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TEST & MEASUREMENT WORLD: PXI and bench instruments evenly matched

When two approaches to the same problem exist in a market, they are bound to compete at some level. For PXI and bench instruments, that competition began in the area of automated testing, where PXI’s PC-based control and modular nature gave it compelling advantages. But improvements in modular instrument technology, along with new software interfaces, are blurring distinctions between PXI and desktop instruments, making the choice of which approach to use more complex.

The introduction by Aeroflex, at Autotestcon 2007, of the 3000A PXI chassis for RF test highlighted this blurring of lines. The chassis includes a built-in touch-sensitive display panel that works with the system controller to turn a populated chassis into a selfcontained instrument. Instead of having the separate keyboard and monitor that make most PXI systems look like computers with an instrumentation peripheral, the Aeroflex chassis seems like a traditional bench instrument.

This type of operation addresses the need for programming that many engineers see as a barrier to using PXI on the R&D engineering bench. But that barrier is collapsing. Software packages from companies like ZTEC and National Instruments are now offering graphical user interfaces that mimic the look and operation of bench instruments.

Instead of requiring users to select from menus or create a program to control operational parameters when setting up and making measurements, these interfaces provide users with the opportunity to press buttons and turn knobs or at least perform the on-screen equivalent. The free ZTEC ZScope software package for the company’s PXI oscilloscope modules, for instance, presents users with the image of an oscilloscope front panel to provide both instrument control and data display.

INCREASING COMPETITION

By allowing users to operate a PXI system like a bench instrument, such interfaces bring the two test methodologies into more direct competition. “PXI has come a long way in the 10 years since its introduction,” said Richard McDonell, senior group manager for PXI and instrument control at National Instruments. “Early units had lower resolution [than bench instruments] and worked at only 100 ksamples/sec or so.” Now, McDonell pointed out, PXI can push the state of the art. At last year’s Autotestcon, NI, BAE Systems, and Phase Matrix demonstrated a jointly developed PXI platform that measures signals as high as 26.5 GHz.

This does not mean that both PXI and bench methodologies have achieved identical performance. “Benchtop instruments typically lead in high-end (28-bit) resolution or ultrahigh frequency ranges,” said McDonell. Further, resolution is only one aspect of performance in test systems. Chris van Woerkom, senior marketing engineer at Agilent Technologies, pointed out that metrics such as measurement throughput can also be important. He noted that many PXI instruments depend on the system’s CPU to turn raw data into meaningful measurements. In a well-populated system, then, the need to share bus bandwidth may limit an instrument’s achievable measurement throughput despite it having a high data rate. Bench instruments do not suffer from such limitations because they have built-in processing.

That kind of built-in processing is making its way into PXI instruments, though. “We’re now seeing PXI modules with processing being performed in the module,” said ZTEC’s director of marketing and product strategy Boyd Shaw. “In the last five years, modular oscilloscopes have gone from being just digitizers to having all their performance in the module. We now have the same kinds of signal conditioning, waveform analysis, and parameter-measurement algorithms as benchtop devices.”

NO CLEAR SUPERIORITY

The result is that from a performance standpoint, neither bench nor PXI instruments have a compelling claim to superiority. Nor is that situation likely to change. Individual products of one type or another will temporarily win the top performance slot, but that honor typically is now trading back and forth.

Further, consolidation within the test-instrument community is creating vendors that offer both PXI and bench products, ensuring that technical advances in one will inevitably make their way into the other.

Agilent, for example, has acquired PXI companies PXIT and Acqiris and is now offering some PXI and bench instruments made with identical boards, parts, software, and specifications. The only differences are the interface and the footprint. “We are differentiating ourselves with superior metrology,” said van Woerkom, “and letting our customers decide which package to purchase.”

This kind of commonality is also eroding another traditional difference between PXI and bench instruments: the ease of porting R&D test development to the production floor. In the end, a user’s choice of platform may simply boil down to preference. Older engineers, whose careers predate PXI and modular instrumentation, may feel more comfortable using buttons and knobs than mouse clicks and keystrokes. Younger engineers, having spent nearly their entire lives using personal computers, may feel precisely the opposite.

A longer version of this article appeared in TMW’s March 2008 issue. You can read it at www.tmworld.com/article/CA6534774.html.