Specifications

Appendix B: Open/Short Compensation
The open/short compensation used in Agilent’s instrument models the residuals of a test fixture or
test leads as a linear four-terminal network (a two-terminal pair network) represented by parame-
ters A, B, C, and D (shown in Figure B-1.) This circuit model is basically same as that used in
open/short/load compensation.
Figure B-1. Four-terminal network circuit model of a test fixture or test cables
The difference between open/short and open/short/load compensation is that the open/short
compensation assumes the unknown network as a “symmetrical network.” From this restriction, the
open/short compensation does not require the load measurement.
The circuit model shown in Figure B-1 is expressed by using the following matrix equation:
(
V
1
)
=
(
A B
)(
V
2
)
(1)
I
1
C D I
2
The relationships between V
1
, I
1
, V
2
, and I
2
are given by the following equations:
{
V
1
= AV
2
+ BI
2
I
1
= CV
2
+ DI
2
The measured impedance of the DUT, Zxm, is expressed as:
Zxm =
V
1
=
AV
2
+ BI
2
(2)
I
1
CV
2
+ DI
2
On the other hand, the true value of the DUT, Zdut, is expressed as:
Zdut =
V
2
(3)
I
2
From equations (2) and (3), the equation that expresses the relationship between Zxm and Zdut is
derived as follows:
A
V
2
+ B
Zxm =
AV
2
+ BI
2
=
I
2
=
AZdut + B (4)
CV
2
+ DI
2
C
V
2
+ D
CZdut + D
I
2
A B
C D
DU T
V
2
Unknown 4-terminal
circuit
Measurement
instrument
Z
du t
I
1
I
2
V
1
B-1