Specifications

Figure 3-13. Coaxial test port circuit configuration
3-7. 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 imped-
ance close to the residual parameter values, variance in the residuals of the test fixture will cause
measurement repeatability problem. 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% difference in mea-
sured value. The variance in residuals and resultant measurement instability is dependent on the
accuracy of positioning the DUT on the test fixture terminals. For repeatable measurements, RF test
fixtures should be able to precisely position DUT across measurement terminals.
The test fixture residuals will have greater effects on measurement at higher frequencies (typically
above 500 MHz) and will narrow 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 inaccuracy and the test
fixture induced errors. Because only the 2-terminal measurement configuration is available, com-
pensation method is crucial to optimize measurement accuracy. The measurement error sources
and compensation techniques are discussed in Section 4.
Each test fixture has unique characteristics and different structure from others. 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.
3-13