Using Software Defined Instruments to Address the Mixed-Signal Test Challenges of Today’s Software Defined Radios
Wednesday, February 01 2012
Page 1 of 4
Software Defined Radio (SDR) represents
an important move forward
for mobile and personal communications,
promising a major increase in
flexibility, capability, and cost efficiency. It
utilizes a combination of field-programmable
gate arrays (FPGAs), digital signal
processors (DSPs), and analog/RF
designs to achieve the radio’s system performance.
The SDR’s core functionality
can be changed by modifying the software
and firmware instead of the hardware.
Because of this flexibility, the radio isn’t
limited to just one transmission scheme or
waveform. Rather, it can be reconfigured
to support new waveforms or to operate as
a different type of radio altogether. For a
true SDR, the waveform stands on its own,
and waveforms can even be ported to different
Figure 1. The ability to compare measurement results using common measurement software at different locations in the radio helps isolate the source of errors.
All of this flexibility comes at a price.
SDR designs require greater integration
of DSP/digital and RF functions and a
wider range of tests, which in turn gives
rise to a number of mixed-signal test
challenges. Consequently, ensuring successful
operation and proliferation of
today’s SDR designs requires use of
modern instruments capable of bridging
the digital-analog divide, while also
addressing any challenges stemming
from use of the SDR technology itself.
Warning: Challenges Ahead
By their very nature, SDR designs
are mixed-signal (signals in both analog
and digital form within the radio),
so testing complexities will arise when
the baseband hardware and RF hardware
are integrated together. This
complexity stems from the impact of
impairments on the SDR’s overall system
performance, which makes issues
difficult to isolate during system integration
Many factors can contribute to error
along the mixed-signal transmitter
chain and in turn, affect waveform
quality and the SDR’s overall error vector
magnitude (EVM) performance.
EVM is a measure of waveform quality
and is typically used as a metric for wireless
transmitter performance. For example, the D/A converter may introduce
nonlinearities and the D/A converter
clock may introduce jitter.
Additionally, local oscillator (LO)
phase noise, IF/RF filters, and nonlinear
gain/phase distortion from the
IF/RF up-converter and power amplifier
may introduce waveform distortion
to the SDR’s EVM performance.
Figure 2. The left, center, and right screen shots show the EVM and constellation measurement of a QPSK radio at IF (EVM = 12.5%), analog IQ (EVM = 6.4%), and digital IQ (EVM = 4.2%), respectively.
Multiple-input multiple-output (MIMO)
technologies further add to the complexity
associated with testing and debugging
mixed-signal SDR designs. SDR orthogonal
frequency division multiple access
(OFDMA) technologies often employ
MIMO as a way to increase data rates relative
to single-input single-output (SISO)
approaches. However, MIMO technology
is highly complex with its spatial multiplexing
algorithms, multiple transmit/
receive IF/RF chains, and multiple
antennas, and MIMO performance can
be impacted by impairments such as timing
errors and cross-coupling between
the multiple channels.
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