T&M Trends For RF Designers


The article looks at the emerging trends and applications in the RF design space which are driving the development of cutting-edge test and measurement tools


RF designers require custom measurements of the device under test
RF designers require custom measurements of the device under test

“A lack of precision is dangerous when the margin of error is small”

—Donald Rumsfeld,the American Secretary of Defense

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JANUARY 2012: These words hold true for any area of operation where a minuscule error can lead to critical repercussions. RF design testing is one such field where lack of accuracy can lead to the failure of a design project.

RF designers in fields such as telecom, aerospace and defence focus on building cutting-edge technologies and applications of the future. The test and measurement (T&M) industry is keping up with these developments.

Recent trends
Demand for compact and sophisticated equipment. The latest trends in T&M equipment for RF design include instruments with higher frequency and higher instantaneous bandwidths in a compact PCI eXtensions for Instrumentation (PXI) form factor.

Abhay Samant, senior RF group manager, NI India R&D, informs, “In the recent months, we have seen PXI RF signal analysers with up to 14GHz frequencies, 50MHz instantaneous bandwidth and a dynamic range of 80 db and higher. With very good interactive software support, designers can use these instruments on desktop for prototype design testing and early validation.” Engineers can also use very powerful instrument drivers for these instruments in LabVIEW, C, CVI or .Net to make custom measurements.

Equipment for the next-generation wireless standards. In the next-generation wireless communications standards, we see continued trends of using more spatial streams, wider channel bandwidths and higher-order modulation types to increase data throughput.

“As more sophisticated wireless standards evolve with more complex modulation/access techniques to address the efficient use of scarce resources like RF spectrum and power, demand for T&M equipment for these standards continues to increase. Apart from this, high-speed multimedia standards like wireless HDMI are also driving the need for advanced RF T&M equipment,” says Vishal Gupta, RF applications consultant, Agilent Technologies.

Samant illustrates, “With IEEE 802.11ac, this trend has resulted in the use of 8×8 multiple-input and multiple-output (MIMO), as much as 160MHz bandwidth and 256-quadrature amplitude modulation. With LTE Advanced, support for 8×8 MIMO configurations and the implementation of carrier aggregation to support as much as 100MHz channel bandwidth are also seen.”

However, while the next-generation wireless standards provide obvious benefits to consumers in the form of faster data transfer rates, design and test of IEEE 802.11ac and LTE Advanced radios pose significant challenges.

Tarun Gupta, business development manager, defense and telecom, NI India, explains, “From building transceivers capable of handling a wider bandwidth to packing more antennae on a single mobile device, next-generation standards come with substantially more difficult hardware requirements. As a result, the measurement and instrumentation required for next-generation wireless standards are more challenging as well.”

Next-generation wireless standards testing with PXI
Next-generation wireless standards testing with PXI

Equipment for in-line processing of RF signals. Another evolution is happening in the in-line processing of RF signals. Samanta explains, “With bandwidth increasing, integrating processors like field-programmable gate arrays (FPGAs) and digital signal processors (DSPs) with RF modules like vector signal analysers (VSAs) and vector signal generators (VSGs) allows users to react to the device under test (DUT) in real time and make changes accordingly.”

Equipment to generate custom-built signals. Another key challenge before RF designers is to generate custom-built signals (for evolving wireless standards and defence applications) to test their systems without sharing the intellectual property. “Wideband arbitrary waveform generators with RF bandwidths up to 5 GHz and microwave vector signal generators up to 44 GHz provide such solutions. Additionally, new signal analysers with up to 900MHz bandwidth and millimetre-wave oscilloscopes with up to 33 GHz of real-time bandwidth overcome the challenge to analyse these signals,” explains Vishal.

Advanced test equipment for strategic electronics. Strategic electronics systems like radars with better speed and range resolutions are pushing the need for millimetre-wave test equipment touching terahertz-range.

RF vector network analyser
RF vector network analyser

Vishal affirms, “Wide-band signal analysers with up to 900MHz intermediate frequency (IF) bandwidth and 160MHz analysis bandwidth have been introduced. The same is the requirement for testing high-bandwidth future radars and evolving wireless standard 802.11ac. The combination of a wide-bandwidth ARB with a microwave vector signal generator provides flexibility to generate custom waveforms and signals of up to 2GHz bandwidth, 44GHz carrier frequency and waveform for virtually any type of wireless standard and electronic warfare signal simulation.”

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Oscilloscopes for complex signal analysis. Conventionally, RF equipment are meant to perform analysis in frequency domain. But there has always been a need to capture and analyse signals at these high frequencies in time domain as well. Introduction of 33GHz millimetre-wave oscilloscopes has enabled RF engineers to capture and analyse these signals in time domain. Along with a versatile analysis tool like VSA, these oscilloscopes make the most convenient tools for complex signal analysis.

Advanced vector network analysers (VNAs). As the demand for faster data transfer rates and wider channel capacities continues to rise, precision RF components are essential to provide high-quality signals required from base-band to RF. For their exceptional accuracy and flexibility, VNAs have become an indispensable tool on the design bench as well as in validation test systems on the production line.

Samant underlines the importance of using VNAs: “The unique hardware architecture of VNA allows to precisely characterise the DUT. VNA allows to understand the electrical contribution of each component, whether in the form of an impedance mismatch or non-linear behaviour. This facilitates design of wireless devices that keep pace with today’s demanding standards.”

N9030A PXA signal analyser
N9030A PXA signal analyser

Introduction of 67GHz single-box VNA with inbuilt second source, pulse generators, pulse modulators, combiner and switch has allowed RF designers to test their radar and satellite components, and also extract non-linear models and X-parameters of their devices and circuits right up to 67 GHz.

“Along with millimetre-wave test heads, it provides the most sophisticated single-sweep, single-connection, multiple-measurement platform up to 110 GHz and in bands up to 2.0 terahertz,” informs Vishal.

Equipment for hardware-in-loop testing. Hardware-in-loop testing of RF sub-systems like telemetry links, radars and software-defined radio systems requires real-time processing of high-bandwidth RF data. One of the major challenges so far had been the backplane throughput, which did not allow streaming of RF data from input/output (I/O) modules to processors at a fast enough rate.

“With PXIe platform, T&M systems have evolved to meet these requirements of in-line signal processing on the acquired data. For example, PXIe backplane offers peer-to-peer streaming capabilities through which high-frequency and high-bandwidth data can be routed to FPGA processors at extremely fast rates, where it can be processed and measured in real-time,” says Tarun.

Test equipment that ensure shorter design cycle. The most critical test challenges before RF design engineers are reducing the design cycle and coming up with an accurate design. Further, RF design engineers want a single and more robust test platform to test multiple parameters of their device with a single connection.

“New equipment like non-linear vector network analysers (NVNAs) and the latest VNAs provide such capabilities where the non-linear behaviour of a device could be modelled more accurately and incorporated in the design to bring down its design cycle drastically. The latest-generation VNA—similar to Agilent’s PNA-X, provides a platform to perform complete characterisation of an active device based on single-connection multiple measurements,” says Vishal.

Equipment for multiple-channel signal analysis. New modular equipment can perform signal analysis for up to six channels.

“T&M industry has experienced the need for multiple-channel RF testing, especially for software-defined radios (SDRs) and wireless standards with MIMO. This has given rise to a new trend of modular test equipment in the RF domain,” says Vishal.

Equipment for custom measurements. As the devices become more complex, there is a need for custom measurements and therefore more flexibility in the instruments to program these custom measurements.

Samanta explains, “In most traditional instruments, measurements are defined by the vendors. Engineers want the flexibility of acquiring raw data and making custom measurements on it. In addition, they like to use an instrument that can be used by their verification and validation engineers as well.”

“The modular software-defined architecture of PXI test equipment makes it a compelling alternative to traditional instrumentation for testing emerging standards and devices with increased complexity,” he adds.

Traditional instruments typically have a fixed software architecture with most of the functionality defined inside the box along with their own mechanical enclosure, processor, power supply and display. On the other hand, software-defined instruments share the processor, chassis, power supply, and display across all instruments in a single PXI chassis. The test software resides on the host PC, allowing user-defined measurements and analysis in real time—a test setup optimised for high-throughput parallel testing.

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Equipment to enable quick prototyping. Another challenge faced by the design engineers at various stages of design is the need for quick prototyping. Tarun explains, “It is not always possible to have ‘golden’ boards or sub-systems that can be used to validate a newly developed sub-system. Thus arises the need for a platform which enables users to prototype the various sub-systems and test the development done by the user quickly.”

PXI platform shifts the complexity in hardware development to software development, which is relatively easier to manage. Thus prototyping becomes much easier and the user can integrate the PXI platform in the entire system to work like hardware-in-loop.

Demand for low-cost and customised T&M. Last but definitely not the least, there has always been a need for low-cost test equipment for the installation and maintenance industry, which continues to grow.


T&M Toolkit: Options Available

The cost of a typical T&M tool kit for RF testing may vary from a few thousand dollars to over a million dollars depending upon the test requirement. For example, if it is a simple test requirement related to an undergraduate-course microwave laboratory of an educational institution (which may not require very sophisticated high-precision, high-performance test equipment), the combination of a general-purpose spectrum analyser and a signal generator will serve the purpose. But if the requirement is to test a more sophisticated system for satellite or radar subsystems, the test requirements are more stringent and the laboratory may comprise a VNA, performance signal analyser, microwave vector signal generator and many more equipment to supplement the testing needs.

There are a growing number of toolkit options available for RF engineers today. Emerging technologies require a flexible platform to keep pace with emerging communication standards. Unlike traditional RF signal generators and analysers that contain fixed features for standards such as GPS, wireless LAN (WLAN) and fixed WiMAX, virtual instrumentation provides equivalent measurement functionality while maintaining ‘Open Source’ software architecture.

Wireless standards toolkits provide a fully-flexible solution to both design and test of wireless devices. Because all waveforms are created and analysed in an Open Source software environment, one can quickly and easily introduce signal impairments, simulate wireless channel conditions and even apply custom filters. Combined with the general-purpose modulation toolkit, GPS, WLAN and fixed WiMAX toolkits can be used with both mixed-signal and RF instrumentation. As a result, engineers designing GPS, WLAN and WiMAX devices have access to a new and fully-flexible design tool for these devices. Many vendors have come up with cost-effective toolkits in the market, which range from $2000 to $6000 per licence.

Impact of new technologies
Experts believe that new technologies in the T&M industry will act as enablers for new developments in the RF domain. RF designers will be able to develop and test new wireless standards that require more accurate and repeatable set of measurements. It will also help them to reduce the design cycle by reducing the development time and hence time-to-market.

Vishal explains, “For example, the new analyser platform has brought down noise floor close to thermal-noise floor of -174 dBm. New techniques in measurement science like NVNA and X-parameter will help RF designers develop more accurate and more efficient amplifiers and RF circuits and hence reduce the overall power requirements of the RF systems without compromising on performance. The new handheld and low-cost instruments allow RF engineers to perform accurate measurements and pin-point the problems in the field. This will, in turn, enable field engineers to troubleshoot the problems on the go without bringing the modules back to repair centres and thus reduce the turnaround time.”

Tarun believes that these new technologies will help the designer in two ways: “First, it will help to rapidly prototype complex RF subsystems using off-the-shelf hardware. High-bandwidth I/O combined with scalable processing will allow users to build prototypes very quickly, reducing the design cycle and time to market the product. Second, the testing can be comprehensive, resulting in better-quality products. The measurements that require processing of a huge amount of data can be done much faster.

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“For example, PXIe backplane’s peer-to-peer streaming feature enables users to combine RF modules like VSA and VSG with FPGA-based processor modules, thus integrating acquisition, analysis and generation for high-bandwidth and real-time operation.”

What’s in the offerings?
A lot of interesting developments are underway. A few of these are listed below:
1. The industry is working towards high-performance test instruments. It is also working towards general-purpose products targeted at specific applications.
2. There is a hairline gap between the RF and digital domains, and with the increasing demand for configurability and flexibility, this gap is going to further reduce over time.
3. The industry is also moving towards building automated test solutions (ATS) catering to specific test needs. These comprise a mix of RF, digital and general-purpose test equipment along with custom-built software. Cost and performance of these ATS could further be optimised by using both box and modular test platforms.
4. Work is underway to introduce new capabilities in high-performance and modular platforms so that both can complement each other to optimise the test platforms.
5. Another major advancement relates to closer integration of design and test. Samanta explains, “The test industry has not innovated as quickly, and many companies have chosen to invest more in their design tools than in test engineering tools. The consequence is, test engineers are typically outmatched when testing the latest software-centric DUT. Pundits in every major industry have envisioned solutions to bridge this gap. A closer look at the existing reconfigurable instrumentation architecture, which is the standard for many industries today, reveals a few common themes: a system-level approach, integration of design and test concepts, and extension of software architectures into FPGAs.”

Tarun adds, “The next phase in integrating design and test is the ability of engineers to deploy design building blocks, known as intellectual property (IP) cores, to both the DUT and the reconfigurable instrument. This capability is called ‘IP to the pin’ because it drives user-defined software IP as close to the I/O pins of next-generation reconfigurable instruments as possible. The software IP includes functions/algorithms such as control logic, data acquisition, generation, digital protocols, encryption, math, RF and signal processing.

“For example, MIMO SoC includes receivers, transmitters, converters, filters, switches and a processor. In addition, it features software IP such as coding, modulation, encryption and communication protocols. To fully validate the functionality of the highly integrated hardware and software sub-components of the SoC, engineers need system-level test capabilities to effectively emulate another communication device in the system. Because many of the IP blocks of the DUT and the test system are common, this presents an ideal case for concurrent design and test with IP reuse.

“The ability of a test engineer to directly embed the SoC design IP in the test instrumentation to perform system-level test can dramatically shorten design verification/validation and improve production test time and fault coverage. There are two key trends that will enable future reconfigurable test systems to deliver this IP-to-the-pin capability: the market shift toward FPGAs and availability of high-level software to program these.”

6. There are also emerging multi-vendor IP ecosystems that feature IP cores from all major FPGA vendors as well as their software and instrumentation partners. The National Instruments’ FPGA IPNet and the Cadence/Xilinx IP microsites are examples of these ecosystems. These contain hundreds of IP blocks and functions including the Xilinx CORE Generator, serial communication protocol cores, advanced encryption standard components as well as peer-to-peer streaming algorithms.

The way forward
RF design challenges and requirements posed by specific verticals are pushing the innovations further. Going forward, we can expect the emergence of more low-cost, sophisticated, customisable, multi-functional and precise equipment to meet the industry requirements.

The author is an executive editor at EFY