Evolution of the PC-Based, Mixed-Signal Test System

Today’s engineers face a growing number of testing challenges, whether it is during a product’s design phase or a high-speed measurement in production. More and more devices contain mixed-signal, or hybrid, components. This added complexity means that traditional test equipment and methods may no longer be cost and time efficient.

In the mid-1990’s the term “mixed-signal” described an integrated circuit that combined analog and digital circuitry onto a single device. While this general meaning of integrated analog and digital circuitry is still the heart of any mixed-signal system, from a design and test standpoint the term now encompasses a lot more. With the advent of wireless technologies and higher speed signals, a mixed-signal system today often includes analog, digital, and RF signals ranging across all frequency and amplitude ranges. Along with being able to interface with these signals, an engineer must also include power, switching, and standard digital multimeter measurements, such as resistance and capacitance.

It is no longer just the semiconductor manufacturers of ADCs and DACs who are faced with the challenges of testing complex mixed-signal devices. Whether you are an engineer for a semiconductor manufacturer, a designer for consumer electronic devices, or a system-level integrator, you are dealing with designs and tests that incorporate a mix of signal types and frequencies.

In this paper we look at what these common challenges are facing engineers across all industries. As a result of these challenges, we look at how the test and measurement industry has responded to these challenges by leveraging existing PC technology to create flexible, high-performance automated test systems.

Challenges of Mixed-Signal Testing

Whether you are talking about testing a cell phone or the electrical system in an automobile, the functionality in these devices has increased rapidly over the past several years. Along with growing device complexity, engineers are challenged to shorten their development time and reduce the cost of test.

The first challenge, increased device complexity, stems from the convergence of numerous technologies, such as new audio, video, and wireless components. A great example of this is the cell phone. If you look at a cell from just a few years ago, they offered none of the standard features found on today’s phones, like mp3 capabilities, wireless internet, streaming video, and color cameras. With all of these features packed into smaller devices, engineers must develop complex test systems for a variety of signals that can accurately correlate the data.

The need for engineers to shorten their development time is a result of growing worldwide competition and the importance of being first to market with the latest technologies. With the proliferation of the web an increasing number of companies can compete successfully in today’s market. Engineers must reduce their development time to accelerate a products lifecycle and get it to market sooner. This reduction in time must be done without increasing the cost of test and, often times, with fewer engineering resources. In order to shorten their time, engineers are challenged to find ways to more quickly incorporate new test equipment and to leverage existing work and test equipment.

The final challenge facing design and test engineers is the need to reduce the cost of tests. As devices become more complex, this naturally means that there is more functionality that must be verified. This requires a more complex and accurate test system; however, the price of the device under test is not being increased to account for the added complexity.

This constant price comes from the fact that it is has become cheaper to manufacture silicon. Figure 1 depicts data from the SIA (Semiconductor Industry Association) which shows that the cost per transistor has steadily decreased over the past two decades. However, the cost to test each transistor over the same period has actually been increasing gradually. Some industries have actually begun to see the cost of testing exceed the physical cost of a device.

As a result of these trends, engineers need to find ways to increase the throughput of their test systems, reduce maintenance costs, and explore lower-cost solutions.

Historical Choices for Mixed-Signal Tests

Until recently, only two main options existed to help engineers minimize these challenges of testing mixed-signal devices: disparate benchtop instruments or expensive proprietary systems. The first option involved traditional analog instrumentation, such as oscilloscopes and signal generators, and standalone digital instrumentation, such as logic analyzers and pattern generators. As circuits and systems began to include an equal mix of analog and digital components, some test vendors responded with a mixed-signal oscilloscope, which added limited digital functionality to an analog instrumentation. While these instruments are suited for some measurements, they have several limitations when it comes to developing a test system. The first limiting factor of a standalone instrument is that it is vendor defined, meaning that all aspects of the instrumentation, such as the analysis capabilities, software, user interface, and controls, are predefined. The same interface and features must be used by the engineer working in a design lab and by the technician on the production floor.

Secondly, a complete test system today almost always contains multiple instruments. As the device under test grows in complexity, more and more instruments must be integrated into the test system. If the system is based on standalone instruments, it often difficult, if not impossible, to accurately synchronize the instruments together and share the necessary signals between the various disparate instruments. No standard exists for synchronizing and correlating the measurements from one vendor’s traditional instrument to another. The second option that has been available to solve mixed-signal testing involves using large, proprietary systems. These complex testers tightly integrate a wide range of functionality together through advanced synchronization techniques. They provide an accurate solution for all types of mixed-signal tests; however, these instruments are complex and prohibitively expensive, which severely limits their use in any environment other than high-volume manufacturing.

The propriety nature of these instruments also makes them difficult to customize and scale. Because you rely on a single vendor to provide all support and upgrades, if that vendor drops support or does not offer the features you need, then you are often left purchasing a new system to address a new test. As a result of the large gap between these two options, engineers have turned to a new, affordable solution to address their challenges.

Introduction of the Modular, PC-Based Platform

To address this gap, test and measurement companies have shifted their focus to a modular, PC-based platform. This new approach combines the key features of disparate instruments and those of proprietary systems with the standard PC to create a flexible, scalable system with tight integration and high performance. Platforms, such as VXI, have historically offered some of these benefits; however, a new platform was needed to address a wider audience then the military and aerospace industry that adopted VXI. In response to VXI and the growth of the PC plug-in instruments in the early 90’s, the CompactPCI form factor was introduced as a rugged, modular architecture. In 1997, this architect paved the way for what has been rapidly adopted by engineers for automated test applications from the design phases to final production tests: PXI.

PXI, or PCI eXtensions for Instrumentation, is an open specification governed by the PXI Systems Alliance (PXISA) that defines a rugged, high-performance platform optimized for test, measurement, and control. PXI combines PCI electrical-bus features with the rugged, modular packaging of CompactPCI and adds specialized synchronization buses. These systems have been adopted in applications such as manufacturing test, military and aerospace, machine monitoring, automotive, and industrial test.

PXI systems are comprised of three basic components – chassis (which includes the PXI backplane), controller, and peripheral modules. The chassis provides the rugged packaging for the system and contains the PXI backplane, which includes the PCI bus and timing and synchronization buses. All PXI systems include a controller, which controls and communicates with the instruments. Controller options include remote control from standard desktop and laptop PCs, 1U rackmount controllers, and high-performance embedded controllers. Embedded controllers are built with normal PC components and run the same operating systems as most desktops, Microsoft Windows or Linux, and the same applications, such as Microsoft Excel and Internet Explorer. The peripheral slots in the chassis accept over 1,200 different modules available from the 70+ members of the PXISA. Modules are available for instrumentation, data acquisition, control, interfacing to buses, image acquisition, and more.

Over the past decade, the availability of PCI, and now PCI Express, instruments has seen continuous growth. Engineers have adopted the PXI platform for all types of test and measurement applications. This quick adoption has led to industry experts predicting 25% growth in the number of PXI systems through 2011. Engineers have used the benefits of PXI to decrease system cost and development time, to improve system bandwidth and latency, and to scale test systems as needed. The following section discusses what a modular, PC-based test system offers to engineers in more detail.

Features and Benefits

Feature	Benefits
Scalable platform	Expand the types and number of instruments in your test system based on the tests required now and in the future. Dynamically adjust the number of test channels available.
Versatile measurements	Develop complex test systems ranging from high-resolution DC measurements to high-speed RF applications.
Tight synchronization	Accurately synchronize and correlate measurements between digital, analog, and RF instruments. Achieve picosecond level synchronization across modules.
User-defined measurements	Create custom measurement and analysis functions to meet all the needs of your application; no need to export data to another system to be processed and presented.
High throughput	Utilize the PCI or PCI Express bus, which offer the highest-throughput and lowest-latency, to acquire, store, and transfer data quickly.
Measurement accuracy	Leverage some of the industry’s highest performance instruments to confidently measure and analyze your unit under test.
Preserve existing investment	Reuse existing test equipment, such as VXI, LAN, and GPIB instruments, to preserve past investment in test equipment and to improve performance by incorporating the latest PC-based instruments.

Benefits of a Modular, PC-Based Test Platform

Decrease Test Time
Increasing yield by lowering the time for test is one of the key challenges facing engineers. A PC-based test platform leverages the fastest commercial processors and application development environments. As companies, such as Intel and AMD, continue to improve desktop performance, then the speed of test systems, like PXI, scale linearly with these advances. No change to the instrumentation hardware or software is required. By standardizing a test system on the PC I/O bus, engineers receive the advantages of the highest-bandwidth, lowest-latency bus. Bandwidth is the rate (MB/s) at which data is sent across the bus. A high bandwidth bus means that data can be transferred between instruments and to and from the host processor significantly faster then through buses, such as LAN, GPIB, and USB, which can drastically reduce the test time per device. This is particular critical for today’s tests where large amounts of digital and analog data must be acquired, correlated, and analyzed for mixed-signal designs.

Latency, which is the inherent delay in data transmission across the bus, is often overlooked when examining a test system’s performance. A bus with low latency introduces less of a delay between the time data is transmitted on one end and processed on the other end. Latency is a critical specifications for applications that involve passing signals between instruments, often referred to as handshaking, repeatedly, such as in applications with digital multimeters (DMM) and switches.


Figure 3 illustrates the bandwidth and latency performance of the most common instrument bus options. The Y-axis shows increasing bandwidth from bottom to top, and the X-axis reveals decreasing or improving latency from left to right. The higher-performance bus in terms of both bandwidth and latency, such as PCI and PCI Express, are plotted in the top, right corner.

Along with higher performance hardware, engineers can decrease test time by increasing the amount of automation in a test system through software. Application development environments, such as National Instruments LabVIEW, are designed to help engineers quickly and easily design, prototype, and deploy test systems. Test management software, like NI TestStand, provides a ready-to-run test management environment for automating tests and validation systems. Rob Walker, the Xbox Accessories Development Manger at Microsoft, recently commented on how PC-based software and hardware can be used to effectively in high-volume manufacturing areas to decrease test time:

“National Instruments hardware and software give us both the performance and flexibility that is critical in meeting our time-to-market demands for the upcoming Xbox 360. With an industry-standard platform based on Microsoft Windows, PXI and NI LabVIEW, we developed a high-performance, low-cost test solution that resulted in a 50 percent reduction in our overall test time.”

Measurement Accuracy
Along with the latency and bandwidth of your test system, the instrumentation is equally important. The two critical components to an accurate measurement system include high performance instrumentation and tight correlation between instruments. PXI instrument vendors develop some of the industry’s highest performance instruments by utilizing the latest commercially available technology, such as FPGAs and the PC I/O bus. Much of the engineering that goes into traditional standalone instrumentation involves designing in and testing the processor, memory, communication buses, and cooling components. Traditional instrument vendors must deal with these issues for each new instrument, while PC-based instrument vendors leave those problems up to computer manufacturers or the PXI Systems Alliance. Many of the industry’s highest performance instrumentation are available on PXI and include the:

• Highest resolution digitizer (NI PXI-5922, Test & Measurement World 2006 Product of the Year)
• Highest performance arbitrary waveform generator (Agilent 6030A)
• Fastest, most accurate 7½ Digit DMM (NI PXI-4071)
• Highest channel count and best synchronization (up to 5,000 dynamic channels)
• Largest matrix density switch (512 cross-points in a single 3U slot)

To achieve a high level of total system accuracy, there is an equally important need to correlate measurements from one instrument to another. By definition mixed-signal test systems require the use of at least two different instruments. The goal of synchronization is to be able to generate and receive data precisely among multiple instruments. Traditional methods for synchronization involve sharing triggers and clock through external wiring, which means that engineers have to give consideration to cable lengths, impedance matching, and matched length cabling.

The backplane of the PXI system handles the synchronization and timing details automatically. The backplane not only routes signals between instruments across a high-performance PC bus, but it also provides specific timing and synchronization signals among slots. These shared signals include a trigger bus, reference clock, and advanced trigger. The synchronization methods on PXI range from the simple sharing of triggers to using phase-lock loops and automatically accounting for propagation delays to achieve picosecond-level synchronization.

Flexible, Modular Instrumentation
A modular test platform is necessary to scale with the demands of today’s complicated devices. A PC-based system enables engineers to add and swap instruments when needed and to reuse instruments from one test system to another. PC-based instruments are software-configurable, which allows the same test system to be used for characterizing digital-to-analog converters one hour and then for analyzing a DVD player the next. This flexibility helps engineers reduce costs by minimizing the number of test stations used. A general purpose, high performance test system can be used as a replacement for several device specific testers. Today, over 1200 PXI products are available from over 70 different vendors. This means that if your future designs require vision inspection, motion control, or a custom bus interface, you can reuse an existing PC-based test platform and add the needed functionality. Complex, tightly integrated test systems, which historically would have been challenging, if not impossible, to develop with separate traditional instruments, can now be created. The standardized timing and synchronization methods of PXI mean that different vendor instruments share the same timing and triggering signals.

Engineers can choose instruments from multiple vendors so that they can leverage each company’s strength. A automated test system can be developed that contains the industry’s best instruments, from analog to digital to RF, while still offering a standard method for synchronization in a single, integrated system. This provides the same synchronization accuracy as that available in tightly integrated proprietary systems without sacrificing the modular, flexible nature of standalone instruments.

User-Defined Measurements through Virtual Instrumentation
Critical to any measurement and automation system is not just the hardware, but also the software, which includes the analysis, control, and presentation aspects of any PC-based instrument. With the PC, standard development environments, such as NI LabVIEW, C/C++, or .NET, are used to fully customize the automated test system. Virtual instrumentation, an approach pioneered by National Instruments, combines software and hardware to build user-defined solutions. Standard PCs and off-the-shelf hardware components are combined to create custom “virtual instruments” for measurement and automation.

To help understand virtual instrumentation, look at a traditional instrument that includes a limited number of vendor-defined measurements. For a box oscilloscope, for example, about 19-20 measurements are included. While these are often the most common measurements, most customers always seem to have a few measurements they need to perform that are difficult or impossible with a vendor-defined box. With a virtual instrumentation approach, you can pass the raw data acquired with the measurement hardware through any number of included functions in software, or through a custom function or algorithm, such as a limit mask or THD (total harmonic distortion) measurement. Virtual instrumentation provides an infinite number of measurements. The user interface of a PC-based instrument, which includes the controls and indicators, is completely defined by the user. The user interface objects available are designed specifically for measurement applications and include the standard controls found on traditional instruments, such as knobs, buttons, analog graphs, and switches. User can also add custom controls and indicators.

Reduce Development Time and Costs
A PC-based approach also addresses the need to decrease the development time of a mixed-signal test system. Graphical programming allows engineers to perform rapid application development using interactive and programmatic tools. Applications ranging from sophisticated benchtop measurements to complicated control algorithms can be developed without the complexity or programming knowledge required by traditional development tools. The PC-centric approach provides a familiar environment for this development.

Along with decreasing development times, an open software platform ties the work done in design with the actual tests. Engineers who develop test conditions and test patterns for their simulation packages do not need to replicate that work when it is time to test hardware. The data can be quickly imported into the PXI system.As devices reach production level testing, the mixed-signal requirements can be addressed by the same hardware and software platform used in the design stages. You no longer need to develop tests for different test systems at each stage in your products lifecycle. For example, by standardizing on an open software and hardware platform, you have the option, for example, to leverage the test code written in the verification and validation (V&V) stage for the final functional test of a product, drastically reducing your development time along with reducing overall maintenance and training costs.

Preserve Existing Investment with Hybrid Systems
Longevity of individual instruments and of the overall test system is a major concern for engineers and, when not taken into consideration, can lead to higher costs and less productivity over the long term. A well-defined system architecture provides for the easy expansion and reuse of existing hardware and software.

A hybrid system, as shown in Figure 4, combines a master PC-based platform with peripheral devices. This allows for the integration of newer, PC technologies with older hardware and software. System components are easily upgradeable and existing hardware investment is maximized. Peripheral devices that use common interfaces, such as Gigabit Ethernet (LAN), USB, GPIB, and RS-232, can be integrated with a PXI system. Hybrid systems address two of the core challenges to mixed-signal tests: increased device complexity and lower test cost. By preserving existing investment, you can help lower the initial cost of developing a new test system. The ability to interface to any bus means that, if your test system has a requirement that cannot be met by existing PXI instruments, you can easily incorporate other instruments without having to redesign the whole system.

Conclusion

Flexible and powerful mixed-signal tests are becoming more critical to design and test engineers as device complexity increases and the pressure to reduce development time and test cost continue to increase. The modular, high-performance capabilities of PXI make it a viable solution to address the challenges of testing mixed-signal devices. The use of PXI can already be found in a wide range of applications and industries spanning from consumer electronics to military & aerospace to medical device testing. The low cost, breath of functionality, and user-defined flexibility is the reason why PXI is seeing one of the widest adoption rates of any instrumentation platform. Along with the hardware, an open software platform, such as NI LabVIEW, helps boost automation, simplify development, and increase efficiency – allowing you to deliver quality products in less time and at a lower cost.

Relevant NI Products and Whitepapers

National Instruments, a leader in automated test, is committed to providing the hardware and software products engineers need to create these next generation test systems.

Software:

• NI TestStand Test Management Framework
• LabVIEW Graphical Programming Environment
• Signal Express Interactive Measurement Software

Hardware:

• Modular Instruments (Oscilloscopes, Multimeters, RF, Switching, and more)
• Multi-function Data Acquisition
• PXI System Components (Chassis and Controllers)
• Instrument Control (GPIB, USB, and LAN)

Whitepapers
NI offers a Designing Next Generation Test Systems Developers Guide. This guide is collection of whitepapers designed to help you develop test systems that lower your cost, increase your test throughput, and can scale with future requirements. To read the entire developers guide, you can: Download the PDF (90+ page) version or view the web-version of the Designing Next Generation Test Systems Developers Guide.

Reader Comments | Submit a comment »