Beyond the Box: A Software-Defined Approach to RF Testing

The traditional engineering re sponse to testing a new wireless standard often involves selecting a box instrument with the closest specifications. For automated test systems with multiple test requirements, this approach usually results in a different box for each measurement requirement in the system. When the test requirements are uniform and non-changing, this method may be sufficient, but it becomes cumbersome, slow, and ultimately more expensive for testing today’s complex radio frequency (RF) devices, which often use multiple wireless standards. A software-defined approach is ideal for automating RF verification, validation, and production tests, while traditional RF box instruments continue to play an important role on the design bench.

Inside the Instrument

Figure 1. Comparison of EVM measurement times of competitive instruments.
Today’s engineers must think beyond the box for their RF test needs. However, to think beyond the box, they first have to know what is inside a typical RF box instrument. Inside each approximately 38,000 cubic centimeters of sheet metal and plastic enclosure is a vendor-defined world of components that constitute an RF box instrument: typically there is a power supply, processor, PC motherboard or backplane, embedded operating system, measurement libraries, and a software display. The traditional appeal of a box instrument is the combination of these matched components applied to a specific set of measurement requirements.

This approach worked when testing RF devices with common test requirements. In recent years, however, the efficiency of a box instrument for automated RF testing has significantly diminished amidst the constant changes in features in wireless devices. The production volume of wireless devices is also exceeding the typical test throughput of traditional RF box instruments due to the slower processors and data buses that are often generations older than current PC technology. A clear understanding of the traditional RF instrument makeup and the challenges of working with fixedmeasurement functionality and suboptimal I/O processing is helping engineers think beyond the box for their automated RF measurement needs.

Software-Defined Approach

The transition to software-defined instrumentation for all types of automated measurement systems, including RF, is growing rapidly with an expected deployment of 100,000 PXI-based systems by the end of 2009, including more than 600,000 software-defined instrument modules. The open, userdefined software and modular, PCbased hardware are ideal for automated RF test applications because they provide the highest-performance processors and data buses, flexible peripheral I/O, compact modular design, smart power distribution and monitoring, and precise timing and synchronization throughout the system.

In other words, the software-defined approach to automated RF test uses similar types of components as a traditional RF box instrument, but applies them in a modular, user-defined architecture. This provides engineers with the highest-performance components, user-programmable I/O and analysis, and a compact form factor with proven reliability in the most demanding RF test environments. The end reward for engineers thinking beyond the box is an RF test solution that is faster, more flexible, and equally accurate – all at a fraction of the cost of stacking traditional RF boxes within a system. To further understand the benefits of software- defined instrumentation for RF, take a look at the following examples that describe how the speed, flexibility, and accuracy of this approach yields significant improvements in meeting today’s RF test needs.

WLAN, Radio, and WiMAX

One of the core benefits of softwaredefined PXI measurement systems is significantly faster measurement times than traditional RF instruments. While this advantage is compounded when testing multiple wireless standards, engineers can also achieve significant speedup when testing a single standard such as wireless local area network (WLAN).

WLAN measurements such as error vector magnitude (EVM) and spectrum mask require a large amount of signal processing. Using multicore CPUs in PXI controllers, engineers can perform these measurements five to ten times faster with software-defined RF instrumentation. Figure 1 shows a comparison between the WLAN measurement times for an EVM and power measurement of a 54 Mbps burst on various RF signal analyzers.

Figure 2. A single Averna URT can broadcast generation and RF record and playback of multiple broadcast radio standards.
A second benefit of software-defined instrumentation is the flexibility to test multiple wireless standards with the same RF hardware. Today’s wireless devices are required to meet an increasing number of standards. For example, the modern smart phone often supports a minimum of six wireless standards such as GSM/EDGE/WCDMA, Bluetooth, GPS, and even WLAN. In addition, some modern broadcast radio receivers support 10 or more wireless standards including AM/FM, RDS/RDBS, Sirius, XM, DAB, IBOC, GPS, TMC over RDS, and even DARC. Thus, there is a clear need in wireless test for instrumentation flexible enough to handle new wireless standards as they emerge.

With software-defined instrumentation, engineers can create any broadcast radio signal within LabVIEW graphical programming software, and download it into the memory of a PXI RF vector signal generator for immediate broadcast testing. For example, Averna offers the PXI-based universal radio tester (URT) to test multiple radio standards using the same RF instrumentation. A typical Averna URT system is shown in Figure 2.

In addition to generating radio broadcast standards, the Averna URT performs RF record and playback. By recording RF signals and playing them back in the lab, engineers can validate how receivers, such as an FM, DVB-T, or GPS receiver, operate in their final deployment environments.

A final benefit of software-defined RF instrumentation is that engineers can achieve highly accurate measurements at a lower cost than traditional instruments. With the emergence of new wireless standards such as WiMAX and 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), many wireless devices must meet stricter RF performance requirements than ever before. For example, the minimum EVM requirement for an 802.11a/g (WLAN) transmitter is -25 dB for the 54 Mbps, 64-quadrature amplitude modulation (QAM) signal type. Newer standards such as 3GPP LTE and WiMAX are subject to even higher RF performance requirements. By contrast, the minimum EVM requirement for an 802.16- 2004 (Fixed WiMAX) device is -31 dB for a similar 64-QAM signal type, requiring better RF performance.

Today’s software-defined instrumentation helps engineers achieve worldclass RF measurement performance at a lower cost than was previously possible. Three years ago, an RF vector signal generator and analyzer that achieved EVM measurements of -45 dB for Fixed WiMAX and adjacent channel leakage ratio (ACLR) measurements of 65 dBc for WCDMA would have cost more than $100,000 USD from any vendor. Today, however, engineers can obtain this level of accuracy for less than $65,000 USD (chassis and controller included) using new PXI instruments that use the latest 16-bit analogto- digital and digital-to-analog converters and a low-phase noise synthesizer over wide instantaneous bandwidths.

With the increasing requirements for measurement speed, flexibility, and accuracy, engineers must continue to think beyond the box for innovative RF test solutions. Fortunately, modular, software- defined instrumentation offers engineers new tools to test the growing number of wireless standards.

This article was written by Richard McDonell, Senior Group Manager for PXI and VXI, and David Hall, Product Manager for RF and Wireless Communications Hardware and Software at National Instruments, Austin, TX.

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