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8/12/2019 NI JulyBasicsOfDesign
http://slidepdf.com/reader/full/ni-julybasicsofdesign 1/5
A Supplement to Microwaves & RF Sponsored by National Instruments
Making the Most of Power
Amplifier Measurements
Evaluating RF/microwave power
amplifier performance requires
an assortment of tests as well as
the right equipment to perform
those measurements quickly and
accurately.Power amplifiers (PAs) are de-
signed to boost the levels of many
different types of signals in commu-
nications and other systems and are
vital components in these systems.
Depending upon the end use of the
PA, engineers will typically charac-
terize this component using a num-
ber of different measure-
ments to determine key
operating parameters.
Understanding essential
PA measurements and
how to accurately per-
form them can provide
greater insight into the
behavior of the compo-
nent and can help guide
practical choices in test
equipment.
PAs cover a great deal
of ground in terms of
frequency, of course,ranging from simple
embedded audio ampli-
fiers at tens of kilohertz to large,
multiple-stage tube or solid-state
RF/microwave amplifiers capable of
kilowatts of output power at tens of
gigahertz. High-frequency solid-
state PAs make use of a variety
of different device technologies,
including silicon bipolar transis-
tors at lower frequencies, GaAs
MESFET devices at microwavefrequencies, and increasingly, gal-
lium nitride (GaN) high-electron-
mobility-transistor (HEMT) devices
for higher-frequency microwave
and millimeter-wave PAs. High-fre-
quency tube amplifiers still make
use of well-established electron
devices, including traveling-wavetubes (TWTs), as the active devices
for amplification.
Performance Criteria Defining a set of measurements
for an RF/microwave PA first
requires establishing typical key
performance parameters for a PA.
Moreover, these key performance
parameters can vary greatly,
depending upon the application.
Although performance criteria such
as output power and gain are impor-
tant for all PA applications, PAs
designed for wireless communica-
tions standards are subject to per-
formance criteria that are specific
to the wireless standard.Output power and gain are the
most fundamental PA metrics, and
engineers typically evaluate these
parameters over a range of operat-
ing temperatures. In addition to
these, other important criteria
include the related output power at
1-dB gain compression and outputpower at the 3-dB gain compres-
sion point. For applications where
power consumption is important,
power-added efficiency (PAE) is
an important criterion. In appli-
cations dealing with modulated
signals, linearity metrics such as
intermodulation-distortion (IMD)
performance is a key
attribute. Finally, even
characteristics such as
the stability factor (K
factor) are important
factors that determine
device behavior under
a range of impedance
conditions. The combi-
nation of these param-
eters helps engineers
to characterize how a
particular PA will behave
in the given application
and make it easier whencomparing different PAs
for a given application.
Knowing how to set up a test
system for evaluating PA perfor-
mance is important from the R&D
laboratory to the manufacturing
production line. For companies
involved with manufacturing high-
frequency PAs or simply evaluating
PA performance for internal use,
it is important to understand the
types of measurements needed andthe type of test equipment that can
complete those measurements.
Traditional PA Test Setup
VSG VSA
PowerSupply
PA
Measures power,EVM, ACLR
(cellular)
Generates CW &Modulated Waveforms(WCDMA, LTE, WLAN)
1. This block diagram shows the use of a vector signal generator (VSG) andvector signal analyzer (VSA) for evaluating a PA.
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Sponsored by National InstrumentsA Supplement to Microwaves & RF
Output Power and GainTraditional PA testing has started
with measurements of gain andoutput power, typically at 1-dB gain
compression, across a specified fre-
quency range. The frequency range
for such measurements has typi-
cally been defined by the targeted
application, with possibly some
tolerance added to ensure coverage
of the full required bandwidth.
Typical commercial cellular
communications channels require
operation in relatively narrow
bands, such as 800 to 1000 MHz or
1800 to 2000 MHz. Although many
of these cellular PAs were histori-
cally designed for single-band usage,
today’s multiple-band PAs now
require engineers to test a single
PA at more bands than before. By
contrast, in defense-related applica-
tions, such as for electronic-warfare
(EW) or RADAR systems, PA gain
and output power are required to be
maintained across a wide frequencybandwidth, such as from 2 to 26
GHz and beyond.
Historically, measuring the gain
and/or output power of a PA was
simply a matter of driving the
amplifier with a known input signal
and measuring its output signal
with a power meter. Here, gain is
found from the simple relationship:
Gain = Pin – Pout
where Pin is the input power to
the amplifier under test and Pout
is the resulting output power after
amplification. However, the use of
wider-bandwidth signals in mod-ern communications applications
as well as in radar systems has
increased the use of vector signal
generators (VSGs) and vector signal
analyzers (VSAs) for performing
power measurements on these
more-complex, wider-bandwidth
signals (Fig. 1). Tools such as a
simple continuous-wave (CW)
signal source and power meter are
useful for measuring the steady-
state power of CW signals, but may
not provide sufficient information
to fully understand an amplifier’s
input-to-output response. Because
a PA may be used in an application
with complex modulation, where
the amplitude and phase of the
amplified signal are changing with
time, or the PA may handle pulsed
signals, it may be more appropri-
ate to measure the input-to-output
signal relationships of a PA undertest with a VSG and a VSA.
The output-power levels of dif-
ferent PAs are usually compared
in terms of the amount of power
available at certain points when the
amplifier begins to become nonlin-
ear. Unlike a small-signal amplifier,
such as a low-noise amplifier (LNA),
which is often exclusively used in
its linear region, PAs frequently
reach points in their operating
range where the output-power-ver-sus-input-power response no longer
follows the linear gain relationship
from input to output ports. In this
scenario, the output versus input
level is reduced by some amount
relative to the nominal gain level.
For standards of comparison, these
reductions in gain/power for PAs
are measured at points referred to
as the 1-dB compression point and
the 3-dB compression point, wherethe PA levels off from its maximum
output-power level by 1 and 3 dB,
respectively.
As the input power increases,
a PA will eventually reach a point
known as saturation, where the
output power no longer increases
no matter how much the input
power is raised. PA measurements
for a given design must find these
different operating points within
the frequency range and input/ output power range, which can be
performed with the appropriate set
of VSG and VSA.
Network Analysis A vector network analyzer (VNA)
for PA measurements can be useful
in terms of determining the four
basic scattering (S) parameters for
a PA, parameters S11, S22, S12,
and S21 which are input and output
measurements which essentially
define through and reflected signal
conditions and impedances for an
amplifier or other two-port network.
In the case of a PA, knowing the val-
ues of the S-parameters can provide
insight into whether an amplifier of-
fers “unconditional stability—which
means that it will not enter a feed-
back mode for any type of source
and load impedance conditions.
At the PA input, the S11 mea-surements provide insight into
input match, input impedance, and
input VSWR. At the output, the S21
measurements reflect PA gain, gain
flatness, phase, and group delay. At
the output, the S22 measurements
denote PA output match, output
impedance, and output VSWR. The
S12 measurements refer to PA’s
reverse isolation.
The stability factor (or K-factor)
of an amplifier is a measure of howstable the amplifier is under differ-
ent operating conditions. It provides
Theory of Adjacent Channel Power
• Measured with a Modulated Signal
• Compression/distortion produces spectral re-growth
• Ratio of “In-Channel” Power and “Adjacent Channel” Power
DUTInput Output
This power ratio is the ACPR (also ACP or ACLR)
SpectralRe-Growth
MainChannel
Adjacent
Channel Adjacent
Channel
2. This plot
is a graphic
representation of the
adjacent-channel
power ratio (ACPR)
parameter for PAs.
8/12/2019 NI JulyBasicsOfDesign
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Sponsored by National InstrumentsA Supplement to Microwaves & RF
insight into how stable an amplifier
will be with different loads. It can
be determined from measurementsof all four S-parameters.
The stability factor of a PA, K, can
be found if the PA’s four input and
output S-parameters are known:
K = (1 – |S11| – |S22| + |∆|2)/
[2|S12S21|]
where
∆ = S11S22 – S12S21.
For an impedance load, an ampli-fier is unconditionally stable if K >
1 and ∆ < 1. Load-pull tuners are
often used when testing different
PA parameters by varying the load
impedance to the PA under test
while measuring the effects of the
changing load impedances on am-
plifier parameters, such as gain or
output power. As part of the design
process, adjusting the impedance
tuner to find the optimum match-
ing impedance and then developing
an impedance matching network
for that amplifier helps to achieve
optimum performance for the PA.
Determining PAEThe efficiency or power-added
efficiency (PAE) of a PA can be an
important performance parameter,
especially for amplifiers intended
for use in portable, battery-powered
applications. In mobile applications,the PA is often one of the most
power-hungry components of a ra-
dio, and a PA with high PAE will run
for a longer time on a given battery
charge than a PA with similar gain
and output power but lower PAE. PA
power efficiency can be determined
as a function of Pout/PDC and the
PAE can be determined from (Pout
– Pin)/PDC :
PAE = [(Pout – Pin)/PDC] x 100%
where PDC is the power supplied
by the test source or battery.
The PAE has an impact on the
amount of power consumed by an
amplifier, along with insight into
how much heat it will generate in an
application (and its heating require-
ments), and how long it will be able
to operate while on battery power.
Metrics for WirelessCommunications
PAs designed for wireless commu-
nications applications, such as LTE
and IEEE 802.11, require metrics
that are defined by the particular
communications standard’s speci-
fications. These PAs must handle
complex modulated signals and
measurements of error vector mag-
nitude (EVM) and adjacent-channel
leakage ratio (ACLR) have becomeimportant parts of testing PAs for
these applications.
EVM measurements are used to
evaluate how well a PA will handle
modulated signals without distor-
tion. EVM performance can offer
insight into a PA’s behavior in a
system’s transmitter section and, if
used at all in the system’s receiver,
the effectiveness of processing
demodulated signals. Measurements
of EVM versus output power canshow where the EVM performance
may degrade with increasing output
power, as a result of degradation
in PA linearity. It may also be
desirable to measure EVM versus
frequency, to better know the EVM
performance at different operating
frequencies or bands.
ACLR, also known as adjacent-
channel power ratio (ACPR),
measures the relative signal power
at some offset frequency above and
below a wireless uplink channel,
usually with a modulated test sig-
nal. The ACLR or ACPR is the ratio
of the “in-channel” power in the up-
link channel to the power levels in
these “adjacent-channel” sideband
channels. It is essentially the ratio
of the power in a certain bandwidth
away from the desired channel to
the power in a bandwidth within
the channel, and basically indicatesthe amount of power that will leak
over into the next channel versus
the signal power being handled by a
PA in its desired channel (Fig. 2).
The capability of a VSA to mea-
sure extremely high ACPR metrics
is a function of the instrument’s lin-
earity and noise floor. In practical
use, engineers must pay careful at-
tention to instrument settings such
as RF attenuation to ensure they
are operating their instrument in its“sweet spot.” In addition, higher-
end VSAs often feature advanced
Typical Test Setup
f 1
f 2
f 1 f 2
DUT ADC
Source
Source Combiner
Signal analyzer
f 1 f 2 f 1 f 2
3. This is an example of a test setup that can be used to measure PA IMD performance.
Knowing how to set up a test system for evaluating PAperformance is important from the R&D laboratory tothe manufacturing production line.
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pullquote
Sponsored by
ACPR measurement settings, such
as high-dynamic-range mode or
noise correction. Noise correction
can be performed with a VSA to
reduce the inherent noise contri-
bution of the VSA. Both advanced ACPR settings typically require
added measurement time but can
enable ACPR measurements of 80
dB or more for WCDMA signals.
When evaluating the IMD perfor-
mance of a PA, in addition to the
usual measurement equipment, a
pair of known RF/microwave signal
sources will be needed to provide
the two-tone test signals, along
with a lowpass filter (LPF) to re-
move harmonic signals that mightinterfere with the IMD measure-
ments, and a precision variable
attenuator to control the test signal
level to the PA under test. As with
the other PA tests, these signal
sources should be linear, accurate,
and repeatable, and cover the
frequency and amplitude ranges of
interest (Fig. 3).
These are just a few of the
measurements typically used to
characterize RF/microwave PAs
for many different applications. A
number of measurements attempt
to explore a PA’s linearity, such as
measurements of the third-order
intercept (TOI) and third-order in-
termodulation distortion (IM3) to
describe the amount of third-order
distortion that a PA is producing.
Such intermodulation distortion
(IMD) can degrade the transmis-
sion of signals with advancedmodulation formats.
Envelope TrackingSome newer PAs incorporate
envelope tracking (ET) technology
to reduce power consumption and
boost PAE, typically in portable,
battery-powered applications. A
PA’s efficiency is highest when the
amplifier is operated as close to
compression as possible. ET tech-
nology is typically implemented ina PA in the form of an envelope-
tracking power supply (ETPS),
which dynamically varies an
amplifier’s supply voltage to track
the input signal’s amplitude and
operate the amplifier as close to
compression as possible for as long
as possible, in the process achiev-
ing high PAE (Fig. 4).
Evaluating a PA with ET tech-
nology can be performed with a
test signal source and an analyzer,
but typically requires an arbitrary
waveform generator (AWG) to pro-
vide the envelope signal for the DC
supply modulator and to study the
effects of power supply modulation
on PA performance. ET technology
is often used in PAs in conjunction
with digital predistortion (DPD)
for optimal performance. In such
cases, a test signal source must be
capable of generating predistortionsignals using techniques such as
the AM-AM/PM memoryless lookup
table or the memory polynomial
model (MPM). As an example,
the PXI test system based on the
NI PXIe-5646R transceiver offers
both ET and DPD capabilities for
exercising PAs under testing with
both technologies.
Typical Test Equipment Traditional measurement
solutions for testing PA perfor-
mance have incorporated some
form of test signal source, signal
analyzer, and controller, and
typically software to provide some
level of automation and processing
speed when used in a production
manufacturing environment. The
development of compact, modular
test instruments, such as in the
PXI format, allows the assembly
of multiple-function measurement
solutions that provide reasonable
measurement speed while occupy-
ing minimal production floor space.
As an example, a PA measure-
ment system based on modular
PXI test instruments from Na-
tional Instruments (www.ni.com)
incorporates an RF vector signal
transceiver (NI PXIe-5646R) which
combines an RF vector signal gen-
erator and vector signal analyzerin one module (Fig. 5). The VSG
is capable of vector signal genera-
tion from 65 MHz to 6 GHz while
the VSA has a frequency range of
65 MHz to 6 GHz. The system can
also include a battery simulator (NI
PXIe-4154), arbitrary waveform
generator (NI PXIe-5451), and
12.5-GSamples/s, 5-GHz digitizer
(NI PXIe-5186) for advanced PA
testing. In addition, these instru-
ments can be tightly synchronizedfor demanding test challenges such
as measurements on ET PAs.
Sponsored by National InstrumentsA Supplement to Microwaves & RF
The Envelope Tracking Approach
LTE Signal inTime Domain
Power Envelope
High PA Efficiency (Close to Compression)
Low PA Efficiency (Far from Compression)
Envelope tracking PAs maximize efficiency by varying the PA’s point of peak efficiency
(by adjusting V CC
) in accordance with the power envelope of the signal.
4. Envelope tracking (ET) increases the PAE of a PA by adjusting the supply voltage as a function
of the input amplitude.
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Armed with LabVIEW design
software, this test system canperform a wide variety of auto-
mated PA tests and measure-
ments, including gain, output
power, power versus time, EVM,
ACLR, harmonics, and open-short
testing. It is also supported by avariety of software programs to
speed and automate PA testing ac-
cording to popular standards, in-
cluding GSM/EDGE, UMTS, LTE,
802.11a/b/g/n/p/ac, Bluetooth, FM/
RDS, and more.
In short, evaluating the per-
formance of a PA can require a
considerable number of different
tests depending upon the intendedapplications for the PA. Some of
the more demanding PA measure-
ments, such as IMD and ACLR/
ACPR, have gained importance as
methods for evaluating a PA’s ex-
pected performance under realistic
conditions in modern wireless com-
munications systems in which a PA
must handle modern, digitally mod-
ulated communications signals.
Understanding the basics of differ-
ent PA measurements and optionsfor test equipment to perform those
measurements can be an excellent
starting point for providing the best
possible next-generation PAs. n
Envelope Tracking PA Test Setup
PA
Measures power,EVM, ACLR
(cellular)
PowerModulator
Power modulator
sources modulated Vcc signal
PowerSupply
Arbitrary WaveformGenerator
Synchronized starttrigger reference
clocks
High SpeedDigitizer
VectorSignal
Transceiver
VSG VSA
5. This test setup
for evaluating PA
performance is
based on a vector
signal transceiver to
both generate and
analyze test signals.
Sponsored by National InstrumentsA Supplement to Microwaves & RF