Instrument inaccuracy, rather than the error factors in the 2T test fixture, primarily limits the mea-
surement range. The effect of residuals increases with frequency and narrows the measurable
impedance range in very high frequencies.
Figure 3-17. Coaxial test port circuit configuration
3.6 RF test fixtures
RF test fixtures are designed so that the lead length (electrical path length) between the DUT and
the test port is made as short as possible to minimize residuals. At frequencies typically below
100 MHz, measurement error due to test fixture residuals is small compared to instrument error and
is normally negligible after compensation is made. But, especially when measuring low or high
impedance close to the residual parameter values, variance in the residuals of the test fixture will
cause measurement repeatability problems. For example, when measuring a 1 nH inductor (a very
low inductance), a slight variance of 0.1 nH in residual inductance will produce a 10 percent
difference in the measured value. The variance in the residual, and resultant measurement instability,
is dependent on the accurate positioning of the DUT on the test fixture terminals. For repeatable
measurements, RF test fixtures should be able to precisely position the DUT across measurement
terminals.
The test fixture residuals will have greater effects on measurements at higher frequencies (typically
above 500 MHz) and will narrow the practical measurement range. Therefore, the usable frequency
range of the test fixture is limited to the maximum frequency specified for each test fixture.
The measurement inaccuracy for the DUT is given by sum of the instrument’s inaccuracy and the
test-fixture induced errors. Because only the 2T measurement configuration is available, the com-
pensation method is crucial for optimizing measurement accuracy. The measurement error sources
and compensation techniques are discussed in Section 4.
Each test fixture has unique characteristics and different structures. Since not only the residuals
but also the surrounding conditions of the DUT (such as ground plate, terminal layout, dielectric
constant of insulator, etc.) influence the measured values of the DUTs, the same type of test fixture
should be used to achieve good measurement correlation.