Software-defined radio for RF instruments

Modern RF instruments have evolved from simple measurement equipment to important system design tools. This development is due to the various technologies triggered by software-defined radio (SDR). The flexibility of software-defined radio is revolutionizing the wireless communication industry and RF test instruments.

In the late 1980s, engineers began to experiment with the idea of ​​software-defined radio. In the past, radios relied on complex analog circuits to send and receive RF and microwave signals and to encode and decode information signals. The original idea of ​​software-defined radio was to use a general-purpose radio to send and receive signals while performing multiple physical layer functions (such as modulation and demodulation) in software.

Walter H. W. Tuttlebee wrote in his article Software Defined Radio: Origins, Drivers and International Perspectives: Some of the earliest typical applications of software radio include military radio communication projects, such as the SPEAKeasy project in the early 1990s. In the design of this project, by developing many modulation and demodulation functions in software, the radio provides interoperability between various wireless interfaces.

However, by the late 1990s, engineers began to actively study the application of software radio technology in commercial systems, such as cellular base stations. One of the most influential papers that described the need for software radio in an increasing number of applications was Software Radios: Survey, Critical Evaluation and Future Directions, published by Dr. Joseph Mitola III in 1993 in IEEE Spectrum. Dr. Mitola is also known as the "Father of Software Radio" for his extensive research.

Perhaps the best example of the benefits of a software-defined radio approach is in modern base stations. As wireless standards evolve from GSM to LTE, it becomes increasingly difficult to add support for new standards through hardware. In addition, base stations perform signal processing and closed-loop control through complex and constantly evolving software. For example, power amplifier (PA) linearization techniques, such as digital predistortion (DPD), are critical to base station performance and continue to evolve over time. This makes a software-defined radio approach ideal for base station design and long-term supportability.

A fundamental change in instrumentation

At the same time, software radio architecture is increasingly being used in the wireless industry, and RF test and measurement equipment is undergoing a major transition. In the early 21st century, the advent of new wireless standards required instruments to provide richer measurement functions, and therefore required a more flexible architecture. Since this requires a large number of RF measurement engineers to achieve, the practice of designing instruments specifically for a few applications has become impractical. Therefore, test vendors began to explore the concept of software-defined RF test equipment.

传统扫频调谐频谱分析仪的发展是整个行业过渡到软件定义仪器系统最典型的例子之一。在传统的频谱分析仪中，分辨率带宽滤波和功率检测等功能是基于模拟组件来实现的。然而，今天的现代射频信号分析仪通过集成通用射频下变频器（无线电）来生成数字化I / Q采样。该仪器能够使用频谱计算等多种方法来处理I / Q采样数据。因此，可能用于执行光谱测量的同一信号分析仪还可以用于解码RADAR脉冲、解调LTE信号或甚至无线记录GPS信号。

如今，测试厂商已经进一步完善射频仪器架构，以不断趋近于软件无线电架构。新一代射频仪器的基本架构不仅结合了通用无线电，还结合了广泛的PC和信号处理技术，如多核CPU和FPGA。今天，RF测试设备的软件无线电化为传统RF测试应用提供了显著的优势，同时也帮助工程师实现了以前无法用射频仪器实现的应用。

The Impact of Moore's Law on RF Test

One of the most obvious advantages of integrating PC technology into RF instrumentation is the continuous improvement of instrument signal processing performance. Moore's Law predicts that the processing power of CPUs will continue to increase, which means that the processing performance of instruments will also continue to increase. Therefore, as CPU manufacturers continue to update processor technology, the measurement speed of PC-based instruments has also continued to increase. For example, a spectrum measurement that took 50 ms ten years ago can now be completed in just 5 ms.

In addition to CPU, modern RF instruments have gradually integrated the core technology of modern software radio - FPGA. FPGA has been used in RF instruments for more than ten years, and a growing trend today is to make the instrument's FPGA user programmable. User-programmable FPGA expands the role of the instrument from a single-function device to an infinitely flexible closed-loop control system.

With the advent of today's FPGA-enabled instruments, engineers can combine the real-time control capabilities of FPGAs with extremely time-critical test functions. For example, in test applications that require device control through digital interfaces, FPGA-enabled instruments can perform digital device control and RF measurements simultaneously. Engineers have achieved 100-fold improvement in test times based on new test methods provided by user-programmable FPGAs.

The tools that support FPGAs have also driven tremendous innovation in FPGA programming. Although some engineers have used hardware description languages ​​such as VHDL for many years, the complexity of FPGA programming has posed a major barrier to widespread adoption of the technology.

Software Radio Promotes the Application of FPGA

Today, architectural elements in RF instruments, such as software-defined radios, have blurred the line between traditional instruments and embedded platforms. Instrument-defining features, such as user-programmable FPGAs, are making RF instruments increasingly popular in embedded applications.

Twenty years ago, it would have seemed unthinkable to put together a million-dollar RF signal generator and RF signal analyzer to prototype a radar system. Not only was the cost and size of such a system high, but the complex programming experience also discouraged engineers from using instruments such as wireless communication equipment.

However, now, smaller and more powerful PC-based instrument platforms such as PXI have become ideal prototyping solutions for electronic embedded systems. PC-based instruments not only meet the size and cost requirements of embedded systems, but also provide engineers with a good software experience that can reconfigure RF instruments to achieve a wide range of applications. Therefore, engineers began to use RF signal generators and analyzers to design embedded systems such as radars, channel emulators, GPS loggers, and DPD hardware.

The ability to fully define and customize RF instrument behavior using software has become critical to solving next-generation test challenges. As a result, future RF instrument architectures will become increasingly difficult to distinguish from software-defined radio architectures.