PXI Modules Offer Speed, Scalability and Small Form Factor for RF Measurement Systems

The emergence of new wireless standards and the growing adoption of wireless radios in the consumer world have created new RF test challenges for engineers. With consumer devices using more radios and engineers performing more RF measurements than ever before, it has become increasingly clear to the industry that the next generation of RF test tools is required to test a wide range of standard-specific signal types—and make these measurements faster.

Background

The PXI (PCI eXtensions for Instrumentation) standard was first introduced in 1997 as a modular platform for test and measurement applications. The PXI Standard was created and is maintained and promoted by an industry consortium known as the PXI Systems Alliance (PXISA). It is comprised of more than 50 industry-leading companies that manufacture and market PXI-based hardware and software solutions. Current Sponsor members include Agilent Technologies, Ampro ADLINK Technology, Gigatronics, Geotest - Marvin Test Systems, National Instruments, Pickering, Teradyne and Aeroflex.

With the need for faster, upgradable instruments clearly defined, several PXI vendors released some of the industry’s first PXI RF instruments nearly a decade ago. Leveraging the raw processing power of the modern CPU, these first RF products were largely designed for production test, offering substantial improvements in measurement speed over rack-and-stack equipment. While the first round of PXI RF equipment competed mostly with low to mid-grade traditional equipment, continued miniaturization of higher performance RF components would soon bring best-in-class performance to PXI.

By the early 2000s, the miniaturization of high-performance DACs and ADCs had enabled new PXI baseband instruments that rivaled the performance of their much larger rack-and-stack counterparts. PXI users largely chose the platform because of the software-defined flexibility, often using tools such as NI LabVIEW and other software to create customized measurement systems. As Moore’s Law describes, the increasing processing power and smaller footprint of modern CPUs made PXI an increasingly attractive target for automated test and measurement applications. To engineers making RF measurements, PXI’s use of a high-performance CPU was particularly attractive, especially as new wireless standards were making RF measurements some of the most processor-intensive measurements in the industry.

At its core, PXI combines the computer industry standard PCI bus with timing and synchronization designed specifically for measurement and automation. Over the years, various extensions to the standard were developed and published by the PXISA. A key new PXI specification was introduced in 2005, the PXI Express Hardware Specification. This made use of a flexible new interconnect, PCI Express (PCIe), for communication to a host PC at very high data rates, with very low latency and high scalability.

Today, recent innovations in PXI instrumentation have not only made measurements faster, but have also enabled PXI instruments to push the boundaries of RF measurement performance. In fact, modern PXI RF instruments deliver measurement accuracy competitive with the best rack-and-stack test equipment in the industry. For example, instruments such as the National Instruments (NI) PXIe-5665 3.6 GHz vector signal analyzer achieve typical phase noise of -129 dBc/Hz (1 GHz carrier, 10 kHz offset), among the lowest of all RF signal analyzers (see Figure 1). In addition, the 6.6 GHz RF vector signal analyzer can measure W-CDMA EVM within 0.5 percent accuracy, again rivaling the best in the industry. Table 1 shows a brief overview of some key PXI RF performance metrics. What is most impressive is that these performance results are achieved in the remarkably small footprint of PXI. One notable example is the NI PXIe-5630, a two-port vector network analyzer (VNA) that boasts a front panel that is less than seven square inches in size.

While today’s newest PXI RF instruments now offer best-in-class accuracy, two of the most compelling reasons for engineers to adopt PXI continue to be measurement speed and flexibility. In PXI, the combination of a high-speed data bus (PCI Express) and high-performance CPU (currently the Intel i7) enable most measurements to be performed five to 20 times faster than traditional rack-and-stack instrumentation. Over the last decade, faster measurement speed has been one of the primary drivers of PXI adoption in applications in which engineers are automating test equipment, where test time is a key factor.

Agilent’s large PXI product introduction in September of 2010 was enabled by this new level of performance available in PXI, improving speed performance over existing parallel bus architectures and addressing scalability to large systems while making use of existing PXI modules. Modular instruments are a way to provide cost-effective solutions by allowing test engineers to purchase exactly what they need, and easily increase (scale) channel count as an application grows.

With system integration in mind, all of Agilent’s modules are either PXIe modules or hybrid-compatible PXI modules and therefore work in PXI hybrid slots. Agilent’s chassis was developed with 16 hybrid slots, and any 32-bit cPCI, PXIe, or PXI hybrid-compatible modules can be placed into any slot, eliminating the need to choose modules according to the connectors in the chassis. Innovative cooling concepts require only 4U mounting without the need of additional space above or below ensuring that rack space is optimized.

Measurements and Applications

The inherently flexible nature of PXI instruments has also enabled their use in many unique applications that are difficult to solve with rack-and-stack instrumentation. One such application is MIMO design and prototyping. With PXI, modularity is one of the chief product design features. As a result, several PXI RF instruments are capable of sharing signals such as reference clocks, ADC/DAC sample clocks, and even local oscillators. Using multiple up-converters, down-converters, or baseband modules, one can configure PXI RF signal generators and analyzers to have two, four, or even eight channels. Not only do PXI multichannel signal generators and analyzers have the benefit of a smaller footprint, they can be configured phase-coherently, unlike many traditional rack-and-stack instruments.

PXI instruments’ flexibility offer significant benefits for applications that require real-time signal processing of IQ samples. Using the PCI Express backplane, today’s PXI RF signal analyzers are capable of streaming IQ samples directly from onboard memory to other PXI modules at rates of up to 800 MB/s. Common targets for these samples include RAID volumes for data storage or even user-programmable PXI FPGA modules. Using this technology, engineers can now program their RF signal analyzer as an FPGA target, using the real-time processing capabilities of the FPGA to perform onboard analysis routines such as FFTs, custom filtering and real-time demodulation. While many of these applications would have required engineers to design custom hardware in the past, today’s FPGA technology saves both time and money by enabling the use of off-the-shelf instrumentation.

Basic measurements are covered by some of the broad application modules introduced. Test engineers can now utilize Agilent’s measurement expertise when developing a PXI-based test and measurement solution. These new modules include a 6.5 digit digital multi-meter, an isolated voltage/current source, a digital I/O module, several high-speed digitizers and high-precision, wideband arbitrary waveform generators. A large number of switching modules have also been introduced covering the range of DC to 26 GHz, filling the needs of almost any test and measurement system.

A new PXI vector signal analyzer from Agilent, with frequency coverage from 50 MHz to 26.5 GHz and with 250 MHz of instantaneous bandwidth, provides very high performance in a small package that was previously thought possible only in larger “box” instruments. This new device leverages Agilent’s current VSA software packages, supporting industry standard software with the newest hardware on the market. Finally, the vector signal analyzer itself is a modular instrument as it is composed of five independent modules, allowing future upward (or downward) scalability as customers may use individual components as they see fit.

The PXI software specification defines standard frameworks for PXI systems. The specification stipulates that the module device driver software must run within a given framework, where the framework is based on the 32-bit Microsoft Windows® operating systems, covering Windows® 2000, XP and Vista. A recommendation of the PXI software specification is for PXI module compatibility with well-established development environments including National Instruments’ LabVIEW and LabWindows/CVI, Microsoft Visual C/C++ and Visual Basic.

The PXI modular hardware is configured and controlled by a device driver implemented using the Virtual Instrument Software Architecture (VISA). The VISA architecture is an I/O software standard, defined by the VXI plug & play specification, and adopted for PXI as well as GPIB, VXI, VME and serial instrumentation. National Instruments’ NI-VISA is the version of VISA used in the Aeroflex PXI 3000. Using VISA promotes interoperability of the software. All Agilent PXI instruments provide IVI and LabView interfaces and a soft front panel, allowing customers the flexibility of using different programming environments including Visual Studio, LabView, LabWindows CVI, MATLAB and VEE.

Advanced Measurement Systems

As part of its strategy to provide flexible and cost-effective test systems, Aeroflex launched the 3000 Series, a PXI-based modular test suite for mobile phone and general-purpose wireless test in October 2003. Today, the Aeroflex 3000 Series of RF Modular Instruments utilizes the speed and modularity of PXI for wireless communications testing, supporting wide bandwidth RF signal generation, RF signal analysis and RF signal conditioning for signals up to 6 GHz with a broad selection of PXI chassis and modular PXI instruments.

The Aeroflex PXI RF signal generator and PXI RF digitizer families offer multiple test system configurations that are entirely customizable to serve many different applications. By combining the appropriate Aeroflex PXI digitizers and signal generators, the overall system bandwidth and frequency range can be set to fit the needs of engineers while allowing for future expansion.

The 3000 series is also supported by the PXI Studio application software for waveform generation and vector signal analysis of complex wireless communications systems. A portfolio of libraries that is continually expanding is now capable of supporting multiple wireless standards including GSM/EDGE, UMTS/HSUPA, LTE FDD, CDMA 2000 and 1xEVDO, TD-SCDMA, WiMAX, WLAN and Bluetooth® wireless technology. The 3000 Series modules harness this speed by incorporating List Mode and Fast Sequence Tuning features to accelerate hardware setup times. Implementation of application-specific multi-threaded algorithms enables the concurrent test of multiple devices transferring speed advantages to the production line.

The Future

Key new PXI infrastructure products provide high speed and robust capabilities to the PXI community. Taking advantage of PCI Express Gen2 signaling and protocol capabilities, the new chassis and remote cabling solutions will allow up to four times the currently available data transfer rates for modules as well as remote hosts via cabled PCI Express. Also included with these infrastructure innovations are advances in backplane architecture allowing efficient and transparent peer-to-peer communication across the entire chassis. This transparency enables the emerging technologies of direct module-to-module communication allowing multiple modules to quickly and efficiently communicate without the overhead of software intervention. With underlying innovations to the modular framework, module designers and test engineers alike are now able to create even more powerful test systems than ever before.

Agilent’s introduction of 48 new instruments in September of 2010 reflects how well suited PXI module development is to the test and measurement products, form-factors and software strategies already in place. Along with other instrumentation platforms such as rack-and-stack, test engineers will have available a widening variety of instrument modules to address a variety of test and measurement needs.

One of those other platforms is AXIe (for more information, see www.axiestandard.org), which can be thought of as a “big brother” to PXI. This new test and measurement standard enables modular applications that require more board space and power than PXI. AXIe modules provide about six times more of each attribute while retaining the identical programming model of PXI. In the future, the AXIe platform might even be a platform for expanding PXI functionality; the AXIe module space could allow for a carrier module with space for two PXI modules, further expanding the integration and interoperability of the PXI standard.

Looking forward, the industry will see widespread adoption of PXI as the standard for automated RF measurement systems. In addition, future products from PXI vendors will continue to introduce measurement performance improvements in microwave RF test equipment, especially at higher frequencies. Finally, the next two to three years are likely to bring user-programmable FPGA technology to the forefront of instrument design, with RF signal analyzers using FPGAs to perform an increasing number of computationally-intensive measurements—like EVM—in essentially real-time.

In summary, test instrumentation based on open modular standards such as PXI and AXIe provides instrument designers with a vehicle to deliver new measurement technologies more rapidly, with a broad selection of test tools enabling customer specific test scenarios. Test engineers’ unique measurement needs will be met faster. The software will continue to advance over time to allow test programmers freedom of choice for their programming environments.