Agilent Technologies AN 1287-6 Installation and operating instructions

Agilent AN 1287-6
Using a Network Analyzer
to Characterize High-Power
Components
Application Note
DUT

3
3
4
5
6
7
9
11
12
14
14
15
15
16
17
17
17
18
18
19
19
19
23
23
25
26
27
Introduction
Defining high power
Why high-power measurements can be challenging
Network analyzer configurations for measuring
high-power devices
Configuration 1
Configuration 2
Configuration 3
Configuration 4
Configuration 5
Configuration 6
Additional configurations
Source leveling
Source leveling using power-meter calibration
Source leveling using external leveling
Calibration — purpose and types
Calibrating tips for best results
Dynamic accuracy
Choosing calibration power levels
Calibrating at one power level versus two power levels
Common problems of high-power measurements
Amplifiers with AGC loops
On-wafer devices (pulsed measurements)
Appendix
Network analyzers—definition and capabilities
Test sets
Calibration
Suggested reading
Table of Contents

3
Introduction
This application note describes linear and nonlinear
measurements of high-power components and how
to use a network analyzer for making them. It covers
the power limitations of a network analyzer, and
special network-analyzer equipment configurations
for high-power measurements. How to improve
the accuracy of high-power measurements and
solve common problems when making high-power
measurements are also described.
To get the most from this note, you should have a
basic understanding of network analyzers and the
measurements you can make with a network ana-
lyzer. or a basic review, please see the Appendix
at the end of this note. Additional network analysis
literature and study materials can be ordered
through Agilent Technologies. A reference list is
included at the end of the note.
Defining high power
What might be considered “high-power device
output” (e.g., 30 dBm or 1 Watt) in one application
can be insignificant in another application, such as
a radar test that uses devices with power levels in
the 60 dBm (1,000 Watt) range. In this note, “high
power” refers to a power level above the compres-
sion level and certainly above the damage level of
a standard network analyzer. Therefore, a power
amplifier with an output beyond the measurement
capability of a standard network analyzer would be
classified as a high-power device. We extend our
definition to also include devices that require a
drive level that is higher than a standard network
analyzer can provide. So a high-power device is
one that delivers more power than a standard
network analyzer can measure, or requires more
input power than the analyzer can provide.

4
Why high-power measurements can be
challenging
Two main challenges exist when measuring high-
power devices:
1. The measurements performed on high-power
devices can be different than those required to
characterize lower-power devices. Measurements
of high-power devices also can be performed dif-
ferently than those made at lower power levels.
Pulsed measurements are a good example.
Measurements typically are not pulsed at lower
power levels since device overheating tends not to
be a problem. High power can heat up a device,
affecting its measured characteristics. Many on-
wafer measurements, for example, require pulsed
R and pulsed DC bias, which reduces the average
power dissipation and keeps the temperature of
the device constant.
2. High-power measurements require special
network-analyzer configurations. This can mean
adding attenuation or a coupler between the out-
put of the device under test (DUT) and the input
of the test instrument to protect the receiver. It
can also mean adding amplification to the stimulus
signal if more power is required. Calibration and
accurate measurements become significantly more
complex as additional equipment is added to the
test setup. In some configurations the additional
hardware can make some types of calibration
impossible, or limit the number of measurable
parameters. or example, reverse S-parameters
cannot be measured in some configurations. The
inability to perform certain calibrations can limit
the accuracy of the measurements.
This application note will show configurations
ranging from those that are easy to assemble but
may have limited accuracy or measurement capa-
bility, to more complex configurations that are
very accurate and can make the same measure-
ments as a standard network analyzer.

5
Network analyzer configurations for measuring
high-power devices
The high-power test configurations described in
this note are designed to boost the power coming
from the network analyzer’s source as necessary,
and also to protect components such as the
receivers, couplers, and switches inside the
network analyzer from excessive power levels.
To select the correct network analyzer configura-
tion, you will need to consider the DUT and the
required measurements and accuracy. This section
shows high-power test configurations including the
necessary hardware, how to set up and calibrate
these configurations, and the advantages and
limitations of each.
The configurations are ordered by the degree of
measurement capability. In general, the increased
capability results in increased complexity in the
configuration. The first configuration is simple —
it uses a standard network analyzer and doesn’t
require high drive power, which means the network
analyzer’s source does not need to be boosted. The
other configurations measure devices that require
high-power inputs and also have high-power out-
puts. Measuring these devices requires amplifying
the network analyzer’s source signal somewhere
along the R path before it reaches the DUT. The
high output power also requires protection for the
couplers, receivers, and switches inside the analyzer.
Available high-power measurements Available calibrations
Forward
Forward transmission
Boosted transmission and reflection Forward/ Full
Configuration Complexity source only only reverse two-port Response
1 low X X X
2 low X X X
3 medium X X X
4 medium X X X
5 high X X X X
6 high X X X

6
Configuration 1
Configuration summary
• Low complexity
• No boosted source
• orward and reverse measurements
• ull two-port or response calibration
Setup and features
The simplest high-power devices to measure are
those that have high gain but don’t require high
levels of drive power, and typically are tested in
their linear region. Since these devices have high
gain, the receiver must be protected from the high
output power. An attenuator or coupler added
between the output of the DUT and the analyzer’s
test port protects the receiver. If using a coupler,
terminate the through arm of the coupler with a
characteristic impedance load. The coupled arm of
the coupler sends a small portion of the input sig-
nal to port 2. or a 20-dB coupler, the signal at the
coupling arm is 20 dB less than the strength of the
signal at the input. Determine the maximum power
out of the DUT, subtract the power level required
at the test port, and then choose the appropriate
attenuator or coupler value. Choose components
that are specified to handle your chosen power level.
Configuration 1 makes both forward and reverse,
reflection and transmission measurements (S11,
S12, S21, and S22) if using an analyzer with an
S-parameter test set.
Calibration
This particular setup allows both forward and
reverse measurements so full two-port error-
correction, the most accurate calibration, can be
performed. Calibration is performed at test-port 1
(or at the end of the cable attached to the port)
and at test-port 2 (with the attenuator and any test
cable). Include the attenuator or coupler on port 2
when performing the calibration to remove any
mismatch between the attenuator and the analyzer’s
test port. Since calibration is performed with all
hardware in place and all error terms corrected,
the measurements can be as accurate as the
standard analyzer.
Configuration 1
The attenuator on port 2 degrades the uncorrected
directivity of port 2 by twice the attenuation value.
This limits the stability of the calibration and
can make S22 measurements very noisy. If an S22
measurement is needed, calibrate at a higher
power level, lower the power level when making
forward measurements, and then raise the power
level for the reverse measurements. There is more
information on calibrating and power levels at
the end of this note.
A common assumption is that the DUT’s displayed
gain should be increased by the amount of the
attenuation, but remember that the calibration
includes the attenuator so the values as displayed
are correct.
Network analyzer
DUT Attenuator
Calibration points
12

7
Configuration 2
Configuration summary
• Low complexity
• Boosted source
• orward transmission measurement only
• Response calibration only
Setup and features
The simplest way to increase the power of the
stimulus signal is to add a booster amplifier at the
test port of a standard network analyzer. The input
of the booster amplifier connects to the analyzer’s
source port and the output connects to the DUT
(see Configuration 2). This configuration boosts
the signal level going into the DUT without modify-
ing the analyzer’s built-in test set or having to add
additional couplers. The analyzer’s source power
plus the gain of the booster amplifier gives the
power available for testing the DUT. Protect the
receiver by adding an attenuator or coupler
between the output of the DUT and port 2 of
the analyzer, as shown in Configuration 1.
Configuration 2
Configuration 2 is convenient because it consists
simply of a standard network analyzer. However,
it has numerous limitations. When boosting the
signal at port 1, only high-power transmission
measurements in the forward direction (S21) and
nonboosted reflection measurements in the reverse
direction (S22) are possible. The position of the
booster amplifier, with its high reverse isolation,
makes forward reflection (S11) and reverse transmis-
sion (S12) measurements impossible. Nonboosted
reverse reflection measurements can be made, but
with limitations as described in Configuration 1.
The accuracy of Configuration 2 is limited because
the network analyzer measures the reference signal
before the booster amplifier (coupling takes place
inside the analyzer). As a result, any mismatch
between the booster amplifier and the DUT is not
ratioed out, and therefore ripple will appear in the
measurement ( igure 1). This ripple can be signifi-
cant due to the poor source match of the booster
amplifier. Even the uncorrected source match of
the network analyzer is likely to be much better
than the source match of the booster amplifier.
Network analyzer
DUT Attenuator
Calibration points
DUT
Booster
amplifier

8
The uncertainty associated with the measurement
will depend upon the quality of the match between
the booster amplifier and the DUT. In addition, any
drift or power fluctuations associated with the
booster amplifier will appear to be from the DUT
since the reference signal is not measured after the
booster amplifier. This configuration is best for
analyzers that do not allow direct access to the R
(reference) channel and when the accuracy of the
measurements is not important. If the analyzer
allows access to the R channel and the reference
signal is coupled after the amplifier, a configura-
tion that allows more accuracy is possible (see
Configuration 3).
Figure 1. Ripple is caused by a mismatch between the
DUT and the booster amplifier.
Calibration
Only transmission response calibration is possible
with this setup. The location of the booster
amplifier does not allow the analyzer to make
transmission measurements in the reverse direction,
making a full two-port calibration impossible.
Perform a response calibration by connecting the
output of the booster amplifier to the attenuator
or coupler on port 2. A basic response calibration
does not remove mismatch errors due to the DUT
— only the frequency response errors and any
mismatch associated with the booster amplifier,
the attenuator, and the analyzer. Since mismatch
is not corrected for during calibration, even
measurements made with a response calibration
are limited in their accuracy.
In this setup you can improve the source match by
adding an isolator between the booster amplifier
and the DUT, or by adding attenuation. Adding an
isolator between the output of the booster amplifier
and the input of the DUT will remove the effects of
any mismatch. If the booster amplifier has enough
gain, a 3-dB or 6-dB attenuator at its output can
be added to improve its output match. Be sure
to include the isolator or any attenuation in the
calibration if you include it in the measurement
setup. Also make sure that the isolator or attenuator
can handle the power level being tested. Instead of
using a high-power isolator, a circulator with a
high-power load on its third port can be used.
CH1 S
21 &M log MAG 2 dB/ REF 11 dB
CENTER 2.500 000 000 GHz SPAN 4.000 000 000 GHz
Cor
PRm
LINE TYPE
1
18 Dec 1997 16:16:33
1
1_: 13.704 dB
2.512 960 000 GHz
True response
True response
with
mismatch ripple

9
Configuration 3
Configuration summary
• Medium complexity
• Boosted source
• orward transmission measurement only
• Response calibration only
Setup and features
Compared to Configuration 2, Configuration 3
improves measurement uncertainty and flexibility
by measuring the incident signal after the booster
amplifier rather than before. Measuring the inci-
dent signal after the booster amplifier ratios out
the effects of any source mismatch.
Boost the analyzer’s source power by connecting
the booster amplifier to the “R out” port on the
analyzer. An external coupler connected to the
output of the booster amplifier directs a portion of
the boosted source signal to the R channel of the
analyzer. This signal becomes the reference signal
used in ratioing (Configuration 3).
Configuration 3
Measuring the reference signal after the amplifier
ratios out any mismatch ripple between the booster
amplifier and the DUT and any drift or power
fluctuations of the booster amplifier. The reverse
isolation of the booster amplifier makes forward
reflection measurements and reverse transmission
measurements impossible.
In configurations where the user provides an
R channel signal, determining the proper level is
important. Too low a signal level can be affected by
noise and can cause phase-locking problems and
inaccurate measurement results. Too high a signal
level can be affected by receiver compression,
causing inaccurate results. The receiver can also
be damaged if the signal is high enough. To find
the proper signal level for the R channel (a typical
range might be 0 to –30 dBm) look at the analyzer’s
specifications. Then determine the signal level at
the coupled arm of the coupler, and add attenua-
tion to achieve the proper signal level at the
R channel.
DUT
DUT
Calibration points
Attenuator
R in
Booster
amplifier
Coupler
Network analyzer
12
Attenuator

10
Normally, the analyzer attempts to phase-lock
near the start frequency of the sweep. When the
analyzer cannot phase-lock at the start frequency
though, the analyzer performs a “pre-tune cali-
bration” routine. During pre-tune calibration the
analyzer attempts to phase-lock at a set frequency
(100 MHz for the Agilent 8753D family), which
could be below the start of the analyzer’s sweep.
The pre-tune calibration routine would be per-
formed, for example, if the analyzer is switched to
external reference mode without a signal attached
to the R channel. When the external R channel
signal is connected the analyzer is attempting to
phase-lock at the pretune frequency (100 MHz).
With a band-limited device in the R channel
path (e.g., a booster amplifier) there may not be
sufficient signal at this pretune frequency and
phase-locking errors can occur on some analyzers.
On the 8753E network analyzer, turning “PLL
Auto” off (located in the Service Modes menu) will
stop the analyzer from performing the “pre-tune
calibration” routine. Turning “PLL Auto” off rarely
affects the accuracy of instrument measurements.
Another way to prevent this phase-locking error
is to use frequency-offset mode in the 8753E. In
frequency-offset mode the analyzer assumes that
a band-limited device is being measured, and as a
result the analyzer never goes outside the sweep
range to phase-lock. Setting LO = 0 Hz when in
frequency-offset mode allows the analyzer to be
used normally.
Band-limited devices in the R channel path may
cause phase-locking problems in the Agilent 8720E
family of network analyzers unless the analyzer
has the high-power option (Option 085) or frequency-
offset option (Option 089). With either of these
options, phase-locking is not a problem. This is
because the internal reference switch switches to
the internal signal when phase-locking. After the
analyzer is phase-locked, the reference switch
returns to the external signal and the analyzer
is ready to make measurements.
Calibration
Calibrating this configuration is similar to the
calibration performed in Configuration 2. Only a
response calibration is possible since only forward
measurements can be made. Perform the response
calibration by connecting the output of the coupler’s
through arm to the attenuator on test-port 2 of
the analyzer.