Specifications
5-10
5.2.2 Causes of measurement discrepancies for inductors
Inductance measurement sometimes gives different results when a DUT is measured using different
instruments. There are some factors of measurement discrepancies as described below:
Test signal current
Inductors with a magnetic core exhibit a test signal current dependency due to the nonlinear magne-
tization characteristics of the core material as shown in Figure 5-13 (a). The level of test signal
current depends on the impedance measurement instrument because many of the instruments out-
put a voltage-driven test signal. Even when two different instruments are set to output the same test
signal (OSC) voltage, their output currents are different if their source resistance, Rs, is not the
same as shown in Figure 5-13 (b).
To avoid the measurement discrepancies, the OSC level should be adjusted for a defined test current
by using the auto level control (ALC) function or by determining the appropriate test voltage setting
from the equation shown in Figure 5-13 (b).
Figure 5-13. Inductor test signal current
Test fixture used
When a metal object is located closely to an inductor, leakage flux from the inductor will induce
eddy currents in the metal object. The magnitudes of the induced eddy currents are dependent on
the dimensions and physical geometry of metal object, as shown in Figure 5-14 (a), causing differ-
ences in the measured values. The eddy current effect is especially important for measuring open-
flux-path inductors. Figure 5-14 (b) shows an example of the difference in Ls – Q measurement val-
ues due to the eddy current effect. When a 40 mm x 40 mm square and 1.0 mm thick brass plate is
placed closely to a 100 µH RF inductor, the measured Ls – Q values decrease according to the
approach of the plate from
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(sufficient distance) to 10 mm and 1 mm. The eddy current effect due
to the leakage flux causes discrepancies in measurement results between different types of test fix-
tures because the test fixtures are also metal objects. To obtain consistent measurement results, it is
necessary to define the test fixture used for inductor measurements. Additionally, the DUT should
be connected at the same position of the same test fixture.