I can still remember my first PC. It was an Intel x286, and it came fully equipped with a 40-Mbyte hard drive, 4 Mbytes of RAM, and host of computer games. Nothing could beat a game of Pac-Man, Gapper, Oregon Trail (if you know what that is, you’re a nerd), or even Outnumbered (if you know what that is, you’re a serious nerd) on a rainy afternoon.

While I loved my first PC, you’d be hard-pressed to find me booting it up again—save for memory’s sake. Why? Because the x286 CPU might have been state-of-the-art at the time, but it’s simply not fast enough to handle the hosts of things that I use my computer for today.

Is it possible that the same can be true of an older RF instrument? If you wouldn’t use a PC that’s 15 years old, why would you use an instrument that is just as old on the production line for a wireless transceiver? Is it ever possible for a test instrument to be outdated?

The Last Decade

To be fair, it’s likely that a circa-1990 VSA may be just as capable of measurements that are accurate enough for production test. However, using it as such might cause you to miss some of the biggest innovations the instrumentation world has seen over the last decade. In fact, at least a few changes observed in today’s RF vector signal analyzers are clear indicators of an industry-wide trend toward lowering measurement speed.

First, we’ve seen an industry-wide transition from YIG-based (yttrium iron garnet) LOs to the faster-tuning VCO. Second, more and more vendors are building instruments with "fast" measurement options for standards such as W-CDMA. For VSAs using digital IF, measurements such as ACLR can actually be performed significantly faster when computed in the time domain. Finally, the past decade has also seen the emergence of PXI software-defined instrumentation, also known as virtual instrumentation, in automated test applications.

In the last five years, virtual instrumentation has become a widely accepted approach to making common RF measurements. While few designers can question the inherent flexibility of virtual instruments, many have overlooked one of the biggest benefits—measurement speed. Now wait a minute. Is it possible that Windows-based CPUs can perform measurements faster than embedded DSPs?

While that might sound crazy, the results might surprise you. Anyone who has ever tried to optimize a VSA or spectrum analyzer for measurement speed will tell you that the biggest bottleneck is processing horsepower. Moreover, the arms race between Intel and AMD over the last 10 years has produced a generation of CPUs that are faster, more power-efficient, and more parallel than ever before.

On The Bench

This all sounds like some great marketing fluff, but can it really be true? Well, I decided that I would run a few experiments to see for myself, so I configured my NI PXIe-5663 RF vector signal analyzer to perform 1000 spectrum measurements (50-MHz span, 100-kHz RBW). Using a PXIe-8105 controller, which has an Intel T2500 Core 2 Duo at 2.0 GHz, my test system averaged 2.8 ms per measurement, which is pretty fast. I’m certain that your circa-1990s VSA can’t come close (100 ms or more).

If my theory is correct, simply swapping out the CPU should reduce measurement time, right? When I traded my PXIe-8105 for a PXIe-8106, which uses an Intel T7400 Core 2 Duo at 2.16 GHz, the exact same test took only 2.1 ms per measurement. Can it really be that easy? The answer is yes. With virtual instrumentation, a simple CPU change can reduce measurement time—in my case by 25%.

To be sure, traditional instruments are by no means going away. In fact, the same innovations have reduced measurement times in both traditional and virtual instruments alike. No matter your preference in instrument type, the industry trends driving faster measurement times should at least turn your head.

David A. Hall is a product marketing manager at National Instruments and is responsible for driving the growth of RF and wireless communications hardware and software. His particular subjects of expertise include digital signal processing and digital communications systems. He holds a bachelor’s of science degree with honors in computer engineering from Penn State University. He can be reached at david.hall@ni.com.

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