PC-Based Instruments Meet Traditional Instruments

Lately, a lot of hype and publicity have been given to PC-based instruments, and in particular to PXI. Yet PC-based instruments account for less than 2% of the test and measurement market. Why?

Faster processors? PC-based instruments use Moore's Law, doubling performance every 18 months by improving processor speed. Faster processors improve analysis speed but provide no gains in the speed, accuracy, or resolution of the measurement hardware. Gaining the most improvement in system throughput requires a change in the measurement hardware, not the processor.

Faster I/O? PC-based instruments have faster I/O than GPIB because they take advantage of the PCI backplane or an MXI link running at higher speeds than GPIB. In reality, the majority of the time spent in a test system is spent waiting for power supplies to settle, switches to close, instruments to change functions, and measurements to complete. GPIB generally isn't the bottleneck.

Cheaper than VXI? Very often, PXI is considered a lower-cost alternative to VXI. This is true if you compare a VXI cardcage to a smaller PXI cardcage or a C-size VXI module to a smaller 3U PXI module. If you compare embedded controllers or MXI links, the costs are nearly the same. If you consider the amount of functionality per plug-in module or the number of switch channels per cardcage, the costs would also be similar.

The bigger question is how PC-based instruments fare against traditional instruments. PC-plug-ins will be the lowest-cost products, but they also have a fraction of the capability of traditional instruments. PXI plug-in cards have similar or more expensive prices with compromises in accuracy, resolution, and functionality. PXI also requires a cardcage and MXI link, which equals about the cost of three basic instruments. So while PXI may be cheaper than VXI, it is not less expensive than traditional instruments.

Higher performance? One Electronic Design article claimed a $1000 PC and a $500 plug-in card could replace a $20,000 instrument ("PC-Based Instruments Reshape The Test Landscape," Sept. 1, 2003). If the $20,000 instrument were used only to make a 16-bit voltage measurement, a PC-based instrument would be a better deal. But the card doesn't contain the circuitry and triggering needed to make real-time measurements and complex data reduction and analysis. It can't replace spectrum analyzers and oscilloscopes that have bandwidths well beyond those of PC-based instruments.

Do more in the PC? PC-based instruments, whether in the PC or in a PXI cage, rely on the PC for their data processing and decision making. This approach works well for simple voltage measurements, scanning through a few channels, making the measurements, and passing the data to the PC for analysis. It also works well for switch modules, digitizers, and D/As. However, the PXI approach can't emulate a traditional instrument, such as a scope, where you can change range, adjust the cursor, zoom in on data, and make complex measurements with the turn of a knob.

In electronic test applications, the "do more in the PC" approach gets much more difficult. Traditional instruments contain man-years of measurement science expertise. It would take a team of experienced programmers and measurement experts to develop a complex program, send strings of commands to various PXI or PCI modules, bring back megabytes of data, run the data through measurement algorithms and decimation routines, filter the data, and store and display the results.

Why PC-based instruments? PXI, like its big brother VXI, makes a fine small-size platform for data-acquisition applications with a variety of measurements. Like VXI, PXI may have a role as a platform for specialized applications, such as vision capture or motion control. But traditional instruments make more-complex measurements and maintain advantages in accuracy, resolution, and bandwidth. If history repeats itself, traditional instruments will dominate test and measurement applications while PC-based instruments will continue to augment them.