Specifications

3-5. Eliminating the stray capacitance effects
When the DUT has high impedance (e.g. Low Capacitance), the effects of stray capacitance are not
negligible. Figure 3-12(a) shows an example of measuring a DUT using 4 terminal contacts. In this
example, Cd is in parallel with the DUT. When a conductive plate is placed under the DUT, the com-
bined capacitance (Ch//Cl) is also in parallel with the DUT, resulting in measurement error. By
placing a guard plate between the high and low terminals, Cd can be minimized (Figure 3-12 (b)).
Also, by connecting the guard terminal to the conductor, the effects of Ch and Cl can be canceled. In
actual measurement setup, the outer shield conductor of coaxial test cables in the 3T, 4T, 5T and
4TP configuration works as the guard terminal. The guarding technique cannot apply to the 2T con-
figuration.
Figure 3-12. Guarding techniques to eliminate the stray capacitance effects
3-6. Terminal configuration in RF region
RF impedance measuring instruments have a precision coaxial test port, which is actually a 2 termi-
nal configuration in principle. The center conductor of coaxial test port connector is active High
side terminal and the outer conductor is grounded Low side terminal, as shown in Figure 3-13. To
measure the DUT, only the simplest 2-terminal connection configuration can be used. Residual
inductance, residual resistance, stray capacitance and stray conductance of the test fixture will add
to measurement results (before compensation). Whether the RF I-V method or network analysis, RF
impedance measurement has lower accuracy as the measured impedance differs greater from 50 Ω.
Instrument inaccuracy rather than the error factors in 2-terminal test fixture primarily limits the
measurement range. The effect of residuals increases with frequency and narrows the measurable
impedance range in very high frequencies.
3-12