Anritsu 37 C Series, Operation Manual, Page: 48 / 603

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 charac-
teristics. 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 sec-
ond 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
, there-
fore, is the signal being injected into port 1 relative to the signal leav-
ing port 1. The four scattering parameters (Figure 3-15):
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 dis-
continuity removed from the important 0 degree area used as the
phase reference.
37XXXC 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
P
HAS
E
FREQUENCY
Figure 3-16. Linear phase-with-
frequency waveform