6-4 PN: 10580-00343 Rev. H S820E UG
6-2 S-Parameters VNA Mode
6-2 S-Parameters
To simplify the description of the types of measurements a VNA can make,
the reflection and transmission measurements are defined in terms of
scattering parameters, or S-parameters. For a 2-port network, four
fundamental S-parameters can be measured, and they are defined as S
XY
.
For a 2-port VNA, measurements of signals leaving Port 1 are called forward
measurements, and those leaving Port 2 are called reverse measurements.
Signals that leave and return to the same port are designated reflection
measurements, and those that leave one port and return to another port are
designated transmission measurements. S-parameters are an abbreviated
designation for these measurements, and are used as shown in the following
list:
• S
11
: Forward Reflection
• S
21
: Forward Transmission
• S
12
: Reverse Transmission
• S
22
: Reverse Reflection
The first number (X) in S
XY
is the port number in which the signal is being
received, and the second number (Y) is the port number from which the signal
is being transmitted. The S-parameter is a ratio of these two signals.
Additional Examples:
S
11
: Forward Reflection represents the measurement in which the
incident signal is transmitted from port 1 and is reflected back to port 1.
S
21
: Forward Transmission represents the measurement in which the
incident signal is transmitted from port 1 and is received at port 2.
S
12
: Reverse Transmission represents the measurement in which the
incident signal is transmitted from port 2 and is received at port 1.
S
22
: Reverse Reflection represents the measurement in which the incident
signal is transmitted from port 2 and is reflected back to port 2.
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