R&S
®
ZVL
System Overview
Measured Quantities
Quick Start Guide 1303.6538.62-01
89
3.2 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.
3.2.1 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
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 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
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:
E
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).
E
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).
