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Network Analyzer Measurements
Now let us consider measuring the DUT. Consider a two port device;
that is, a device with a connector on each end. What measurements
would be of interest?
First, we could measure the reflection characteristics at either end
with the other end terminated into 50 ohms. If we designate one end
as the normal place for the input that gives a reference. We can then
define the reflection characteristics from the reference end as forward
reflection, and those from the other end as reverse reflection
(Figure 3-14).
Second, we can measure the forward and reverse transmission
characteristics. However, instead of saying “forward,” “reverse,”
“reflection,” and “transmission” all the time, we use a shorthand. That
is all that S-parameters are, a shorthand! The “S” stands for
scattering. The second number is the device port that the signal is
being injected into, while the first is the device port that the signal is
leaving. S
11
, therefore, is the signal being injected into port 1 relative
to the signal leaving port 1. The four scattering parameters
(Figure 3-15) are:
q S
11
Forward Reflection
q S
21
Forward Transmission
q S
22
Reverse Reflection
q S
12
Reverse Transmission
S-parameters can be displayed in many ways. An S-parameter consists
of a magnitude and a phase. We can display the magnitude in dB, just
like a scalar network analyzer. We often call this term log magnitude.
We can display phase as “linear phase” (Figure 3-16). As discussed
earlier, we can’t tell the difference between one cycle and the next.
Therefore, after going through 360 degrees we are back to where we
began. We can display the measurement from –180 to +180 degrees.
The –180 to +180 approach is more common. It keeps the display
discontinuity removed from the important 0 degree area used as the
phase reference.
37xxxE OM 3-9
NETWORK ANALYZERS, A PRIMER NETWORK ANALYZERS
FORWARD
REFLECTION
REVERSE
REFLECTION
PORT 1 PORT 2
DUT
Figure 3-14. Forward and
Reverse
Measurements
S12 REVERSE TRANSMISSION
PORT 1 PORT 2
S11 FORWARD
REFLECTION
S22 REVERSE
REFLECTION
S21 FORWARD TRANSMISSION
DUT
Figure 3-15. S-parameters
+180
0
-180
PHASE
FREQUENCY
Figure 3-16. Linear
Phase-with-frequen
cy Waveform