Specifications

4-7-3. A difference in open/short compensation conditions
Improper open/short measurements deteriorate accuracy of compensated measurement results. If
the open/short measurement conditions are not always the same, inconsistent measurement values
will result. Particularly, each short device has its inherent impedance (inductance) value and, if not
defined as zero or an appropriate value, the difference of the short device used will produce resul-
tant measurement discrepancies. Effective impedance of the short device will vary depending on
how it contacts to the measurement terminals. When the bottom-electrode test fixture is used, con-
tact points on the measurement terminals will be different from the case of the parallel-electrode
test fixture, as shown in Figure 4-19. Besides, if the short device is not straight (slightly curved), the
measured impedance will be different depending on which side of the device comes upside. These
effects are usually small, but should be taken into considerations especially when performing a very
low inductance measurement, typically below 10 nH.
Figure 4-19. Difference in short impedance by test fixture types
4-7-4. Electromagnetic coupling with a conductor near the DUT
Electromagnetic coupling between the DUT and a metallic object near the DUT varies with mutual
distance and causes variance in measured values. Leakage flux generated around inductive DUT
induces an eddy current in a closely located metallic object. The eddy current suppresses the flux,
decreasing the measured inductance and Q factor values. The distance of the metallic object from
the DUT is a factor of the eddy current strength as shown in Figure 4-20 (a). As test fixtures contain
metallic objects, this is an important cause of measurement discrepancies due to test fixture. Open-
flux-path inductors usually have directivity in generated leakage flux. As a result, measured values
will vary depending on the direction of the DUT. The difference in the eddy current due to the leak-
age flux directivity is illustrated in Figure 4-20 (b), (c) and (d).
If a parasitic capacitance exists between the DUT and an external conductor, it is difficult to remove
the effect on measurement because guarding technique is invalid. Thus, the DUT should be separat-
ed from the conductor with enough distance to minimize measurement errors.
4-21