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 hardware platforms.

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 testing.

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.