Many EEs feel that test-and-measurement flexibility is best achieved with modular instruments, But, they're only one of many ways to reach the goal. Grant Drenkow, Solutions Planner at Agilent Technologies's Test and Measurement Group explains why he believes there are tradeoffs.
By Grant Drenkow, Solutions Planner, Agilent Technologies, Test and Measurement Group, Loveland, Colorado. E-mail: firstname.lastname@example.org. Phone: (970) 679-3128
A common belief is that test-and-measurement flexibility is best achieved with modular instruments, but they're only one of many ways to reach the goal. Let's examine the tradeoffs.
One of the most desired attributes in a test system is flexibility. You can save your company money by developing systems that can test more than one product. In R&D you look for instruments and systems that can add functionality and achieve higher performance to keep up with the testing of new technologies.
In production you want test systems that can easily change configurations to test new products coming down the manufacturing line.
A common belief today is flexibility is best achieved with modular instruments. In fact, modular instruments are only one of many ways to achieve flexibility. Let's look at the few ways to add flexibility, and see what the tradeoffs are with each approach.
Those Modular Instruments
Let's begin with modular instruments. When most people hear the word modular they think of physically modular. Products such as VXI and PXI come to mind. Their flexibility comes in the form of plug-in modules that can measure, source, and switch analog and digital signals.
Modular products come in two flavors: proprietary modular and open-standard modular. Open-standard modular formats such as VXI, VME, CompactPCI (cPCI), and PXI impose strict rules in terms of physical size, electrical connections, cooling, and EMI. That's done so that many companies can provide plug-in modules that can co-exist in one cardcage. They're extremely flexible because the products from many companies provide a lot more choices for you.
CompactPCI and PXI, for example, are open-standard formats with small plug-in modules (i.e. A/D and D/A converters, and switching) that work well for data acquisition applications. VXI (C-size in particular) has plug-in modules roughly four times larger, so it excels at high-speed, high frequency, and high-complexity applications such as those found in aerospace, telecom, and surveillance applications.
One company, on the other hand, produces proprietary modular products. They bring the advantage of being low cost. Such modular products are popular as switch/control units or DMM/switch (digital multimeter/switch) combinations that don't need expensive cardcages with complex backplanes, large power supplies, and sophisticated controllers.
Proprietary modular products are popular in manufacturing test applications where they offer a significant cost savings over open standard approaches.
Enter System-Ready Instruments
In my opinion, the most popular approach to measurement flexibility continues to be traditional GPIB (General Purpose Interface Bus IEEE-488) instruments, sometimes referred to as rack-and-stack, but more popularly called system-ready instruments these days.
The rack-and-stack designation comes from the fact they're stacked on R&D benches, and racked in manufacturing test systems. As these instruments transition away from GPIB interfaces to high-performance LAN (local area network) and USB (universal Serial Bus) interfaces, they're probably more accurately named system-ready instruments.
Price vs. Performance Leaders
System-ready instruments, used by R&D and manufacturing, are the price/performance leaders on the market. They pack a large number of measurements in a highly tuned instrument package with a front panel, at a price point below their equivalent in a modular format.
For example, a DMM can make DC voltage, DC current, AC voltage, AC current, resistance, frequency, and sometimes temperature measurements, often with just the push of a button. The mixed-signal oscilloscopes (MSO), such as Agilent's 54600 Series, combine a conventional (call them traditional) oscilloscope with a logic analyzer, forming a highly flexible tool for design engineers.
The combination permits designers to see how analog signals in an electronic device are responding to digital logic signals. Features such as Agilent's MegaZoom, where the display updates in realtime by turning a knob, are only possible on a highly tuned system-ready oscilloscope.
There are numerous additional examples. High-performance power supplies not only output precise voltages and current but they can contain voltmeters or digitizers to monitor output voltage or characterize current draw from the supply (in order to emulate batteries).
Similarly, network analyzers can contain both source and measurement functions, letting you analyze RF circuitry.
All these system-ready instruments exhibit flexibility that makes it possible to perform a multiple measurements from one instrument and cover the needs of multiple applications.
Another way to get test flexibility comes in the form of software-flexible instruments. These change their measurements or output signals via software personalities.
By downloading analysis routines and waveforms into such instruments, the instruments can be easily reconfigured to test a new technology. For example, Agilent's spectrum analyzers have personalities for testing Bluetooth, CDMA, GSM, GPRS, and cable TV.
RF signal sources might download CDMA2000, GSM, Bluetooth, and wireless LAN (WLAN) waveforms. This flexible software approach is quite effective for RF and microwave instruments, where purchasing separate instruments for each RF format can be too cost prohibitive.
The highly-tuned internal architectures of these instruments makes it possible to get precise triggering, superb shielding, and highly accurate RF and microwave calibration, things not possible in modular hardware formats. The ability to download new personalities to these instruments extends their lives through many technology "rolls," too.
Don't Forget PC-Based Products
What about PC-based products? A variation on software-flexible instruments, these include products such as PC plug-in cards or their cousins, PXI modules. These cards and modules contain simple A/D and D/A circuitry and high-speed links direct to a computer's backplane.
They don't have any built-in functionality and can't operate on their own without the PC, though. They rely totally on the PC to set-up a measurement, trigger the reading, analyze data, format data, and display final results.
These products achieve their flexibility through the PC software that analyzes the raw data coming back from the card or module. For example, a digitizer can send back data that the PC software can analyze to determine peak voltage, risetime, period, or frequency.
PC-based products are especially popular in mechanical test, vision capture, and other low-frequency applications.
So, where is modular best used? Modular instruments are quite cost-effective in data acquisition applications that require a wide variety of simple measurements and outputs. The plug-in modules in these instruments are typically A/D converters, D/As, digital I/O, and switches that make physical measurements such as temperature, pressure, and strain.
Proprietary modular instruments, such as the Agilent 34970A or the Keithley Instruments Model 2701, provide the lowest cost approaches, while open-standard modular instruments based on PXI and VXI provide the widest selection of plug-in modules.
The decision of going proprietary or open-standard generally therefore comes down to finding the proper mix of modules for your application. Low-cost modular solutions generally use standards such as GPIB or LAN, while the more expensive approaches often opt for MXI 2 or MXI 3 (Multisystem Extension Interface).
(The popular VXI architecture, introduced back in 1988, defined a modular instrumentation architecture where multiple instrument modules fit into a single standardized mainframe or chassis. With VXI came the introduction of a technology called the Multisystem Extension Interface, or MXI. It linked PCs to modular instrumentation at rates more than 20 times faster than the original IEEE-488 links. Because of its high performance and seamless operation for multi-mainframe expansion, MXI-2 has surpassed IEEE-488 as the most popular PC-to-VXI interface today.--Ed.)
Modular instruments are also popular as switches in electronic functional test applications. The ability to combine multiplexer, matrix, general purpose, and digital switches in one modular frame make it quite cost effective.
Proprietary switch approaches are generally more popular in electronic test because they don't need the added expense of large power supplies, high-speed backplanes, and MXI interfaces.
Not A Good Fit?
Where is modular not a good fit? Small modular instruments, popular in simple data acquisition applications, are very limiting in electronic test applications.
To perform all but the most basic measurements requires multiple modules in a PXI or cPCI format. The expense of multiple modules, printed circuit boards, connectors, faceplates, and backplane interface circuitry can cause cost to go up dramatically.
Performance is also usually compromised, because timing between modules is less precise, signal paths aren't always well defined, and additional programming is usually required to initialize and trigger each module.
If modules come from different companies, you must often be a "measurement expert" to get the modules working together. As a result, modular solutions are only a good fit where the modules are simple enough to work autonomously.
That brings us back to system-ready instruments, Where are system-ready instruments popular?
System-ready instruments are quite pervasive across all applications. In R&D they're popular because with the push of a button they can make an extremely sophisticated measurement, and you get to see the results on-screen without writing a program.
Multi-function instruments are popular choices for making measurements that would be time-consuming and extremely difficult if using individual instruments. Engineers who don't want to become measurement experts use system-ready instruments so they can take advantage of years of expertise built into the instruments.
In manufacturing---where you're usually not restricted by the size of a cardcage and modules, the output limits of the power supply, or the bandwidth of the backplane---system-ready instruments make it possible to mix simple, low-cost products with sophisticated, high-performance instruments in one system. Engineers get the benefit of finely tuned measurement hardware coupled to powerful measurement science software, providing accurate and calibrated results.
Engineers and technicians who want to reduce programming code often prefer system-ready instruments, where a single command can invoke measurements and get back a result.
System-ready instruments also help align R&D testing to manufacturing testing, as system-ready instruments can readily travel from the bench to design validation to the manufacturing test system.
So, where do software-flexible instruments fit? Software-flexible instruments are most commonly used in applications with emerging standards. As the standards change, the measurement algorithms and downloadable waveforms change. Upgrades of the downloadable software can be handled very efficiently via the Web.
Software-flexible instruments are also popular in manufacturing applications, where a change in the product being tested can be made very efficiently by downloading new measurement routines into software-flexible instruments. Since the instruments work independently and in parallel, the highest overall throughput is attained because the PC and the I/O paths from the instruments are no longer a bottleneck.
So what really makes a test system flexible? A test system is made flexible by the instruments in the system!
Modular instruments make it possible to mix a wide variety of simple measurements and switches, depending on the application requirements. Software-flexible instruments make it possible to keep up with technology changes by downloading new measurements and waveforms into a high-performance instrument.
In my opinion, the next generation of flexibility will most likely come in the form of system-ready instruments. Instead of achieving flexibility via modules in a cardcage, the test system of the future will be system-ready instruments run from a high-speed LAN-based backbone. Instruments will be free to go beyond the limitations of a cardcage with its plug-in modules, internal power supply, and backplane to world-class system-ready instruments that simply plug-and-play to a PC from anywhere in the world.
High throughput will be a combination of highly tuned instruments, software-flexible instruments, and low cost modular instruments---all operating on the same high-speed backbone.---eePC/AMM