TEST FUNCTIONS CONTINUE TO GROW IN COMPLEXITY, such as those evaluating multiple-input multiple-output (MIMO) systems for technologies like Long Term Evolution (LTE) cellular networks. For RF and microwave engineers, however, it is essential that the equipment performing such tests be easy to understand and use. In today's cost-sensitive business environment, companies also are looking to invest conservatively in test equipment. To satisfy their needs for increased test performance and make sure they are not paying for more than they need, engineers are increasingly opting for a modular measurement approach. Compared to traditional "rack and stack" test gear, modular instruments are relying on standards like AXIe, PCI, and VPX to add function cards to a mainframe. Th e modular approach also provides a cost-e ective, smooth upgrade path as engineers look to satisfy future design requirements.

Test and measurement makers point to a variety of reasons for the rise in the demand for modular approaches. According to Donn Mulder, VP/General Manager of Anritsu Co.'s Microwave Measurement Division, "Two key reasons modular test instruments are gaining market share are the continual advancement of wireless technologies and the desire for companies to control the cost of test. The proliferation of wireless standards and the continual need for additional test requirements due to these standards, as well as other reasons, makes it imperative that a test instrument has the flexibility to expand analysis capability as standards evolve." Mulder emphasizes that in today's economy, designers and developers of wireless products and systems need to control the cost of test. In his words, "A modular instrument allows engineers to cost efficiently add a new board to their existing instrument to perform new tests rather than purchase a new instrument every time a standard changes or a new series of tests needs to be made."

Hiren Joshi, Product Manager at Aeroflex Test Solutions, points to additional benefits of modular design. For example, a modular approach leads to the faster development of instruments that meet market needs. As a result, test and measurement companies can focus on individual module development with faster time-tomarket to respond to the market's quickly shifting needs. Modular design also lowers capital cost of ownership while protecting the investment, as upgrades can be ordered as needed. Mike Millhaem, RF and Microwave Applications Engineer for Agilent's Modular Products Operation, emphasizes that modular products typically provide advantages in smaller footprint, lower cost, and faster execution speed. According to Millhaem, "The smaller footprint and lower costs come from allowing multiple measurement functions to coexist in one chassis. For example, a PXI mainframe has 18 card slots in a 4U-tall, 19-in. chassis (see sidebar, "Standards Befit Modular Approaches). The Agilent PXI microwave vector signal analyzer (VSA) uses seven to eight slots, depending on configuration (Fig. 1). Many moderately complex test systems can be developed in a single chassis. The PCI Express interface to the PXI and AXIe chassis allows for fast data transfer between modules and from the modules to the host PC, which can reduce test time. Specifically in the area of RF and microwave instruments, I think that flexibility is also an advantage for modular products. For example, a customer could purchase a local oscillator module and a digitizer module and use this with a downconverter that they developed internally."

Of course, the rise in modular instrumentation could not have come about without the evolution of RF components. As noted by David Hall, Product Manager for RF and Wireless Test at National Instruments, "The timing of the trend toward modular instrumentation is driven by customer demand and enabled by the miniaturization of high-performance off-the-shelf RF components. RF instruments have historically been known for being extremely largepartly from their use of custom application-specific integrated circuits (ASICs). However, today's modular RF instruments are smaller than ever before. One notable example in PXI is the NI PXIe-5630, which is a two-port, 6-GHz vector network analyzer (VNA). While VNAs were once known for being the size of an old meat slicer, this two-slot module is about the size of a large deli sandwich (Fig. 2)."

POTENTIAL PROBLEMS AND SOLUTIONS
As much as modular approaches can be heralded for their benefits, however, some obvious issues can arise in their implementation. Most obvious is the fact that all of the different modules have to work together seamlessly. Aeroflex's Joshi cautions simply, "The integration issues of the modules may affect performance."

Agilent's Millhaem explains that some challenges need to be overcome to allow customers to effectively use modular products for complex functions, such as a spectrum or signal analyzer. He notes, "A typical instrument consists of five major functions two in hardware and three in software. In hardware, you have signal-generation/acquisition hardware and data-processing hardware. In software, you have hardware control software, measurement or analysis software, and a user interface. When a customer purchases a box instrument, the instrument manufacturer has done all of the work to integrate these different sections together, providing the customer with a complete solution. Box instruments, such as signal/ spectrum analyzers, also provide built-in measurements like error vector magnitude (EVM), adjacent channel power (ACP), and spectral emissions mask (SEM). They can be controlled remotely via Standard Commands for Programmable Instruments (SCPI) code, allowing the user to quickly develop remote applications in an automated test environment. The built-in display means the instrument can be operated from the front panel on a benchtop or even as a diagnostic tool in an automated test environment. A myriad of measurement applications are normally available for box instruments to quickly enable additional functionality for specific applications like noise figure, phase noise, pulse, and standard-specific applications like LTE. When a customer purchases modules or a modular instrument, they may not get the same level of integration."

Of course, some engineers do not mind completing this integration themselves. Yet Millhaem knows of several instances in which people have purchased modular RF products but could not use them either due to missing measurement software, difficulty in integrating them into their test environment, or underestimating the scope of the integration tasks. Of course, test and measurement makers are working on providing such integration. With the M9392A, for example, each module has an application programming interface (API) that allows the user to program every setting, such as switch positions and attenuator values. In addition, an "instrument layer" API allows the customer to define the setup in terms of the measurement parameters, such as center frequency, bandwidth, and input power. The interface is therefore similar to traditional box instruments. The instrument layer software programs the module-specific settings of parameters, such as the local-oscillator (LO) frequency and individual attenuator settings. Agilent also included support for its 89600 vector-signal- analyzer (VSA) software, giving customers the same measurement software in the modular signal analyzer as they get with the box instruments.

Although modular instruments are already being used for validation and production testing, the question is whether they will be able to satisfy the demands of R&D laboratories. David Asquith, another Aeroflex Product Manager, predicts, "Boxed instruments will continue to provide the instant control, flexibility, and simplicity provided by a custom-made user interface. This need not be seen as a barrier between R&D and manufacturing, as boxed instruments will employ the modular instrument(s) internally and share much common driver software, thus easing the migration from the laboratory to the factory."

In fact, the firm recently debuted a button-free instrument that plans to serve engineers from the field to the laboratory or production line. The SGA can serve as a general-purpose signal generator in addition to delivering critical receiver measurements or targeting manufacturing. It employs an 8.5-in. touchscreen user interface (Fig. 3). The SGA-3 covers 100 kHz to 3 GHz while the SGA-6 spans 100 kHz to 6 GHz. It delivers an output of +13 dBm. A +20-dBm option is available as well. The SGA boasts single-sideband phase noise of -135 dBc/Hz at 1 GHz at 20 kHz offset from the carrier. It settles in 100 s. A synthesizer input/output is provided for the phase locking of multiple signal generators. Because the SGA uses PXI modularity inside the chassis, it can easily be upgraded with new features and capabilities.

Over the next couple of years, test and equipment manufacturers will continue to invest in the modular-instrument space. It looks like PXI will most likely be the standard for automating microwave and RF measurements. Tim Carey, Aeroflex Product Manager, predicts that the distinction between the performance of modular and boxed RF instruments performance will largely disappear. He notes, "There is fundamentally no reason why modularity itself should compromise RF performance or functionality. The constraints imposed by working within a specific modular standard are a challenge for design engineers and not a difficult restriction. Modularity is itself not a distinguishing factor, as boxed instruments are invariably modular in their design. The issue is more about how open' the product's hardware and software architecture is, and whether it is described as a module' or an instrument.' In this area, I think we can expect some convergence. Modular components will remain open. But where modular components are combined to form an instrument function, some lower- level detail of individual modules becomes hidden. For boxed instruments, convergence is from the other direction. Increasingly lower-level functionality is exposed to the user. This occurs already with some VSA-type instruments, which can internally process measurements but can also export raw unprocessed data for external manipulation."