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3-10 ES User’s Guide
Making Measurements
Measuring Devices with Your Network Analyzer
NOTE
When making any type of S-parameter measurement with a two-port
calibration, the analyzer takes both a forward and a reverse sweep while
collecting data.
S
11
and S
21
are determined by measuring the magnitude and phase of
the incident, reflected, and transmitted signals when the output is
terminated in a perfect Z
0
(a load that equals the characteristic
impedance of the test system). This condition guarantees that a
2
is zero.
S
11
is equivalent to the input complex reflection coefficient or impedance
of the DUT, and S
21
is the forward complex transmission coefficient.
Likewise, by placing the source at port 2 and terminating port 1 in a
perfect load (making a
1
zero), S
22
and S
12
measurements can be made.
S
22
is equivalent to the output complex reflection coefficient or output
impedance of the DUT, and S
12
is the reverse complex transmission
coefficient.
The accuracy of S-parameter measurements depends greatly on how
good a termination we apply to the port not being stimulated. Anything
other than a perfect load will result in a
1
or a
2
not being zero (which
violates the definition for S-parameters). When the DUT is connected to
the test ports of a network analyzer and you don’t account for imperfect
test port match, you haven’t satisfied the condition of a perfect
termination. For this reason, two-port error correction, which corrects for
source and load match, is very important for accurate S-parameter
measurements (two-port correction is covered in more detail in
Chapter 6, “Calibrating for Increased Measurement Accuracy” ).