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Modern
Tools for Today’s Radio
By John Barfuss, Agilent Technologies
Radios, and the instruments used to test
them, have both benefited from advances in digital signal
processing (DSP) and converter technology. The result is
more versatile designs that can be adapted to multiple uses
because most of the functionality is implemented in software
and firmware rather than hardware.
This article will discuss the versatility in today’s
test tools and how that versatility contributes to the design
and test needs of the modern radio.
Software Defined Instruments (SDI)
A software defined instrument is analogous to the software
defined radio (SDR). The core functionality of an SDR is
implemented in digital signal processors (DSPs), field programmable
gate arrays (FPGAs), and/or general purpose processors (GPPs)
and can be changed by modifying the software and firmware.
As a result, the radio isn’t limited to just one function
but is flexible and can be modified to support a new standard
or to operate as a different kind of radio altogether. Further,
for a true SDR, the software stands on its own and can be
transported to different hardware platforms.
Modern test instruments are versatile and
can perform many functions by simply adding or changing
software. Measurements traditionally done with hardware
circuitry are now implemented in DSP. The result is instruments
that are largely software based. And, as we will see in
this paper, in some cases the measurement software can stand
on its own and be transported to radically different platforms.
An example of a software defined instrument is a spectrum
analyzer that incorporates an all digital intermediate frequency
(IF) section.
One such instrument is the MXA signal analyzer from Agilent.
In the analog front-end of this instrument, the input signal
is down converted to an IF, and then digitized. From there,
the core functions of the analyzer are implemented using
DSP. With the right software, the analyzer operates as a
complete swept tuned spectrum analyzer. But, it can also
be an FFT analyzer, a modulation analyzer, or a full-fledged
Vector Signal Analyzer (VSA). You can expect more functions
to be added to the MXA in the future.
This does not mean that the MXA hardware is completely generic.
As found with the software defined radio, cost, function,
and performance restraints shape the hardware. SDIs are
not without their practical boundaries.
As an aside, a type of generic SDIs exist in the form of
synthetic instruments. Synthetic instruments build on the
idea of a general purpose SDI by using modular high performance
building blocks, including digitizers, frequency converters,
processing units, and software to synthesize all measurements.
The modularity of these components allows them to evolve
independently. Due to the large upfront costs of developing
such a test system, today’s synthetic instruments
are used primarily for large military projects that may
mandate the use of them to meet long-term cost, reuse, and
maintenance criteria. For the purpose of this article, I
won’t go any further into synthetic instruments other
than to make mention of them as part of the big picture
in instrumentation trends.
Changing Signal Forms
A challenge with testing modern radios is the changing signal
formats. A conventional radio transmitter typically uses
a baseband integrated circuit (IC) that outputs an analog
baseband signal which modulates an IF, which is then upconverted
to RF and amplified.
Today, more of the radio is being implemented in the digital
realm. It is therefore more common to be working with digital
representations of the signal, such as digital IQ or digital
IF. These digital signals can be formatted in various ways,
such as parallel or serial, two’s complement or binary,
or packetized in the form defined by one of the digital
interface standards such as DigRF. Note that these “digital”
signals are not the same as digital data. In other words,
the ones and zeros do not represent the data directly. Rather,
the digital values represent a digitized form of the modulated
analog waveform.
Test equipment vendors are responding to these changing
signal formats by providing digital interfaces to traditionally
analog test tools.
Consistent Measurements Everywhere
An example of a traditional analog instrument with digital
signal I/O capability is the Agilent ESG signal generator.
The ESG has the versatility to provide test stimulus in
any format needed. Not only can the instrument output signals
at RF, but it can also provide the same test signals at
IF, analog IQ, digital IF or digital IQ. For the digital
signal output, the generator utilizes a digital signal interface
module (DSIM) that is reconfigurable to various digital
formats and clock rates. (see Figure 1)
The power in such a solution is the ability
to provide consistent test stimulus to any part of the radio
and to independently verify the performance of each component
or section. Since the ESG is arbitrary waveform generator
(AWG) based, it has the flexibility to recreate, with the
right software, any signal within its performance restraints.
Further, impairments, such as noise or channel effects,
can also be modeled into the signal using software processing.
Similar flexibility also exists on the analysis
side. (see Figure 2) The following example
is one of transportable measurement software that can operate
on different platforms.
The Agilent 89601A VSA software is a flexible measurement
tool that supports many demodulation formats and measurements.
This transportable VSA software is not only able to run
on its native signal analyzer but can also operate on the
oscilloscope and the logic analyzer. By doing so, its measurement
capabilities are unlocked and can provide insight for signals
of any format including RF, IF, analog baseband, digital
baseband, or digital IF.
A great benefit of being able to consistently measure the
signals anywhere within the radio with the same test tool
is that it allows you to directly compare the signal quality
in different parts of the radio.
To illustrate, measurement screen shots A,
B, and C show the error
vector magnitude (EVM) and constellation measurement of
a QPSK radio at IF, analog IQ, and digital IQ respectively,
using the VSA software running on the signal analyzer, oscilloscope,
and logic analyzer. This is a basic QPSK signal, but the
concept works for any supported modulation format including
WLAN, WiMax, CDMA, GSM or generic QAM signals.
The measurement results show that we gain about 6% EVM error
going from analog IQ to IF and about 2% EVM error going
from digital IQ to analog IQ. (see Figures
A, B, C)
Closer examination using the detailed analysis functions
of the VSA software reveal the cause of the errors. In this
case, the majority of the error between the IF and analog
IQ is quadrature error introduced by our IQ modulator. The
error gained between our digital IQ and analog IQ signals
is largely the result of dispersion introduced by analog
filters located just after the DAC. The 4% EVM of our digital
IQ signal is primarily due to the ripple in the passband
of the digital filter implemented within our FPGA. The key
point is that being able to compare measurement results
at different locations in the radio helps isolate the source
of the errors.
Connected Solutions – Tying
Simulation with the Real World
The versatility does not stop with multiple instrument platforms.
The VSA measurement software, can also operate within a
software design environment.
In the case of the Agilent VSA software it has direct support
for the Agilent ADS and The Mathworks Simulink simulation
and design environments.
Figure 4 is an example of an ADS simulation
of the RF section of a radio. Note the VSA icons placed
into the model. By adding the VSA tool into the simulation,
an engineer is able to measure signals with the same algorithms
and functions that will eventually be used to test the hardware
implementation of the simulated circuit.
Notice, in the example above the VSA software is acting
as both a measurement tool and a source within the simulation.
This is possible because of the record and playback capability
of the VSA that allows it to record, store, and play back
signals in either the physical world or in simulation. The
capability allows designers to test their simulated system
using real-world signals. The connection from simulation
to the real world is a powerful tool in transitioning designs
from the software development environment to the real world.
In summary, the flexibility of today’s test tools
greatly improves the efficiency of radio designers by providing
flexibility to use common measurement tools throughout the
radio and through all stages of development. This flexibility
in test complements the trends in modern radio designs that
utilize more DSP, require greater functionality and more
rapid development.
About the Author
John Barfuss, an applications engineer for Agilent’s
Aerospace & Defense business, joined Agilent in 1999.
Prior to working for Agilent, John was a test engineer for
3Com Corporation. He graduated from the University of Utah
with his BSEE in 1997 and his MBA in 2000.
AGILENT
TECHNOLOGIES
www.agilent.com
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