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RF instruments complement PCB ATE

Rick Nelson, Senior Technical Editor- August 1, 2001

As wireless and broadband applications pervade electronic products, PCBs are increasingly populated by microwave and high-speed digital and optical components. Those components can stretch the capabilities of traditional board-test strategies, requiring you to adapt your test equipment to handle the new products.

The types of tests you’ll need to perform on your wireless- or broadband-enabled boards and assemblies depend on product design as well as on the standards your product must meet. Test requirements for unlicensed, low-power Bluetooth capabilities might be fairly limited, focusing on transmit power and receiver sensitivity. Cellular communications capabilities will impose much tougher test requirements, mandating tests for such factors as adjacent-channel power measurements.

Whatever tests you employ, you’ll want to use as much of your current test equipment as possible. If your test line includes x-ray and optical inspection equipment, in-circuit test (ICT) systems, and functional board testers, you should be able to adapt it for high-speed products.

Inspection equipment can be the easiest to adapt, because it doesn’t care about the operating frequencies of the parts being inspected. X-ray equipment can readily inspect the solder joints of RF components soldered to your boards, and optical inspection equipment can look for correct part numbers and proper part alignment.

But you may want to tighten the tolerances on your inspection systems. A component misalignment that may have no affect on low-frequency circuit performance might prove to be unacceptable at high frequencies—a situation you would want to identify well before final functional test, when the product is already encased in a plastic shell, making the misaligned component unreachable and irreparable.

In-circuit-test limits

In-circuit test can be more difficult to use for high-frequency boards. Nevertheless, ICT remains a valuable part of the manufacturing process. For instance, if you’re adding wireless-communications capability to an existing computer peripheral board, you could continue employing in-circuit test to the low-frequency nodes, deferring RF tests until final functional test. Of course, delaying the RF tests could ultimately increase costs, as you would not identify faulty RF components until late in the production process. That risk is manageable, however, if your RF circuitry includes only a few components, and you can count on ICT to detect faults among the many baseband components on your board.

A single-chip Bluetooth radio, for example, might not be amenable to in-circuit test on your existing ATE systems, but the many other, low-frequency, components will be. You can assume the Bluetooth chip itself has been tested and screened to whatever defects-per-million quality level your company has agreed to. (An article in the September 2001 issue will discuss Bluetooth testing—from chip to final product—in more detail.) Inspection stages before in-circuit test can ensure the proper Bluetooth part is installed and properly soldered, while final test can verify the basic functionality of the chip. It’s unlikely that the manufacturing process would have altered the device’s performance with respect to factors such as protocol. Consequently, you can defer a single quick test of the Bluetooth device until final functional test while applying ICT to the many low-frequency devices on your board to catch the vast majority of manufacturing defects in a timely manner.

In-circuit test can remain valuable, too, in multiple-component implementations of more demanding wireless technologies, ranging from cellular technologies to wireless LAN implementations. Here, in-circuit test can perform simple electrical tests on components such as inductors; once again, RF tests can be deferred to final functional test while ICT detects faults in low-frequency devices.

Adding RF boxes

Figure 1. Benchtop instruments remain the mainstay to many RF test applications. Courtesy of Rohde & Schwarz.

Ultimately, you’ll need RF instrumentation (Figure 1) to test the full functionality of your boards, subassemblies, and completed products. One option is to add your own RF instrumentation to basic functional test systems, relying on standard instrument drivers from firms such as National Instruments to interface these instruments to your test environment. The Interchangeable Virtual Instrument (IVI) Foundation (www.ivifoundation.org) is developing standards to ensure compatibility among instrumentation of various classes; it began by defining requirements for low-frequency instruments such as DMMs and is working to develop standards for high-frequency instruments such as spectrum analyzers by year’s end.

In addition to choosing general-purpose instruments such as signal sources and spectrum analyzers, you can choose ones dedicated to a particular optical network standard or wireless communications protocol. Most such approaches involve IEEE 488 instruments, but firms including ZTEC are starting to offer RF instrumentation in CompactPCI format. ZTEC offers digitizers that target pulsed RF signal measurements plus modulation and spectral analysis.

If you’re looking for a turnkey functional test system, you can employ dedicated test systems tailored to your product. Several board-test ATE vendors such as Agilent Technologies, GenRad, and Teradyne have teamed up with RF instrument makers to offer turnkey test platforms. GenRad’s Versa Cell Phone tester is one such system. In an effort to help contract test houses meet customers’ requirements for specific RF instrumentation, GenRad will integrate its Versa Cell Phone system with instruments from firms including Agilent Technologies, Anritsu, IFR, Racal Instruments, and Rohde & Schwarz. GenRad also targets Versa at test of high-speed OC-192 optical network products and has teamed with Tektronix to integrate the latter firm’s OTS family of optical test instruments with the Versa functional-test platform.

Winning combination

Figure 2. Combinational testers combine bed-of-nails PCB access for in-circuit test with functional test capabilities. Courtesy of Teradyne.
Figure 3. Fixturing is a key aspect of high-speed board tests. Dual-stage fixture schemes can enable in-circuit tests while minimizing the adverse antenna effects that a bed-of-nails fixture can create. Courtesy of Agilent Technologies.

You can also chose alternatives that perform traditional in-circuit test as well as some functional test for your high-speed boards. Teradyne ( Figure 2) and other firms call these systems combinational testers. Agilent Technologies eschews the term “combinational test,” but nevertheless provides similar performance. These systems provide bed-of-nails access to low-frequency components and also establishes pathways to high-speed nodes for external instrumentation. Agilent Technology’s 3070 Performance Port, for example, lets you connect benchtop oscilloscopes, signal sources, and spectrum analyzers to boards being tested on one of the company’s 3070 Series in-circuit testers.

Fixturing is a critical component of the in-circuit test strategy for high-speed boards. The contacts of a bed-of-nails fixture can act as antennas, adversely distorting high-speed signals under investigation. To mitigate these adverse affects, you can employ a dual-stage fixture scheme—such as that employed in Agilent Technologies’ Quick Press fixtures (Figure 3). With the Quick Press approach, long probes engage to permit device-specific functional test. When these tests complete, the fixture compresses to bring short probes in contact with the board as well, permitting full-board ICT.

Including high-speed components on PCBs and subassemblies complicates manufacturing test, but emerging test products can help you adapt. As wireless and broadband functions creep into ever more products, you’ll find capable test hardware and software emerging to help you make short work of test chores. T&MW

Rick Nelson received a BSEE degree from Penn State University. He has experience designing electronic industrial-control systems. He served as the managing editor of EDN, and he became a senior technical editor at T&MW in 1998. E-mail: rnelson@tmworld.com.