Digital Signal Processing Reference
In-Depth Information
VNA
DUT
R
Error
Port1
Error
Port2
DUT
R
Desired measurement
reference planes
Actual measurements are referenced internal to the VNA
and include cable, probe and connector losses and phase delays
Figure 9-35
Calibration removes the unwanted parts of the measurement, such as the
cable and probe effects.
a
1
b
2
b
1
a
2
Z
L
Error
Port1
Γ
in
L
Figure 9-36
The simplest way to calibrate a VNA is to characterize the port error by
driving different loads (
Z
L
): an open, a short, and a resistive load.
precision resistive load. For example, consider Figure 9-36, which represents
one port of a VNA driving a load
Z
L
. The equations for the
S
-parameters can be
written in terms of the reflection coefficient at the load where the incoming wave
at port 2 (
a
2
) is simply the reflected portion of the exiting wave (
b
2
)
a
2
=
b
2
L
.
b
1
=
S
11
a
1
+
S
12
a
2
=
S
11
a
1
+
S
12
b
2
L
b
2
=
S
21
a
1
+
S
22
a
2
=
S
21
a
1
+
S
22
b
2
L
S
21
a
1
b
2
(
1
−
S
22
L
)
=
S
21
a
1
→
b
2
=
(9-51a)
1
−
S
22
L
S
21
a
1
b
1
=
S
11
a
1
+
S
12
−
S
22
L
L
(9-51b)
1
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