The SDRification of RF instrumentation

The flexibility of the software-defined radio (SDR) is revolutionising not only the wireless industry but also RF test equipment.

In the late 1980s, engineers began to experiment with the idea of the SDR. Historically, radios relied on complex analog circuitry not only for the transmission and reception of signals at RF and microwave frequencies but also for the encoding and decoding of the message signal.

The idea of the SDR was to use a general-purpose wireless radio for transmission and reception while executing many of the physical layer functions (such as modulation and demodulation) in software.

Some of the first significant incarnations of the SDR were military radiocommunications programs such as the SPEAKeasy program in the early 1990s, according to Software Defined Radio: Origins, Drivers and International Perspectives by Walter HW Tuttlebee. Radios designed as part of this program offered interoperability between various air interfaces by implementing many of the modulation and demodulation functions in software.

However, by the late 1990s, engineers were actively researching the use of SDR technology in commercial systems such as cellular base stations. One of the most influential papers that described the requirements of SDRs for an ever-increasing range of applications was ‘Software Radios: Survey, Critical Evaluation and Future Directions’ by Dr Joseph Mitola III and published in IEEE Spectrum in 1993. As a result of his extensive research, Dr Mitola is widely known as the ‘Father of SDR’.

Modern base stations are perhaps the best embodiment of the benefits of the SDR approach. As wireless standards evolved from GSM through LTE, it became increasingly more difficult to add support for new standards with more hardware. In addition, base stations use sophisticated and evolving software for signal processing and closed-loop control.

For example, power amplifier (PA) linearisation techniques such as digital predistortion (DPD) are not only essential to the base station’s performance but also constantly improving over time. As a result, the SDR approach is ideal for base station design and long-term supportability.

Fundamental changes to instrumentation

At the same time the adoption of the SDR architecture was increasing in the wireless industry, RF test and measurement equipment was undergoing a significant evolution. In the early 2000s, the onslaught of new wireless standards required instruments to offer an increasing breadth of measurement capabilities, which led to a more flexible architecture.

Given the variety of RF measurements engineers were required to make, the historical practice of designing an instrument for a relatively narrow range of applications became impractical. As a result, test vendors began to explore the concept of software-defined RF test equipment.

The evolution of the traditional swept-tuned spectrum analyser marks one of the most dramatic examples of an industry-wide transition to software-defined instrumentation. In a traditional spectrum analyser, functions such as the resolution bandwidth filters and power detection were implemented using analog components.

Today, however, the modern RF signal analyser incorporates a general-purpose RF downconverter (a radio) to produce digitised I/Q samples. Internally, the instrument processes I/Q samples in a variety of ways including the computation of a spectrum. As a result, the same signal analyser that engineers might use to perform a spectrum measurement can also be used to decode a radar pulse, demodulate an LTE signal or even record a GPS signal off the air.

Today, test vendors have further refined the architectures of RF instruments to increasingly resemble that of the SDR. The fundamental architecture of the new generation of RF instruments incorporates not only a general-purpose radio but also a wide range of PC and signal processing technologies such as multicore CPUs and FPGAs.

This ‘SDRification’ of today’s RF test equipment provides substantial benefits in traditional RF test applications while helping engineers use applications that were previously impossible to solve with RF instrumentation.

The impact of Moore’s law on RF test

The consistent improvement in instrument signal processing performance is one of the most obvious benefits of integrating PC technology into RF instrumentation. Moore’s law predicts constant improvements in CPU processing power, which means similar improvements in instrument processing performance.

Thus, as CPU vendors continue to innovate on processor technology, PC-based instruments benefit by achieving faster measurement speeds. For example, the same spectrum measurement that took 50 ms a decade ago can now be performed in less than 5 ms.

In addition to the CPU, modern RF instruments increasingly incorporate a core technology of the modern SDR - the FPGA. Although RF instruments have used FPGAs for more than a decade, an evolving approach is to make the instruments’ FPGAs user programmable. User-programmable FPGAs are expanding the role of instrumentation from a single-function device to an infinitely flexible closed-loop control system.

With today’s FPGA-enabled instruments, engineers can marry the real-time control capabilities of the FPGA with the time-critical functions of testing. For example, in test applications that require device control through a digital interface, an FPGA-enabled instrument can synchronise digital device control with the execution of the RF measurements.

As a result of the new testing approaches offered by user-programmable FPGAs, engineers can see test time improvements of up to 100X.

The benefits of FPGA-enabled instruments have also driven significant innovation in the FPGA programming experience. Although some engineers have used hardware description languages such as VHDL for years, the complexity of FPGA programming is often a barrier for widespread adoption.

Expanding applications from SDRification

Finally, the SDR-like architectural elements of today’s RF instrumentation have blurred the line between instrument and embedded platforms. Defining instrument characteristics such as a user-programmable FPGA have led to a rapid rise in the number of RF instruments used in embedded applications.

Twenty years ago, it seemed unimaginable to assemble a million-dollar collection of RF signal generators and RF signal analysers to prototype a radar system. Not only was such a system cost and size prohibitive, but the instrument programming experience prevented engineers from using the instruments like a radio.

Today, however, more compact and powerful PC-based instrument platforms such as PXI are ideal prototyping solutions for electronic embedded systems. PC-based instruments not only meet the size and cost requirements of embedded systems, but they also offer a software experience that helps engineers reconfigure an RF instrument for a wide range of uses. Now engineers are designing embedded systems such as radars, channel emulators, GPS recorders and DPD hardware with RF signal generators and analysers.

The ability to fully define and customise the behaviour of RF instrumentation with software is a key element to solving the next generation of test challenges. As a result, the architecture of tomorrow’s RF instruments will look more and more indistinguishable from that of the SDR.