Concepts and Features
R&S
®
ZNB/ZNBT
116
User Manual 1173.9163.02 ─ 53
5.3.1
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
direction. 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 necessa-
rily 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. An addi-
tional a
2
contribution occurs in forward measurements, and an a
1
contribution occurs in
reverse measurements. The 7-term calibration types Txx take these additional contri-
butions 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
22
21
12
11
2
1
a
a
S
S
S
S
b
b
Meaning of 2-port S-parameters
The four 2-port S-parameters can be interpreted as follows:
●
S
11
is the reflection coefficient of DUT port 1, i.e. the ratio between outgoing wave
b
1
and incident wave a
1
in a forward measurement with matched DUT port 2:
S
11
= b
1
/ a
1
, if |a
1
| > 0 and a
2
= 0
●
S
21
is the forward transmission coefficient, defined as the ratio between outgoing
wave b
2
and incident wave a
1
in a forward measurement with matched DUT port 2:
S
21
= b
2
/ a
1
, if |a
1
| > 0 and a
2
= 0
●
S
12
is the reverse transmission coefficient, defined as the ratio between outgoing
wave b
1
and incident wave a
2
in a forward measurement with matched DUT port 1:
S
12
= b
1
/ a
2
, if |a
2
| > 0 and a
1
= 0
●
S
22
is the reflection coefficient of port 2, i.e. the ratio between outgoing wave b
2
and
incident wave a
2
in a forward measurement with matched DUT port 1:
S
22
= b
2
/ a
2
, if |a
2
| > 0 and a
1
= 0
Measurement Results
