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Agilent
Network Analyzer Basics
Abstract
This presentation covers the principles
of measuring high-frequency electrical
networks with network analyzers.
You will learn what kinds of
measurements are made with network
analyzers, and how they allow you to
characterize both linear and nonlinear
behavior of your devices. The session
starts with RF fundamentals such
as transmission lines and the Smith
chart, leading to the concepts
of reflection, transmission and
S-parameters. The next section covers
the major components in a network
analyzer, including the advantages
and limitations of different hardware
approaches. Error modeling, accuracy
enhancement, and various calibration
techniques will then be presented.
Finally, some typical swept-frequency
and swept-power measurements
commonly performed on filters
and amplifiers will be covered.
An appendix is also included with
information on advanced topics,
with pointers to more information.
2
Network Analysis is Not....
This module is not about computer
networks! When the name "network
analyzer" was coined many years ago,
there were no such things as computer
networks. Back then, networks
always referred to electrical networks.
Today, when we refer to the things
that network analyzers measure,
we speak mostly about devices and
components.
What Types of Devices Are
Tested?
Here are some examples of the types
of devices that you can test with
network analyzers. They include
both passive and active devices (and
some that have attributes of both).
Many of these devices need to be
characterized for both linear and
nonlinear behavior. It is not possible
to completely characterize all of these
devices with just one piece of test
equipment.
The next slide shows a model covering
the wide range of measurements
necessary for complete linear and
nonlinear characterization of devices.
This model requires a variety of
stimulus and response tools. It takes
a large range of test equipment to
accomplish all of the measurements
shown on this chart. Some instruments
are optimized for one test only (like
bit-error rate), while others, like
network analyzers, are much more
generalpurpose in nature. Network
analyzers can measure both linear
and nonlinear behavior of devices,
although the measurement techniques
are different (frequency versus power
sweeps for example). This module
focuses on swept-frequency and
swept-power measurements made
with network analyzers
3
Device Test Measurement Model
Here is a key to many of the
abbreviations used at right:
Response
84000 8400 series high-volume RFIC tester
Ded. Testers Dedicated (usually one-box) testers
VSA Vector signal analyzer
SA Spectrum analyzer
VNA Vector signal analyzer
TG/SA Tracking generator/spectrum analyzer
SNA Scalar network analyzer
NF Mtr. Noise-figure meter
Imped. An. Impedance analyzer (LCR meter)
Power Mtr. Power meter
Det./Scope Diode detector/oscilloscope
Measurement
ACP Adjacent channel power
AM-PM AM to PM conversion
BER Bit-error rate
Compr'n Gain compression
Constell. Constellation diagram
EVM Error-vector magnitude
Eye Eye diagram
GD Group delay
Harm. Dist. Harmonic distortion
NF Noise figure
Regrowth Spectral regrowth
Rtn Ls Return loss
VSWR Voltage standing wave ratio
4
Lightwave Analogy to RF Energy
One of the most fundamental concepts
of high-frequency network analysis
involves incident, reflected and
transmitted waves traveling along
transmission lines. It is helpful to
think of traveling waves along a
transmission line in terms of a
lightwave analogy. We can imagine
incident light striking some optical
component like a clear lens. Some of
the light is reflected off the surface
of the lens, but most of the light
continues on through the lens. If
the lens were made of some lossy
material, then a portion of the light
could be absorbed within the lens.
If the lens had mirrored surfaces,
then most of the light would be
reflected and little or none would
be transmitted through the lens.
This concept is valid for RF signals
as well, except the electromagnetic
energy is in the RF range instead of
the optical range, and our components
and circuits are electrical devices
and networks instead of lenses and
mirrors.
Network analysis is concerned with
the accurate measurement of the
ratios of the reflected signal to the
incident signal, and the transmitted
signal to the incident signal.
5
Why Do We Need to Test
Components?
Components are tested for a variety
of reasons. Many components are
used as "building blocks" in more
complicated RF systems. For example,
in most transceivers there are amplifiers
to boost LO power to mixers, and
filters to remove signal harmonics.
Often, R&D engineers need to measure
these components to verify their
simulation models and their actual
hardware prototypes. For component
production, a manufacturer must
measure the performance of their
products so they can provide accurate
specifications. This is essential so
prospective customers will know how
a particular component will behave
in their application.
When used in communications
systems to pass signals, designers
want to ensure the component or
circuit is not causing excessive signal
distortion. This can be in the form of
linear distortion where flat magnitude
and linear phase shift versus frequency
is not maintained over the bandwidth
of interest, or in the form of nonlinear
effects like intermodulation distortion.
Often it is most important to measure
how reflective a component is, to ensure
that it absorbs energy efficiently.
Measuring antenna match is a good
example.
6
The Need for Both Magnitude
and Phase
In many situations, magnitude-only
data is sufficient for out needs. For
example, we may only care about the
gain of an amplifier or the stop-band
rejection of a filter. However, as we
will explore throughout this paper,
measuring phase is a critical element
of network analysis.
Complete characterization of devices
and networks involves measurement
of phase as well as magnitude.
This is necessary for developing
circuit models for simulation and
to design matching circuits based
on conjugatematching techniques.
Time-domain characterization
requires magnitude and phase
information to perform the inverse-
Fourier transform. Finally, for best
measurement accuracy, phase data
is required to perform vector error
correction.
Agenda
In this section we will review reflection
and transmission measurements. We
will see that transmission lines are
needed to convey RF and microwave
energy from one point to another with
minimal loss, that transmission lines
have a characteristic impedance, and
that a termination at the end of a
transmission line must match the
characteristic impedance of the line
to prevent loss of energy due to
reflections. We will see how the
Smith chart simplifies the process
of converting reflection data to the
complex impedance of the termination.
For transmission measurements,
we will discuss not only simple gain
and loss but distortion introduced
by linear devices. We will introduce
S-parameters and explain why they are
used instead of h-, y-, or z-parameters
at RF and microwave frequencies.
7
Transmission Line Basics
The need for efficient transfer of RF
power is one of the main reasons
behind the use of transmission
lines. At low frequencies where the
wavelength of the signals are much
larger than the length of the circuit
conductors, a simple wire is very
useful for carrying power. Current
travels down the wire easily, and
voltage and current are the same no
matter where we measure along
the wire.
At high frequencies however, the
wavelength of signals of interest are
comparable to or much smaller than
the length of conductors. In this case,
power transmission can best be
thought of in terms of traveling waves.
Of critical importance is that a
lossless transmission line takes on
a characteristic impedance (Zo). In
fact, an infinitely long transmission
line appears to be a resistive load!
When the transmission line is
terminated in its characteristic
impedance, maximum power is
transferred to the load. When the
termination is not Zo, the portion of
the signal which is not absorbed by
the load is reflected back toward
the source. This creates a condition
where the envelope voltage along the
transmission line varies with position.
We will examine the incident and
reflected waves on transmission
lines with different load conditions
in following slides
8
Transmission Line Z0
RF transmission lines can be made in
a variety of transmission media.
Common examples are coaxial, wave-
guide, twisted pair, coplanar, stripline
and microstrip. RF circuit design on
printed-circuit boards (PCB) often use
coplanar or microstrip transmission
lines. The fundamental parameter of a
transmission line is its characteristic
impedance Zo. Zo describes the
relationship between the voltage
and current traveling waves, and is
a function of the various dimensions
of the transmission line and the
dielectric constant (r) of the non-
conducting material in the transmission
line. For most RF systems, Zo is either
50 or 75 ohms.
For low-power situations (cable TV,
for example) coaxial transmission
lines are optimized for low loss, which
works out to about 75 ohms (for
coaxial transmission lines with air
dielectric). For RF and microwave
communication and radar applications,
where high power is often encountered,
coaxial transmission lines are
designed to have a characteristic
impedance of 50 ohms, a compromise
between maximum power handling
(occurring at 30 ohms) and minimum
loss.
9
Power Transfer Efficiency
Before we begin our discussion about
transmission lines, let us look at the
condition for maximum power trans-
fer into a load, given a source imped-
ance of Rs. The graph above shows
that the matched condition
(RL = RS) results in the maximum
power dissipated in the load resistor.
This condition is true whether the
stimulus is a DC voltage source or an
RF sinusoid.
For maximum transfer of energy into
a transmission line from a source or
from a transmission line to a load (the
next stage of an amplifier, an antenna,
etc.), the impedance of the source and
load should match the characteristic
impedance of the transmission line.
In general, then, Zo is the target for
input and output impedances of
devices and networks.
When the source impedance is not
purely resistive, the maximum
power transfer occurs when the load
impedance is equal to the complex
conjugate of the source impedance.
This condition is met by reversing
the sign of the imaginary part of
the impedance. For example, if
RS = 0.6 + j0.3, then the complex
conjugate RS* = 0.6 - j0.3.
Sometimes the source impedance is
adjusted to be the complex conjugate
of the load impedance. For example,
when matching to an antenna, the
load impedance is determined by the
characteristics of the antenna. A
designer has to optimize the output
match of the RF amplifier over the
frequency range of the antenna so
that maximum RF power is transmitted
through the antenna
10
Transmission Line Terminated
With Z0
Let's review what happens when
transmission lines are terminated
in various impedances, starting with
a Zo load. Since a transmission line
terminated in its characteristic
impedance results in maximum
transfer of power to the load, there
is no reflected signal. This result is
the same as if the transmission line
was infinitely long. If we were to look
at the envelope of the RF signal versus
distance along the transmission line, it
would be constant (no standing-wave
pattern). This is because there is
energy flowing in one direction only.
11
Transmission Line Terminated
with Short, Open
Next, let's terminate our line in a
short circuit. Since purely reactive
elements cannot dissipate any power,
and there is nowhere else for the
energy to go, a reflected wave is
launched back down the line toward
the source. For Ohm's law to be
satisfied (no voltage across the short),
this reflected wave must be equal in
voltage magnitude to the incident
wave, and be 180