System Overview
R&S
®
ZVT
68
Getting Started 1300.0045.62 ─ 10
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 so that an
additional a
2
contribution occurs in forward measurements, an a1 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 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 quanti-
ties 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 quanti-
ties 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 refer-
ence impedance Z
0
)
|a
2
|
2
Available incident power at the output
Measured Quantities
