Software-Defined Radio (SDR) Technology Drives the Evolution of Test Instruments
Test equipment manufacturers consistently face the challenge of developing new testing solutions that meet the latest product testing demands. Traditionally, they address this by designing specialized hardware. In the communications market, the rapid evolution of new standards often requires novel signal generation and measurement capabilities, presenting even greater challenges. To keep pace with these developments, test instrument vendors must find ways to shorten development cycles to ensure their instruments can address emerging measurement needs. Software-Defined Radio (SDR) technology has emerged as a powerful enabler in this context.
SDR is a wireless communication system that uses software to modulate and demodulate radio signals. Its widespread adoption is driven by cost-effectiveness: compared to traditional analog designs, SDR systems offer significantly greater flexibility at a lower cost.
In a strict sense, digital-to-analog (D/A) and analog-to-digital (A/D) conversion would occur directly at the carrier frequency without analog upconversion or downconversion. However, current SDR applications typically include at least one stage of analog frequency conversion. Clearly, A/D and D/A converters are critical components of SDR systems. The speed and resolution of these converters determine the extent of analog frequency conversion required. Converters must have sufficient resolution (bits) to accurately generate or capture modulated data, with more complex modulation formats demanding higher resolution. Converter speed also limits the maximum signal frequency that can be generated or sampled. Ongoing advancements in converter technology now enable both high resolution and high frequency simultaneously.
Digital signal processing is another key element of SDR systems, as it replaces many functions traditionally implemented in analog circuitry, including frequency conversion, modulation, demodulation, and filtering. It also enables features like waveform predistortion and decimation, delivering performance superior to analog designs. For example, transmit waveform predistortion accounts for analog circuit nonlinearities by adjusting the baseband signal to compensate, resulting in higher-quality modulated signals.
There are three fundamental approaches to implementing digital signal processing:
Software-Based Processing: Utilizing general computing resources (e.g., PCs) to handle all signal processing in software.
FPGA-Based Processing: Designing a logic circuit to perform signal processing and implementing it using a field-programmable gate array (FPGA).
Programmable Hardware: Employing specialized programmable hardware such as digital signal processors (DSPs) and digital upconversion (DUC)/downconversion (DDC) devices.
All three methods achieve the primary design goal of SDR: high flexibility. However, cost considerations - both development and per-unit costs - are critical. The cost of each approach varies significantly based on real-time bandwidth requirements. Higher bandwidth demands greater processing power, increasing costs. For moderate performance needs, FPGAs may be the most expensive option, while DSP-based systems often offer the lowest cost.
Frequency generation is a key element of all communication systems. Direct Digital Synthesis (DDS) is a technology that uses D/A converters to generate sine waves with extremely precise frequencies. It enables rapid frequency switching at low cost. Advances in semiconductor technology have driven the rapid development of DDS, with modern devices capable of generating sine waves up to several hundred megahertz with microhertz-level frequency resolution.
SDR technology is increasingly favored for applications requiring both economy and flexibility, such as military communication systems and multifunctional cellular base stations. These applications typically share the following characteristics:
Moderate to high flexibility requirements.
Small to medium deployment scale.
Moderate to high complexity.
Test instruments share many traits with other SDR applications. They are often highly complex, as measuring cutting-edge systems with adequate margin requires exceptional performance. Production volumes for test instruments are generally small to medium compared to high-volume devices like mobile phones or base stations. Flexibility remains a crucial feature for test instruments, especially in communications. Key technical requirements for communication test instruments include high modulation bandwidth, large dynamic range, and high data throughput.
In recent years, digital communication systems - particularly in modulation formats - have evolved rapidly. New standards necessitate test instruments capable of generating novel modulated waveforms and analyzing them. Different standards often have unique performance parameters, requiring new analysis routines.
These challenges demand test instruments that can be quickly upgraded to support new modulation standards. Upgradeability is critical for cost savings and reducing time-to-market. Communication system and device manufacturers cannot afford to wait for the development of next-generation test equipment. Moreover, communication standards often change during development, necessitating updates to signal generation and analysis routines.
These requirements make SDR technology an ideal choice for test instruments. The cost-performance trade-offs developed for general SDR applications are equally applicable to test instruments. First-generation SDR test instruments used software processing or FPGA-based approaches. Advances in digital signal processing devices (e.g., DSPs and DDC/DUC) now provide the processing power needed for test instruments. This approach offers the best balance of cost and performance for test equipment.
SDR-based test instruments provide several benefits to manufacturers and users:
Easy Upgrades for New Standards: Signal generation and analysis are primarily implemented via routines programmed into digital signal processors. When new standards emerge, new DSP routines can be developed and distributed to existing instruments via firmware updates.
Higher Throughput: Faster frequency switching and signal analysis. High-bandwidth A/D converters and rapid DSP processors efficiently handle large FFTs. For example, under wide spans and narrow resolution bandwidths, DSP-based analyzers can measure orders of magnitude faster than traditional spectrum analyzers. DDS enables much faster frequency switching than traditional methods, improving throughput for signal generators and analyzers.
Faster Time-to-Market: Test equipment manufacturers can leverage advanced commercial signal processing components to achieve instrument-grade performance, significantly reducing development effort. Additionally, core digital designs can be reused across instruments, further lowering development costs.
Communication standards will continue to evolve. Meanwhile, pressure to reduce testing costs will compel test equipment vendors to deliver more cost-effective instruments. The combination of SDR technology and high-end signal processing components provides a powerful toolset for meeting these demands.