Specifications

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 characteris
-
tics 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 “trans
-
mission” 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 network
analyzer port that the signal is being injected into,
while the first is the network analyzer 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):
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. There
-
fore, 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 ap
-
proach is more common. It keeps the display discon
-
tinuity removed from the important 0 degree area
used as the phase reference.
NETWORK ANALYZERS, NETWORK
A PRIMER ANALYZERS
MS462XX OM 3-9
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-
Frequency Waveform