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PA P E R
Software-defined radio:
The next wave in RF test instrumentation?
Michael Millhaem, Principal RF Application Engineer
Keithley Instruments, Inc.
Test equipment manufacturers are constantly challenged to develop new solutions for
testing their customers’ latest devices, but they’ve traditionally developed specialized
hardware to meet this challenge. The communications market is still more challenging
due to the rapid development of new standards, which often require new stimulus
and measurement capabilities. To keep pace, test vendors must find new approaches
that reduce instrument development times and allow instruments to adapt to new
requirements. Software-defined radio is one technique that can help.
“Software-defined radio” (SDR) can be defined as a radio communication
system that uses software to modulate and demodulate radio signals. Economics is
the driving force behind the growing use of SDR. These systems can achieve high
flexibility at a lower cost than traditional analog designs. Figure 1 illustrates an SDR
system.
Baseband
Input
Baseband
Processing
DAC
RF
Front-End
RF
Front-End
transmitter
ADC
Baseband
Processing
Baseband
Output
receiver
Figure 1. Software-defined radio system.
In the purest sense, digital-to-analog (D/A) and analog-to-digital (A/D)
conversion would occur at the carrier frequency and no analog up- and downconversion would be required. Today’s SDR applications typically have at least one
analog up- and down-conversion stage. Clearly, the A/D and D/A converters are key
elements of an SDR system. The speed and resolution of the converters will determine
how much analog frequency conversion is required. Converters need sufficient
resolution (bits) to produce or capture the modulation data adequately, and more
complex modulation formats will require converters with even greater resolution. The
speed of the converters will limit the maximum signal frequency that can be created
Keithley Instruments, Inc.
28775 Aurora Road
Cleveland, Ohio 44139
(440) 248-0400
Fax: (440) 248-6168
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or sampled. Converter technology continues to advance, providing higher combinations of resolution
and frequency.
Digital signal processing is another key element of SDR because it performs several
functions traditionally performed with analog circuitry, including frequency conversion, modulation,
demodulation, and filtering. Digital signal processing also allows better performance than analog
designs by supporting functions such as waveform pre-distortion and decimation. Pre-distortion of
transmitted waveforms takes into account the known non-linearity of the analog circuitry and modifies
the baseband waveform to compensate for it, producing a better quality modulated signal.
Three basic approaches can be used to implement digital signal processing. The first approach
is to do all the signal processing in software using generic computing resources, such as those
provided in PCs. The second approach is to define a logic circuit to perform the signal processing,
then program that circuit into a field programmable gate array (FPGA). The third approach is to
use programmable hardware devices designed to implement the functions required for digital signal
processing. These devices include digital signal processors (DSP) and digital up-conversion (DUC)
and digital down-conversion (DDC) devices.
All three approaches can fulfill the primary objective of SDR: providing a highly flexible
system. However, to control costs, the other primary objective of SDR, it’s important to consider both
development and per-unit costs. The cost of the solutions will vary, influenced largely by the system’s
real-time bandwidth requirements. Wider bandwidths will require more processing power, driving
up costs. However, for a moderate level of performance, the FPGA approach will likely be the most
expensive solution, while the DSP system will likely be the least expensive.
Frequency generation is a key element of any communications system. Direct digital synthesis
(DDS) is a technique for using a D/A converter to create sinewaves at very precise frequencies.
Direct digital synthesis allows for very fast frequency switching at a low cost. Advancements in
semiconductor technology have led to rapid progress in DDS technology as well. Today’s DDS devices
can produce sinewaves with frequencies of several hundred megahertz with microhertz frequency
resolution.
SDR approaches are increasingly popular for applications that demand economical flexibility,
such as military communications systems and multi-function cellular base stations. These applications
tend to share the following characteristics:
•
Moderate to high flexibility requirements
•
Low to moderate volumes
•
Moderate to high complexity
Test instrumentation shares many characteristics with the other applications that employ
SDR techniques. Test instrumentation tends to be very complex because of the level of performance
required to measure leading-edge systems with a reasonable margin. Test instrument volumes can be
considered low to medium when compared with high volume items like cell phones or base stations.
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Flexibility continues to be an important characteristic in test instrumentation, especially in the area of
communications.
The key technical and economic requirements in test instrumentation related to
communications include wide modulation and demodulation bandwidth, wide dynamic range, and fast
throughput.
Digital communication systems have changed rapidly in recent years, particularly in the area
of modulation formats. New standards mean that test instruments like signal sources must be able to
generate new modulation waveforms and signal analyzers must be able to demodulate and analyze
them. Key performance parameters tend to vary from standard to standard, so new analysis routines
are often required.
These changes have created a demand for test instruments that can be upgraded quickly
and easily to add new modulation standards rather than forcing their owners to replace them. While
upgradeability is obviously desirable from a cost standpoint, it’s equally desirable from a time-tomarket perspective. Communication system and device manufacturers can’t afford to wait for the
next generation of test equipment to be developed. Communications standards also change frequently
during development, which can require modifications to signal generation and analysis routines.
These requirements make SDR a very desirable approach for test instrumentation. The same
cost and performance tradeoffs that apply to generic SDR applications apply to test instruments. The
first SDR test instruments used either the software processing or the FPGA approach. Advances in
digital signal processing devices like DSPs and DDC/DUCs provide the power to make that approach
viable for test instruments. This approach can provide the best balance of cost and performance for
test equipment.
Test instruments employing SDR techniques offer advantages to both equipment
manufacturers and their customers:
•
Easy upgradeability to new communication standards. Signal generation and analysis
are largely performed by routines programmed into the digital signal processor. When new
standards emerge, it’s easy to create new DSP programs for the new functions and distribute
them to the owners of existing instruments via firmware upgrades.
•
Improved throughput due to faster frequency switching and signal analysis. Wide
bandwidth A/D converters and fast DSP devices can process large FFTs very efficiently.
For example, a DSP-based analyzer can provide measurement times several orders of
magnitude faster than traditional spectrum analyzers, under conditions of wide spans and
narrow resolution bandwidths. Direct digital synthesis provides significantly faster frequency
switching than traditional approaches allow. Fast frequency switching will improve the
throughput of both signal generators and signal analyzers.
•
Faster time to market for test instruments. Test equipment manufacturers can leverage
the capability of leading-edge, commercially available signal processing devices and achieve
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instrument-level performance from them. This reduces the amount of development required
for test instruments dramatically. Also, the basic digital design can be shared across a range
of instruments, further reducing development costs. For example, Figure 2 shows the digital
architecture used in Keithley Instruments’ Model 2810 Vector Signal Analyzer and Model
2910 Vector Signal Generator. Both instruments share the same digital processing design.
µP
Host
256M
DRAM
RF
Front-End
Input
ADC
RF Front-End
Hardware
Control
DSP
Dual
DAC
FPGA
I
Q
Output to RF
Front-End 40MHz
Complex BW
DDC/
DUC
Figure 2. Digital architecture of the Model 2810 Vector Signal Analyzer and Model 2910 Vector Signal Generator.
Communication standards are likely to continue evolving. At the same time, test cost pressures
from communication system and device manufacturers will keep forcing test equipment vendors to
provide cost-effective instruments that offer continuing performance value. Together, SDR techniques
and high end signal processing devices provide test equipment manufacturers with invaluable tools to
meet these requirements.
Specifications are subject to change without notice.
All Keithley trademarks and trade names are the property of Keithley Instruments, Inc.
All other trademarks and trade names are the property of their respective companies.
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© Copyright 2006 Keithley Instruments, Inc.
Printed in the U.S.A.
No. 2695 05063KGW