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.
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.