Measured Quantities R&S
®
ZVA/ZVB/ZVT
1145.1084.12 3.26 E-1
Measured Quantities
This section gives an overview of the measurement results of the network analyzer and the meaning of
the different measured quantities. All quantities can be selected in the Trace – Meas. submenu.
The definitions in this and the following sections apply to general n-port DUTs. An analyzer with
a smaller number of test ports provides a subset of the n-port quantities.
S-Parameters
S-parameters are the basic measured quantities of a network analyzer. They describe how the DUT
modifies a signal that is transmitted or reflected in forward or reverse direction. For a 2-port
measurement the signal flow is as follows:
Extensions to the signal flow
The figure above is sufficient for the definition of S-parameters but does not necessarily show the
complete signal flow. In fact, if the source and load ports are not ideally matched, part of the transmitted
waves are reflected off the receiver ports so that an additional a
2
contribution occurs in forward
measurements, an a
1
contribution occurs in reverse measurements. The 7-term calibration types Txx
take these additional contributions into account.
The scattering matrix links the incident waves a
1
, a
2
to the outgoing waves b
1
, b
2
according to the
following linear equation:
∗
=
2
1
2221
1211
2
1
a
a
SS
SS
b
b
The equation shows that the S-parameters are expressed as S
<out>< in>
, where <out> and <in> denote
the output and input port numbers of the DUT.
Meaning of 2-port S-parameters
The four 2-port S-parameters can be interpreted as follows:
• S
11
isthe input reflection coefficient, defined as the ratio of the wave quantities b
1
/a
1
, measured at
PORT 1 (forward measurement with matched output and a
2
= 0).
• S
21
is the forward transmission coefficient, defined as the ratio of the wave quantities b
2
/a
1
(forward measurement with matched output and a
2
= 0).
• S
12
is the reverse transmission coefficient, defined as the ratio of the wave quantities b
1
(reverse
measurement with matched input, b
1,rev
in the figure above and a
1
= 0) to a
2
.
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