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

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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

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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

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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.

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Configuration 4
Configuration summary
• Medium complexity
• Boosted source
• orward transmission and reflection
measurements only
• Response calibration only
Setup and features
High-power reflection and transmission measure-
ments in the forward direction can be made on
analyzers that provide access to the A channel and
the R channel. orward reflection measurements
are possible by adding a reverse coupler between
the coupler used for the R channel and the DUT.
This reverse coupler, which is connected to the A
channel, allows the forward reflection measurement.
Calibration
Again only a response calibration is possible since
a reverse measurement cannot be made. When per-
forming the calibration, connect the output of the
second coupler to the attenuator or output coupler
so that the calibration includes all hardware.
Configuration 4
8753E with Option 011
DUT
Reverse
coupler
Forward
coupler Calibration points
Booster
amplifier
RABRF out
Attenuator
AttenuatorAttenuator

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Configuration 5
Configuration summary
• High complexity
• Boosted source
• orward and reverse measurements
• ull two-port or response calibration
Setup and features
The configurations presented up to now have made
use of an analyzer’s build-in test set, or include
additional external hardware to make high-power
measurements. Another approach is to modify
the standard test set to make it more suitable for
high-power measurements. Typically modifications
to the internal test set are options that Agilent
provides at time of purchase.
Configuration 5
High-power option
An example of a modified test set is the high-power
option, Option 085, for the Agilent 8720E family
of vector network analyzers. This option provides
four features that allow for the measurement of
higher power levels. These are: 1) access to the R
path between the source and the transfer switch;
2) direct access to the R channel; 3) direct access
to the R path between the transfer switch and the
test ports; and 4) step attenuators between the
couplers and the samplers on the A and B channels.
The 8720E with Option 085 allows input power and
output power up to +43 dBm at the test ports of
the analyzer. or more information on the set-up
of 8720E, Option 085, refer to the 8720E manual
section Making High Power Measurements with
Option 085.
DUT
DUT
Isolator
Isolator
Coupler
switch
switch
Coupler 12
R in
RF in
RF out
Calibration
points
Attenuator
8720E
with Option 085
Coupler
Booster
amplifier

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Forward and reverse measurements by amplifying the
source signal
Access to the R path between the source and
the transfer switch allows the source signal to be
amplified and then be switched between port 1 or
port 2, allowing forward and reverse high-power
measurements. The input of the booster amplifier
connects to the “R out” connector on the analyzer,
and the output of the booster amplifier connects
to the coupler. The coupled arm of the coupler
connects to the R-channel input to provide the
reference signal used for ratioing. To insure an
optimum power level at the R receiver, add any
needed attenuation between the reference coupler
and the analyzer. The optimum power range for the
receiver is provided by Agilent. The through arm
of the coupler connects to the “R in” connector
on the analyzer. The amplified signal goes through
the transfer switch, which directs it to either test
port. Jumpers between the transfer switch and
the test ports give access to the R signal path.
They allow the user to add high-power isolators
to protect the transfer switch. Without isolators,
signals with too much power can damage the
transfer switch. Isolators on both sides of the
transfer switch ensure that after the signal has
been measured by the coupler any signal will be
terminated, thereby protecting the switch.
Protecting the receivers
The final feature of this high-power option is the
internally controlled step attenuators that protect
the receivers. Located after the couplers and
before the receivers, these 55-dB step attenuators
(with 5-dB increments) reduce the signal to an
optimum level for the receiver. These attenuators
are controlled from the front panel of the analyzer.
Calibration
A major benefit of this configuration, besides the
ability to do both forward and reverse measure-
ments, is the ability to do full two-port vector error
correction. Calibrating in this manner takes into
account the effects of the hardware that has been
added to the setup (isolators, amplifiers, couplers,
etc.) and all errors associated with the analyzer
and the measurement setup up to the point of
calibration. Perform the calibration at the point
where the DUT will be connected.
When using this configuration, it is extremely
important not to damage any of the internal
components because high power levels are inside
the analyzer itself. Since amplification takes place
before the transfer switch, the power-handling
capabilities of the switch must be known. Analyzers
that allow signal amplification before the transfer
switch must specify how much power the switch
can handle. Agilent specifies maximum power for
the transfer switch for two conditions, when the
switch is switching, and when the switch is not
switching. Typically the nonswitching power-rating
is higher than the switching power rating. The
power-handling capability of all components in
the R path must be considered when making
high-power measurements. When using this
configuration, it is important to understand
how the analyzer works and the power-handling
capability of each component in the signal path.

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Configuration 6
Configuration summary
• High complexity
• Boosted source
• orward transmission and reflection
measurements only
• Response calibration only
Setup and features
Configuration 6 uses the same high-power test set
as Configuration 5, but the hardware is rearranged
so that measurements can be made at even higher
power levels. Configuration 5 is desirable because
it allows high-power measurements in both direc-
tions, but the maximum power level does not reach
the test ports. Due to losses in the test set and the
power limitations of the transfer switch, the power
at the test ports is less than the couplers can handle.
To test at power levels up to the level that the
couplers can safely handle (+50 dBm), the
Agilent 8720E with Option 085 is used in an
alternate setup.
Amplification is done after the transfer switch
and before the test port to get the maximum
power possible at the test ports. This configuration
allows higher power levels than possible using
Configuration 5, but high-power measurements,
both transmission and reflection, are possible
only in the forward direction.
Configuration 6
Calibration
Two-port vector error calibration cannot be used
in this configuration. Since the external R channel
is being used and is connected only to port 1,
the reference signal is not accurate when doing
a reverse sweep. This means only a response
cali-bration can be done between test-port 1 and
test-port 2. Since full two-port calibration cannot
be done, this configuration should only be used if
more power is needed than can be provided by
Configuration 5.
Additional configurations
Often Agilent can design test sets to specifically
match your needs. Contact your local sales
representative if you would like to investigate a
specially configured test set. or example, solid-
state switches may be substituted for mechanical
switches in a high-power test set. High-power test
sets usually use mechanical switches to handle
higher power levels. If continuous switching is
required, solid-state switches need to be used.
Special configurations can include solid-state
switches in high-power test sets if needed.
Other special configurations include high-power
test sets that allow through-reflect-line (TRL)
calibrations for noncoaxial measurements.
Special configurations for applications that
increase the power range over which a network
analyzer sweeps such as compression measure-
ments, are also available.
DUT
8720E with Option 085
RF out RF in
Attenuator
Switch
Coupler
Switch
Coupler
R in
DUT
12
Calibration
points
Coupler
Booster
amplifier

15
Source leveling
When amplifiers operate in their nonlinear region,
the measured response may differ from the true
response of the amplifier. If the amplifier is operating
in its nonlinear region, variations in the stimulus
signal might not be duplicated at the output of the
amplifier. In contrast, the response of an amplifier
tested in its linear range is not affected by this
level variation. By ratioing the response to the
stimulus, the network analyzer removes the effects
of the stimulus variation and displays the true
performance of the amplifier.
However, if the amplifier is operating in its nonlinear
range, at or near saturation, the amplifier does not
produce an output signal with a variation propor-
tional to the variation of the signal present at the
input. The ratioing process in the network analyzer
then creates an erroneous display ( igure 2).
To optimize measurements in the nonlinear region, a
leveling scheme must be used to produce a stimulus
signal with a frequency response that is as flat as
possible.
Figure 2. When a device operates linearly, its true
response can be measured. However, a device that oper-
ates nonlinearly is affected by nonlevel input signals.
Source leveling using power-meter calibration
Using power-meter calibration in continuous sampling
mode is one way to level the source ( igure 3). The
power level at each frequency point in the sweep is
measured with a power meter. The network analyzer,
connected to the power meter via GPIB, adjusts its
source power until the power meter measures the
desired power level. Then the measurement is
made. Since the accuracy of the power meter is
very high (uncertainty in the tenths of a dB range),
you can have confidence that the power level is
accurate as well as flat.
Figure 3. Power-meter calibration levels the source
signal and removes the nonlinear ripple.
8753E with Option 011
DUT
Power meter
GPIB
RA BRF out
Power sensor
Power
splitter

16
Source leveling using external leveling
Using power-meter calibration for source leveling
works well, but can slow down the measurements
too much for some applications. A faster method of
source leveling is external leveling. External leveling
makes use of an external amplitude modulation
(AM) input, available on some network analyzers.
Adjusting the voltage at the AM input adjusts the
analyzer’s source power. By creating a leveling
circuit that is external to the network analyzer,
and adjusting the voltage to the external AM input,
source power can be leveled on a real-time basis.
The external leveling circuit can be implemented
in several ways. One common implementation is to
create a circuit consisting of a detector diode and
an operational amplifier. Changes in the power
measured by the diode are detected by the opera-
tional amplifier circuit. The circuit connected to
the AM input of the analyzer adjusts the source
power by adjusting the voltage at the AM input
( igure 4).
Figure 4. This circuit shows a common way to implement
external leveling.
When using either power-meter calibration or
external leveling, keep the source-leveling process
active, just as it will be during the measurement.
Then perform a response calibration as you would
if source leveling was not used.
8753E with Option 011
Attenuator
DUT
RA BRF out
Power
splitter
Ext AM
+
–+
–
REF
50 Ω
Loop gain
RF filter
and load
Detector

17
Calibration—purpose and types
Calibrating an analyzer eliminates systematic
errors and makes measurements more accurate.
This note has shown calibrations for various con-
figurations and where in the test setup to perform
these calibrations. Of the two types of calibration
discussed, full two-port calibration is the most
accurate because it removes all systematic errors
in the measurement setup, ideally up to where the
DUT connects to the analyzer. Response calibration
is less accurate and removes only the frequency-
tracking errors. requency-tracking errors are
associated with the differences in frequency
response between one channel and another. In a
full two-port calibration, the analyzer sweeps in
both forward and reverse directions, while in a
response calibration, the analyzer sweeps only
in the forward direction.
Calibration tips for best results
In addition to understanding what type of calibra-
tion is possible for each configuration and where
to perform the calibration, it is important to
understand how to get the most out of a calibra-
tion. The following topics discuss techniques and
considerations for achieving the best calibration
results.
Dynamic accuracy
By examining the dynamic accuracy response of
an analyzer, you can optimize the power levels for
calibration and measurement. Dynamic accuracy
refers to the uncertainty associated with calibrating
at one signal level and measuring at another.
The dynamic accuracy plot for the Agilent 8753E
shows extremely low uncertainty (0.02 to 0.06 dB)
if receiver power is between –10 and –50 dBm
independent of the calibration power level ( igure 5).
Perform calibration and measurements so that the
signal level at the receiver is in this high-accuracy
range. At lower signal levels, noise is a factor; at
higher levels, receiver compression is a factor.
Figure 5. Agilent 8753E dynamic accuracy
–10 dBm
–20 dBm
–30 dBm
Reference power level
Accuracy (dB)
1
0.1
0.01
10 0–10 –20 –30 –40 –50 –60 –70 –80 –90 –100
Test port power (dBm)

18
Choosing calibration power levels
If you are measuring an amplifier and the output
power level is higher than the input power level,
at what power level should you calibrate: at the
higher power level, the lower power level, some-
where in between the two levels, or at a different
level for each port? Calibrating and measuring
with the test port powers in the range of lowest
uncertainty, –10 to –50 dBm for the Agilent 8753E,
results in accurate measurements as shown above.
Calibrating at the input (lower) power level can
result in a power level at the output receiver that
is too low, because attenuation is required to pro-
tect the analyzer during measurement of the DUT.
Calibrating at the output (higher) power level
might not be possible since the gain of the DUT
can result in an output power level that the net-
work analyzer’s source together with the booster
amplifier cannot generate. Therefore, you might
need to perform some calibrations at a power
level between the input power level and the
output power level. In general, to reduce noise
calibrate at the highest possible power below the
onset of receiver compression.
Calibrating at one power level versus two power levels
A response calibration is done at only one power
level since only one forward sweep is made. ull
two-port calibration, on the other hand, can be
done at different power levels since both forward
and reverse sweeps are made. The analyzer can
calibrate one port using one power level and the
other port using another power level to more
accurately match the power levels present during
the measurement of a DUT.
As an example, the open, short, and load standards
for port one and the reverse through measurements
may be made at the lower power level. The open,
short, and load standards for port two and the
forward through measurements could then be
made at the higher power level. It is also possible
to perform a full two-port calibration at one power
level as in a response calibration. The power level
used would be determined from the dynamic
accuracy specification of the analyzer.
A final note on calibration: power-handling
capabilities of the calibration standards can
become an issue when calibrating at high power
levels. The common standards used for calibration
are the open, short, load, and through. The open,
short, and through standards are not a problem
since they do not dissipate any energy. The load,
how-ever, does dissipate energy so when you are
calibrating make sure the standards can handle
the power level.

19
Common problems of high-power
measurements
Some amplifiers contain an automatic-gain control
(AGC) loop. AGC loops attempt to keep the output
power of the amplifier constant by adjusting gain
to account for variations at the input of the ampli-
fier. Amplifiers with AGC loops can pose a problem
when measured on some network analyzers, espe-
cially at high power levels.
Amplifiers with AGC loops
Network analyzers sweep across the selected
frequency range while holding the power at the
desired constant power level. At the end of the
sweep, some network analyzers might blank, or
turn off the source, during the time it takes the
analyzer to reset itself and set up for another
sweep — the retrace time. Turning off the source is
a problem for amplifiers containing an AGC loop.
When an amplifier with an AGC loop is measured
and the input signal is turned off at the end of the
sweep, the AGC loop of the amplifier compensates
for the turned-off signal by increasing its gain to
keep the output power level constant. When the
sweep begins again, the network analyzer restores
its signal, and there is power again at the input of
the amplifier, which has ramped-up its gain. The
momentary high output power can cause damage
or destroy the amplifier or the analyzer’s receiver
if the AGC loop cannot respond quickly enough.
Using a network analyzer that keeps its power
constant during retrace will reduce the possibility
of destroying the device or damaging the analyzer.
The 8753E keeps power constant except when
switching frequency bands at 300 kHz and 3 GHz.
The 8720E family of network analyzers allows the
user to keep the power constant or to blank during
retrace (the default is to have the power remain
constant). The 8720E family will briefly blank
during band changes at 2.55 GHz. The 8722D blanks
at 20.05 GHz as well. Be aware that this blanking
occurs if the analyzer sweeps across these bands.
On-wafer devices (pulsed measurements)
A problem commonly encountered when meas-
uring high-power on-wafer devices is the heating
up of the DUT. Devices on-wafer tend to heat up
quickly because they lack sufficient heatsinking.
This heating up requires that the temperature
of the DUT be controlled in some way since the
response of a DUT may change as the temperature
of the DUT increases. Two common ways to control
heating up is to pause the network analyzer between
measurements, or pulse the R and/or DC bias
signals so that a constant DUT temperature is
maintained.

20
Presuming that a device will not overheat during
a single sweep, pausing between sweeps can keep
the average temperature within bounds. A test-
sequencing program that contains a user-defined
pause between each measurement will do this auto-
matically. More information on test sequencing is
available in your network analyzer’s user’s guide.
Pausing between sweeps dramatically slows down
testing time and might not be practical in some
situations. Pulsed measurements can be a better
solution to the heating problem.
In some cases pulsed R and DC-bias measure-
ments might be needed rather than the usual CW
measurements made with a network analyzer.
Pulsed measurements are used for several reasons.
As noted above, pulses can be configured so that
an isothermal measurement is achieved. A pulsed
signal might also be used because it is representative
of the signals that the DUT encounters in actual
use (radar is a good example of this), or if there
is interest in the transient response of a device
stimulated with a pulse, etc.
Pulsed measurements typically requires a test set
designed for this purpose. The Agilent 85108 is an
example of a network analyzer system designed for
pulsed measurements.
While not common, it is possible to make pulsed
R measurements with a standard network analyzer
under certain conditions. The ability to make pulsed
measurements depends on the pulse repetition
frequency (PR ) of the pulse used, relative to the
I bandwidth and the sampling rate of the network
analyzer and the duty cycle of the signal. There
are three cases to consider when determining if
pulsed measurements are possible using a network
analyzer. (Note that we are referring to finding a
DUT’s steady-state frequency response under pulse
conditions, not a DUT’s transient time-variant
response to a pulse. A standard network analyzer
cannot be used to measure the transient time-variant
response to a pulse.)
1. The first case is when the PR of the pulse is
less than the I BW of the analyzer. When the PR
is less than the receiver’s I BW, the pulse side-
bands pass through the I filter and the modulated
R (the pulse) cannot be measured ( igure 6). So
for pulsed measurements with a PR less than the
I BW of the analyzer, a standard network analyzer
will give inaccurate results.
Figure 6. When the PRF is less than the IF bandwidth,
pulse sidebands are measured in addition to the center
spectral line.
t
t
IF BW
PRF
Pulsed RF IF Filtering Modulated IF
f
PRF
PRF