Specifications

3.4.2 Guarding technique to remove stray capacitances
By inserting a shielding plate between the DUT and the conductor, and by connecting it to the guard
terminal of the instrument as shown in Figure 3-15 (b), the leakage current flow through the stray
capacitances can be eliminated. Since the Low side of the DUT has a potential of zero volts (virtual
ground) equal to the guard potential, the voltage difference that yields the stray capacitance on the
Low side is extinguished. Basically, the guard terminal is the outer shielding conductor of the test
cables.
Note: If the conductor yielding the stray capacitances is isolated from the ground and is free of
noise, it may be directly connected to the guard terminal without using the additional
shielding plate. On the contrary, if the conductor has a noise potential, this method should be
avoided because noise current flows into the outer shielding conductor of test cables and may
disturb measurements.
When a stray capacitance in parallel with the DUT is present between High and Low terminals, as
shown in Figure 3-16 (a), it can be removed by inserting a shielding plate between the High and Low
terminals and by connecting the plate to the guard terminal (as shown in Figure 3-16 (b).)
Figure 3-16. Guarding technique (2)
3.5 Terminal configuration in RF region
RF impedance measuring instruments have a precision coaxial test port, which is actually a 2T
configuration in principle. The center conductor of the coaxial test port connector is active (High
side) terminal and the outer conductor is grounded Low side terminal, as shown in Figure 3-17. To
measure the DUT, only the simplest 2T connection configuration can be used. Residual inductance,
residual resistance, stray capacitance, and stray conductance of the test fixture will add to measure-
ment results (before compensation.) Whether using the RF I-V method or network analysis, RF
impedance measurement has lower accuracy as the measured impedance differs greater from 50 Ω.
H
p
H
c
L
c
L
p
V
Null
detector
DUT
(b) Removing the stray capacitance
Insert a
shielding plate
Connect to
guard
H
p
H
c
L
c
L
p
V
Null
detector
DUT
Leakage
current
(a) Stray capacitance between
High and Low terminals
Stray C
3-16