Transcript Vector Network Analyzer (VNA)

Introduction to Vector
Network Analyzer (VNA)
Vector Network Analyzer
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Vector Network Analyzer (VNA)
• Introduction
• Transmission line basics
• Reflection and transmission parameters
• S parameter definition
Vector Network Analyzer
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Introduction –
Low
Integration
High
Types of Devices Tested by VNA
Duplexers
Diplexers
Filters
Couplers
Bridges
Splitters, dividers
Combiners
Isolators
Circulators
Attenuators
Adapters
Opens, shorts, loads
Delay lines
Cables
Transmission lines
Waveguide
Resonators
Dielectrics
R, L, C's
Passive
RFICs
MMICs
T/R modules
Transceivers
Receivers
Tuners
Converters
VCAs
Amplifiers
Antennas
Switches
Multiplexers
Mixers
Samplers
Multipliers
Diodes
Device type
Vector Network Analyzer
VCOs
VTFs
Oscillators
Modulators
VCAtten’s
Transistors
Active
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Reasons for testing component
• To verify specifications of building blocks in a
complex RF systems such as amplifiers and filters
in a transceiver
• Measured hardware prototype compared to
simulation model
• To ensure component or circuit cause no distortion
in the transmission of communications signals
• Linear : constant amplitude, linear phase /
constant group delay versus frequency
• Nonlinear
:
harmonics,
intermodulation,
compression, AM-to-PM conversion
• To ensure good matching for absorbing energy
efficiently (such as good matching antenna)
Vector Network Analyzer
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Lightwave Analogy to RF Energy
• 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.
Incident
Reflected
Transmitted
Lightwave
DUT
RF
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Transmission Line Basics
+
Low frequencies
I
-
wavelengths >> wire length
 current (I) travels down wires easily for efficient power transmission
 measured voltage and current not dependent on position along wire

High frequencies
wavelength » or << length of transmission medium
 need transmission lines for efficient power transmission
 matching to characteristic impedance (Zo) is very important for low
reflection and maximum power transfer
 measured envelope voltage dependent on position along line

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Transmission Line Zo
•
•
Zo determines relationship between voltage and current waves
Zo is a function of physical dimensions and r
Zo is usually a real impedance (e.g. 50 or 75 ohms)
1.5
attenuation is lowest
at 77 ohms
1.4
1.3
1.2
normalized values
•
50 ohm standard
1.1
1.0
0.9
0.8
power handling capacity
peaks at 30 ohms
0.7
0.6
0.5
10
20
30
40
50
60 70 80 90 100
characteristic impedance
for coaxial airlines (ohms)
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Power Transfer Efficiency
RS
RL
For real impedances, maximum power
transfer occurs when RL = RS
R
s
+
j
X
Load Power
(normalized)
1.2
1
j
X
0.8
0.6
R
L
0.4
0.2
0
0
1
2
3
4
5
6
7
8
9
10
RL / RS
Maximum power is transferred when RL = RS
For complex impedances,
maximum power transfer occurs
when ZL = ZS* (conjugate
match)
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Transmission Line Terminated with Zo
Zs = Zo
Zo = characteristic impedance
of transmission line
Zo
Vinc
Vrefl = 0! (all the incident power
is absorbed in the load)
For reflection, a transmission line terminated in Zo
behaves like an infinitely long transmission line
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Transmission Line Terminated with
Short, Open
Zs = Zo
Vinc
o
Vrefl In-phase (0 ) for open,
o
out-of-phase (180 ) for short
For reflection, a transmission line terminated in a short or
open reflects all power back to source
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Transmission Line Terminated with
25 W
Zs = Zo
ZL = 25 W
Vinc
Vrefl
Standing wave pattern does not go to zero as with
short or open
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High Freq. Device Characterization
Incident
Transmitted
R
B
Reflected
A
TRANSMISSION
REFLECTION
Reflected
Incident
=
SWR
S-Parameters
S11, S22
Reflection
Coefficient
G, r
A
Transmitted
R
Incident
Return
Loss
Impedance,
Admittance
R+jX,
G+jB
=
B
R
Group
Delay
Gain / Loss
S-Parameters
S21, S12
Vector Network Analyzer
Transmission
Coefficient
T,t
Insertion
Phase
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Reflection Parameters
Reflection
Coefficient
G
Vreflected
=
=
Vincident
r
Return loss, RL = -20 log (r),
Emax
Emin
F
=
=
G
r
ZL - ZO
Z L + ZO
VSWR =
Emax
Emin
=
1+r
1-r
Full reflection
(ZL = open, short)
No reflection
(ZL = Zo)
0
r
1
 dB
RL
0 dB
1
VSWR

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Transmission Parameters
V Incident
V Transmitted
DUT
Transmission Coefficient =
T
=
V
Insertion Loss (dB) = - 20 Log
V
Gain (dB) = 20 Log
V
Trans
V
V Transmitted
V Incident
Trans
= - 20 log
=
t
t
Inc
= 20 log
t
Inc
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Smith Chart Review
+jX
Polar plane
90
o
1.0
.8
.6
0
+R
.4

+ 180 o
-
o
.2
0

0
-jX
-90 o
Rectilinear impedance
plane
Constant X
Constant R
Z L = Zo
Smith Chart maps
rectilinear impedance
plane onto polar plane
G=
0
ZL =
Z L = 0 (short)
G= 1
±180
G =1
O
(open)
0
O
Smith chart
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Characterizing Unknown Linear 2-port Devices
Using parameters (H, Y, Z, S) to characterize devices at low frequency:




gives linear behavioral model of our device (or network)
measure parameters (e.g. voltage and current) versus frequency under
various source and load conditions (e.g. short and open circuits)
compute device parameters from measured data
predict circuit performance under any source and load conditions
H-parameters
Y-parameters
Z-parameters
V1 = h11I1 + h12V2
I2 = h21I1 + h22V2
I1 = y11V1 + y12V2
I2 = y21V1 + y22V2
V1 = z11I1 + z12I2
V2 = z21I1 + z22I2
h11 = V1
I1
V2=0
(requires short circuit)
h12 = V1
V2
I1=0
(requires open circuit)
Extending measurements of these parameters to high frequencies is
not very practical !
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Why Use S-Parameters?





relatively easy to obtain at high frequencies
 hard to measure total voltage & current at the device ports at high
frequency
 measure voltage traveling waves with a vector network analyzer
 don't need shorts/opens which can cause active devices to oscillate or
self-destruct
relate to familiar measurements (gain, loss, reflection coefficient ...)
can cascade S-parameters of multiple devices to predict system performance
can compute H, Y, or Z parameters from S-parameters if desired
can easily import and use S-parameter files in our electronic-simulation
tools
S 21
Incident
Transmitted
a1
S11
Reflected
b1
b2
DUT
S22
Port 2 Reflected
Port 1
a2
Incident
S12
Transmitted
b1 = S11 a1 + S12 a 2
b 2 = S21 a1 + S22 a 2
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Measuring S-Parameters
a1
Forward
S 21 =
b1
Incident
a2 = 0
b1
= a
1
b
a2 = 0
S 22 =
2
= a
1
a2 = 0
S 12 =
a1 = 0
Z0
DUT
Load
b1
Load
DUT
Reflected
Transmitted
b2
Transmitted
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Z0
S 11
Reflected
Incident
S 11 =
S
Incident
Transmitted
S 12
Vector Network Analyzer
Reflected
Incident
Transmitted
Incident
S 22
b2
= a
2
b
a1 = 0
1
= a
2
a1 = 0
b2
Reverse
Reflected
a2
Incident
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Equating S-Parameters with Common
Measurement Terms
S11 = forward reflection coefficient (input match)
S22 = reverse reflection coefficient (output match)
S21 = forward transmission coefficient (gain or loss)
S12 = reverse transmission coefficient (isolation)
Remember, S-parameters are inherently complex, linear
quantities. They are expressed as real and imaginary or
magnitude and phase pairs
However, we often express them in a log magnitude format
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Network Analyzers Vs Spectrum Analyzers
.
Measures
known signal
Amplitud
e
Amplitude Ratio
8563A
Frequency
SPECTRUM ANALYZER
GHz
Measures
unknown
signals
Frequency
Network analyzers:
 measure components, devices,
circuits, sub-assemblies
 contain source and receiver
 display ratioed amplitude and phase
(frequency or power sweeps)
 offer advanced error correction
Spectrum analyzers:
 measure signal amplitude characteristics
carrier level, sidebands, harmonics...)
 can demodulate (& measure) complex signals
 are receivers only (single channel)
 can be used for scalar component test (no
phase) with tracking gen. or ext. source(s)
Vector Network Analyzer
9 kHz - 26.5
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Network Analyzer Hardware Generalized Network Analyzer Block
Diagram
Incident
Transmitted
DUT
SOURCE
Reflected
SIGNAL
SEPARATION
INCIDENT
(R)
REFLECTED
(A)
TRANSMITTED
(B)
RECEIVER / DETECTOR
PROCESSOR / DISPLAY
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Types of Network Analyzer
Scalar
Vector
• Magnitude only
• Phase and Magnitude
• Broadband
Detector with higher
noise floor
• Tuned Detector with
lower noise floor
• Lower Price
• Higher Price
• Normalization –
Less Accurate
• Complete Error
Correction – More
Accurate
• Measures RL,
SWR, Gain/Loss
• Measures all
Vector Network Analyzer
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Basic Antenna
Parameters
Antenna Parameters
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Antenna Parameters
• Introduction
• Antenna - Reflection Coefficient / Return Loss ?
• Bandwidth
• VSWR
• Impedance Matching
• Other Parameters?  in class
• Demo and Hands On
Antenna Parameters
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Introduction
Antenna Parameters
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Reflection Coefficient
If 50 % of the signal is absorbed by
the antenna and 50 % is reflected
back, we say that the Reflection
Coefficient. is -3dB. A very good
antenna might have a value of 10dB (90 % absorbed & 10 %
reflected).
Antenna Parameters
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Bandwidth
Typically,
bandwidth
is
measured
by
at
looking
SWR, i.e., by finding the
frequency range over which
the SWR is less than 2.
Bandwidth also can be measured by looking at the
frequency range where reflection coefficient value
dropped below than -10 dB.
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VSWR
VSWR is a measure of impedance mismatch between the
transmission line and its load. The higher the VSWR, the
greater the mismatch. The minimum VSWR, i.e., that
which corresponds to a perfect impedance match, is unity.
The result is presented as a figure
describing the power absorption of
the antenna. A value of 2.0:1 VSWR,
which is equal to 90 % power
absorption, is considered very good
for a small antenna.
Antenna Parameters
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Impedance
An ideal antenna solution has an impedance of 50
ohm all the way from the transceiver to the antenna,
to get the best possible impedance match between
transceiver, transmission line and antenna. Since
ideal conditions do not exist in reality, the impedance
in the antenna interface often must be compensated
by means of a matching network, i.e. a net built with
inductive and/or capacitive components.
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Hands On 1 - Antenna
Characteristic Measurements
You will be given few types of antennas, please
measure the following :
• Reflection Coefficient, S11
• VSWR
• 10 dB bandwidth and % bandwidth
• Impedance at resonance
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Hands On 2 – Antenna
Measurement Environment
Display the S11 of your antenna between your frequency range of
interest. Place the marker at the minimum.
Observe the trace when the following objects are brought close to
it from different directions:
• metallic object (e.g. steel rule, metal rod)
• human hand
• insulator (e.g. book, plastic)
Observe the trace when the antenna is rotated by 90 degrees and
when it is moved around relative to surrounding.
Antenna Parameters
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