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Copyright © 2017, Tektronix. All rights reserved. Tektronix products are covered by U.S. and foreign patents, issued and pending. Information in this publication supersedes that
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03/17 EA 2D-61077-0
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Vector Network Analyzer
Fundamentals
––
POSTER
Vector Network Analyzer Fundamentals
Types of Measurement Error
Common S-Parameter Names
WARNING: To reduce errors that affect measurement results, it is important to calibrate a VNA setup
regularly. Calibration reduces the impact of systematic and drift errors.
I
m
(
z
)
=
-
1
I
m
(
z
)
=
1
Inductive
(+jX)
Capacitive
(-jX)
Z = ∞
(Open)
Z = 1
Impedance
Matched
Z = 0
(Short)
Im (z) = 0
Impedance (Z = R+jX)
R
e
(
z
)
=
1
R
e
(
z
)
=
2
Away From
Generator
(Toward Load)
Toward
Generator
(Away From Load)
I
m
(
Y
)
=
-
1
I
m
(
Y
)
=
1
Inductive
(-jB)
Capacitive
(+jB)
Away From
Generator
(Toward Load)
Toward
Generator
(Away From Load)
Y = ∞
(Short)
Y = 1
Admittance
Matched
Y = 0
(Open)
Im (Y) = 0
Admittance (Y = G+jB)
R
e
(
Y
)
=
1
R
e
(
Y
)
=
2
Smith Chart 101
The Smith chart is a very useful tool used to determine complex impedances and admittances of RF
circuits. Most network analyzers can automatically display the Smith chart, plot measured data on it, and
provide adjustable markers to show the calculated impedance.
Impedance Smith Chart
1. The circles touching the right corner are
constant-resistance circles.
2. The curves stretching from the right corner to
the outer edges of the impedance Smith chart
are constant-reactance curves.
3. The center of the circle is the Zo point. In
most cases, Zo = 50 ohms. This is also the
20-millisiemens (mS) point.
Admittance Smith Chart
1. The circles in the Smith chart that touch the
left corner are constant-conductance circles.
2. The curves stretching from the left corner
of the Smith chart to the outer edges of
the admittance Smith chart are constant-
susceptance curves.
SYSTEMATIC ERROR RANDOM ERROR DRIFT ERROR
• Imperfections in the test
equipment or in the test setup
• Typically predictable
• Can be easily factored out by
a user calibration
• Examples that occur across
the frequency range:
- Output power variations
- Ripples in the VNA
receiver’s frequency
response
- Power loss of RF cables that
connect the DUT to the VNA
• Error caused by noise emitted
from the test equipment or test
setup that varies with time
• Determines the degree of
accuracy that can be achieved
in your measurement
• Cannot be factored out by a
user calibration
• Examples include:
- Trace noise
• Measurement drift and
variances that occur over time
in test equipment and test
setup after a user calibration
• The amount that the test setup
drifts over time determines
how often your test setup
needs to be recalibrated
• Examples include:
- Temperature changes
- Humidity changes
- Mechanical movement of
the setup
Calibration Methods
Basic VNA Operation
A VNA contains both a source, used to
generate a known stimulus signal, and a set
of receivers, used to determine changes to
this stimulus caused by the device-under-
test or DUT. This illustration highlights the
basic operation of a VNA. For the sake of
simplicity, it shows the source coming from
Port 1, but most VNAs today are multipath
instruments and can provide the stimulus
signal to either port.
For simplicity, a single source is shown, but most VNAs today are multipath instruments and can
provide the stimulus signal to either port.
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S-Parameter Basics
S-Parameter Definition: Scattering parameters or S-parameters describe the electrical properties and
performance of RF electrical components or networks of components when undergoing various steady
state electrical signal stimuli. They are unitless complex numbers, having both magnitude and phase, and
are related to familiar measurements such as gain, loss, and reflection coefficient.
For more information on S-parameters go to tek.com/VNAprimer
Key VNA Parameters
Frequency Range
Consider not only your
immediate needs but also
potential future needs.
Trace Noise
Random noise generated
by VNA that may affect
measurement accuracy.
Dynamic Range
Make sure DUT noise
floor is at least 10 dB
above VNA spec.
Measurement Speed
Critical for high volume
manufacturing, less so for
most other applications.
03/17 EA 2D-61077-0
Understanding VNA Calibration
DUT
Port 1 Port 2
User Calibration
Reference Plane
VNA
• Covers up to the
Port 1 and Port 2
connectors
• Ensures output
signals meet
specs and input
signals will be
represented
accurately
• Factors out the
effects of cables,
adaptors, and
most things used
in the connection
of the DUT
• Allows for exact
measurement
of the DUT
performance
alone
Factory
Calibration
User
Calibration