NI JulyBasicsOfDesign

8/12/2019 NI JulyBasicsOfDesign

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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

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.

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.

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|]

∆ = 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

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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NI JulyBasicsOfDesign

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