Test & Measurement
Figure 4: With two parallel transmission lines, the
characteristic impedance of the combined
transmission line is the parallel combination of
each transmission line’s characteristic
impedances
Figure 2: C-V device accurately. In this figure, the device to be measurement cabling system based on an
measurement characterized has two terminals. understanding of two transmission concepts. The
32
connection scheme for Figure 2 illustrates the cabling for C-V first concept is that if there are two parallel
Kelvin connections measurements using four coaxial cables. The transmission lines, the characteristic impedance
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outer shells are connected together to control of the combined transmission line is the parallel
.eur the characteristic impedance the signals see. The combination of each transmission line’s
oasiasemiconductor
outer shells of all four cables must be inter- characteristic impedances, as shown in Figure 4.
connected near the device under test (DUT). The second transmission line concept is that
Pulsed measurements require the highest if there are two transmission lines connected in
bandwidth of the three measurement types, so series, as shown in Figure 5, the characteristic
to prevent reflections off the DUT from reflecting impedance of the combined transmission line is
off the source, the cable must have a the sum of the characteristic impedances of the
.com
characteristic impedance that matches the two individual transmission lines.
source impedance. Pulsing does not use a This series arrangement can be observed in
remote sense cable. Figure 3 illustrates a typical a triaxial cable, as shown in Figure 6. A triaxial
square4
Issue VI 2009
connection to a two-terminal DUT. Pulsing is the cable is actually two concentrically arranged
only one of the three measurement types that transmission lines. The inner shield and the
connects the DUT to the outer shield of the centre conductor form one transmission line (Z
1
)
cable. and the inner shield and the outer shield form a
second transmission line (Z
2
). The centre
Characteristic Impedances conductor to outer shield interaction has a
Figure 3: Pulse testing Given the challenges created by these differing characteristic impedance (Z
S
) equal to the sum
connection scheme for cabling requirements for different measurement of the two transmission lines that share the inner
Kelvin connections types, Keithley has developed a multi- shield.
Taking these two transmission line concepts
into account, Keithley has developed a cabling
kit that supports making I-V, C-V and pulsed I-V
measurements with the same set of cables. This
reduces the effort required from operators of
parameter analyzer systems, who would
otherwise be forced to go through the laborious
process of re-cabling the connections from the
bulkhead/feedthrough to the probe tips each
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