System Overview
R&S
®
ZVA
75
Quick Start Guide 1145.1090.62 ─ 10
●
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
.
●
S
22
is the output reflection coefficient, defined as the ratio of the wave quantities b
2
(reverse measurement with matched input, b
2,rev
in the figure above and a
1
= 0) to
a
2
, measured at PORT 2.
Meaning of squared amplitudes
The squared amplitudes of the incident and outgoing waves and of the matrix elements
have a simple meaning:
Table 3-1: Squared S-parameters
|a
1
|
2
Available incident power at the input of a two-port (= the power pro-
vided by a generator with a source impedance equal to the reference
impedance Z
0
)
|a
2
|
2
Available incident power at the output
|b
1
|
2
Reflected power at the input of a two-port
|b
2
|
2
Reflected power at the output
10*log|S
11
|
2
(= 20*log|S
11
|)
Reflection loss at the input
10*log|S
22
|
2
Reflection loss at the output
10*log|S
21
|
2
Insertion loss at the input
10*log|S
12
|
2
Insertion loss at the output
3.3.1.1
Multiport S-Parameters
The multiport S-parameters extend the standard 2-port S-parameters to a larger number
of incoming and outgoing waves. For a 4-port DUT,
a
a
a
a
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
S
b
b
b
b
4
3
2
1
44
43
42
41
34
33
32
31
24
23
22
21
14
13
12
11
4
3
2
1
where again a
I
(I = 1 to 4) denote the incident, b
I
(I = 1 to 4) denote the outgoing waves,
and the S-parameters are expressed as S
<out>< in>
.
The indices of the S-parameters described so far number the output and input ports of a
DUT; the parameters are referred to as single-ended S-parameters. The S-parameter
description can also be used to differentiate between different propagation modes of the
waves at the output and input ports. This results in the so-called mixed mode S-param-
eters. The analyzer measures either single-ended or mixed mode S-parameters.
Measured Quantities
