4
Introduction
What is a Network Analyzer?
One of the most common measuring tasks in RF engi-
neering involves analysis of circuits (networks). A net-
work analyzer is an instrument that is designed to handle
this job with great precision and efciency. Circuits that
can be analyzed using network analyzers range from
simple devices such as lters and ampliers to complex
modules used in communications satellites.
A network analyzer is the most complex and versatile
piece of test equipment in the eld of RF engineering. It
is used in applications in research and development and
also for test purposes in production. When combined
with one or more antennas, it becomes a radar system.
Systems of this type can be used to detect invisible ma-
terial defects without resorting to X-ray technology. Using
data recorded with a network analyzer, imaging tech-
niques were used to produce the following gure which
shows a typical material defect. (Figure 1.1.2)
A similar system can be used to verify the radar visibility
which forms the basis for a dependable ight control
system. For such purpose the radar cross section (RCS)
of an aircraft is an important quantity. It is typically mea-
sured on a model of the aircraft like the following result.
(Figure 1.1.3)
For measurements with less demanding technical re-
quirements such as measurement of a ll level without
physical contact or determination of the thickness of
layers of varnish, simpler approaches are generally used.
Wave Quantities and S-Parameters
The so-called wave quantities are preferred for use in
characterizing RF circuits. We distinguish between the
incident wave
a
and the reected wave
b
. The incident
wave propagates from the analyzer to the device under
test (DUT). The reected wave travels in the opposite
direction from the DUT back to the analyzer. In the fol-
lowing gures, the incident wave is shown in green and
the reected wave in orange. Fig. 1.2.1 shows a one-port
device with its wave quantities.
The true power traveling to the one-port device is given
by |a|
2
and the true power it reects by |b|
2
. The reection
coefcient Γ represents the ratio of the incident wave to
the reected wave.
Γ = b/a (1.2-1)
It is generally a complex quantity and can be calculated
from the complex impedance Z. With a reference imped-
ance of typically Z
0
= 50 Ω
1
, the normalized impedance
Fig. 1.1.2 Material defect Fig. 1.1.3 ISAR image of a
Boeing 747 model
Fig. 1.2.1 One-port device with
incident and reected waves.
1) In RF engineering and RF measurement a reference impedance of 50 Ω is used. In broadcasting systems a reference impedance of 75Ω is preferred. The impedance of
50 Ω offers a compromise which is closely related to coaxial transmission lines. By varying the inner and out conductor diameter of a coaxial transmission line we achieve
its minimum attenuation at a characteristic impedance of 77 Ω and its maximum power handling capacity at 30 Ω.
