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Combining Simulation and Test to
Accelerate SDR Development
Software-defined capabilities create versatile test
platforms that enhance design and test
Application Note
The goal of software-defined radio In SDR design, development and This application note reviews the
(SDR) technology is to provide a test, tracing an issue back to its root major attributes of SDRs then looks
foundation for seamless interopera- cause can be a complex endeavor. at three key topics: the use of mul-
tion between diverse communication This is often due to differences tiple waveforms during SDR design,
systems. This level of interoperation between the baseband and radio the use of simulation to add flexibility
is easy to envision but difficult to frequency (RF) sections of the design. to SDR testing, and the use of logic
achieve. For example, the hardware The complexity is often compounded analyzers and digital oscilloscopes
and software elements required to by three organizational factors: any to address baseband/RF testing chal-
enable greater flexibility introduce separation between the baseband, RF lenges. These techniques can create
major challenges that reach back and digital design teams; the diver- a common ground that helps mitigate
to the earliest stages of the design gent skill sets of analog and digital the risks caused by literal and figura-
process. Later, as prototypes or designers; and differences in the tive disconnects between baseband,
end-user devices become available, design and test tools used by each RF and digital design teams and their
interoperation introduces challenges type of engineer. Addressing these designs.
and tradeoffs in testing methods and issues early in the design process will
test systems. help ensure timely completion and
introduction of the end-user device.
Reviewing SDR characteristics and Their implications
As context, let's define an SDR as follows: It's a radio in which the baseband
processing--the physical or PHY layer--is implemented in re-configurable
hardware and software. The PHY layer includes field-programmable gate arrays
(FPGAs), as well as RF hardware such as mixers, filters, modulators, demodula-
tors, and amplifiers. The software that emulates these devices may be a C pro-
gram running in a digital signal processing (DSP) or general-purpose processor
(GPP) chip, or could be VHDL code for an FPGA design
To the outside world, the result is a receiver/transmitter that can handle
multiple types of formats and modulation schemes. This has two key benefits:
flexibility and portability. An SDR offers greater flexibility through interoperation,
which comes from the ability to support multiple waveforms, legacy formats,
and new or future formats. Said another way, an SDR can be a backward-com-
patible and future-ready device. The technology provides portability by ensuring
the ability to use an SDR waveform across platforms from a single vendor, or
across platforms from multiple vendors. From a design perspective, portability
also includes the ability to reuse waveform components across multiple devices
or platforms.
Within the SDR, there is a technology shift: the amount of analog circuitry is
decreasing while the quantity of digital technology is increasing. As a result, the
digital circuitry is getting ever-closer to the antenna--and this transformation
has important implications for both design and test.
Outlining the implications for design and test
In an SDR, RF performance is determined by not just the hardware but also the
software elements. For example, a seemingly trivial bug fix in software could
have a ripple effect that changes the radio's RF performance. On the hardware
side, platforms from different vendors may offer different levels of functionality
and performance. As a result, the hoped-for outcome of "write once, run any-
where" is not yet true in practice.
Within an SDR's mixed-signal path, there can be a mix of analog, RF, and digital
probe points. Traditionally, digital and analog/RF teams have had their own
design and test methodologies and tools, making it difficult to test along the
SDR's mixed-signal path and introducing system integration risks. Addressing
this issue has a two-pronged implication: designers need multiple ways to probe
the signal path; however, to ensure valid comparisons, they also need a consis-
tent way to measure the signals, both in design and test.
An SDR's actual RF performance depends on three aspects: baseband process-
ing, radio configuration and the RF hardware. As a result, it can be difficult to
isolate the root cause of a performance problem. This suggests the need for a
consistent way to quantify RF performance in hardware and software.
To create a consistent approach to testing, practical experience suggests the
use of three main elements: simulation software digital/baseband, and RF/IF
test instruments. Specifically, the measuring instruments would be a logic ana-
lyzer, an oscilloscope, and vector signal analyzer (VSA) software. Additionally, a
vector signal generator (VSG) with arbitrary waveform capabilities can be used
to produce input signals.
2
Figure 1 shows a consistent and seamless approach to evaluate a mixed-signal
Supporting future design in simulation and when testing the SDR hardware. VSA measurement
elements can be used in simulation to evaluate the SDR design along the mixed-
waveforms signal path. The VSA software can also be used with logic analyzers, oscil-
loscopes, and RF signal analyzers to evaluate the SDR hardware performance
In many instances, an SDR design
along the mixed-signal path during the hardware system integration testing
will be expected to support field
phase.
upgrades of radio functionality.
This raises an issue: What can be
done in the present--in design and
manufacturing--to ensure con-
fidence that the SDR will deliver
sufficient performance with future
waveforms and configurations?
Certainly, a different test strategy
is needed, and there are two key
elements to consider. First is the
need to test not just supported
waveforms but also possible radio
configurations that might be used
with future waveforms. Second is
the need to identify a consistent
set of tests that may include
measurements such as adjacent-
channel power (ACP), bit error
ratio (BER), error vector magnitude
(EVM), spurious and noise figure.
Focusing on possible waveforms, Figure 1. Using VSA software in design and test to evaluate SDR mixed-signal performance
three approaches will be useful.
One is to test the SDR with real,
Considering the longer-term implications
present-day waveforms and the
required radio configurations. Stepping back to view the larger intent of SDR, there is a more imposing
The next is to test with a broad challenge: The need to design--and ultimately test--hardware capable of sup-
selection of representative porting waveforms and algorithms that are yet to be realized. As discussed in
waveforms such as 64QAM. The the sidebar "Supporting future waveforms," it is possible to use representative
third approach is to create custom and custom waveforms and to exercise a variety of radio configurations and
waveforms that exercise different assess relevant performance characteristics. This technique can be used during
hardware configurations and make development, diagnosis and manufacturing test.
it easier to observe and character-
ize performance metrics such as
phase noise and intermodulation
distortion (IMD).
During the design process, simula-
tion is an ideal vehicle for this test
strategy. For example, a transmitter
design could be tested with a
variety of waveforms, defects, and
impairments such as noise, DC
offset, quadrature error, delay mis-
match and distortion. The results
of these simulations can be used
to set the test limits used during
actual measurements on prototype
or final-article SDRs.
3
Using a variety of waveforms in SDR design
Looking inside with
To illustrate the preceding ideas, let's look at how different types of waveforms
FPGA dynamic probe and waveform sources can be used in the design of an SDR RF transmitter. The
waveforms will be either new or legacy formats. The four waveform sources are
FPGA Dynamic Probe is a flex-
HDL code, FPGA hardware, simulation models and algorithm code.
ible tool that allows you to look
inside an FPGA and view internal
The block diagram in Figure 2 applies to Examples 1 and 2. Using Agilent
design signals. Virtual access is
SystemVue electronic system-level (ESL) design software, the RF transmitter
accomplished by connecting one
design was constructed using RF models of filters, mixers, local oscillators (LOs)
or more actual FPGA pins to a
and amplifiers. SystemVue also supports the simulation of potential issues such
logic analyzer. This approach has
as amplifier gain compression, filter impairments and LO phase noise.
three important benefits: