System Overview
R&S
®
ZVA
74
Quick Start Guide 1145.1090.62 ─ 10
3.3 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 ana-
lyzer with a smaller number of test ports provides a subset of the n-port quantities.
3.3.1 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 direc-
tion. For a 2-port measurement the signal flow is as follows.
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 a1 contribution occurs in reverse meas-
urements. 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
accord-
ing to the following linear equation:
2
1
22
21
12
11
2
1
a
a
S
S
S
S
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
is the 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).
Measured Quantities
