Check electronics success with RF/microwave test
As RF/microwave designs grow in popularity, so does the importance of validating and verifying their implementations.
Greg Reed, Contributing Editor -- Test & Measurement World, 5/12/2008 9:23:00 AM
As more and more wireless capabilities are added to various consumer products, including automotive systems, RF/microwave testing gains increased importance. Manufacturers of cellular, Bluetooth, and WLAN devices need to ensure their products meet customer demands and also conform to the governing standards.
"Quality wireless products strongly affect the end-user experience, as is evident in both cellular phone commercials and in our collective experience," said Ken Harvey, product line architecture manager at Teradyne. "Thus, semiconductor component quality is essential to ensure continued high-value product delivery and avoidance of product-yield problems."
Consumers use numerous wireless devices every day: cellphones, PCs, earbuds, garage door openers, key fobs, and RFID card readers. "Only one of these examples—the cellphone—uses a licensed band," said Darren McCarthy, marketing development manager of RF testing at Tektronix. "Most others use the ISM band. Digital RF implementations are necessary in the ISM bands to avoid device collision and interoperability."
Additionally, high-speed serial chips, printed-circuit boards (PCBs), and backplanes need to be tested with RF/microwave tools to maintain signal integrity. "PCBs that will be used with high-frequency signals are often tested with a vector network analyzer [VNA] to verify that the impedance is correct—a manufacturing process issue," said Eric Hakanson, product marketing manager of the microwave measurement division of Anritsu. "This also confirms there are not ‘nulls' in the frequency response caused by reflections—a design issue. Nulls can cause signals to ‘disappear' on PCBs, which clearly would make circuits not work."
Components and related subsystems are often tested with RF and microwave test equipment to verify not only impedance, but also signal amplitude, distortion, and jitter (phase noise). Examples include capacitors, resistors, inductors, transistors, amplifiers, cables, connectors, filters, mixers, frequency converters, modulators, demodulators, attenuators, switches, couplers, isolators, digital device packages, and high-speed optical components.
Components that do not handle high data rates, high-speed signals, or RF/microwave signals, however, do not always need to be tested with RF/microwave test equipment. Such test equipment is generally more expensive that test tools like oscilloscopes and digital multimeters. "A power-supply capacitor is typically tested only to a few hundred kHz," said David Ballo, product manager of the components test division at Agilent Technologies. "Garden-variety op-amps usually only require testing to a few hundred MHz. PCBs that handle low-data rate signals (like an RS-232 link) usually are not tested beyond several hundred MHz."
Testing production and design verification
A typical test program for a cellular transceiver will have many hundreds of tests in production and thousands of tests during design verification and characterization. Although the tests will vary from manufacturer to manufacturer, they are commonly related to the specific fault coverage needs of a given RFIC design and to the specification conformance needs of the end customer.
More than 1 billion RF transceivers for cellular handsets were tested last year, and Bluetooth is projected to reach that volume in 2010 and WLAN just two years afterward, according to market-research firm IC Insights. "As devices are integrated into either RFSOC or RFSIP format, the semiconductor manufacturer is able to gain cost and price efficiencies in the fabrication of die and packaging assembly," said Teradyne's Harvey. "Unfortunately, the test problem actually grows. Not only are there more capabilities to test, but also the cost of yield-loss is higher due to the higher value of the silicon and the package assembly. As the RFSOC/SIP devices are more complex, the testers are therefore required to have more instrumentation. Without a move to multisite and the assumption of high-parallel efficiency in the tester, test costs will expand proportionally to the complexity increase."
To ensure the proper operation and reliability of today's devices, modules, and systems, a test environment that simulates actual operating environments is essential. For devices that operate at RF frequencies, you must characterize their electrical behavior over the frequencies at which they will be used.
Getting good accuracy when testing at RF and microwave frequencies can be challenging, and Agilent's Ballo contends a solid understanding of RF fundamentals is needed to avoid common pitfalls. Most RF devices are tested in 50-ohm environments, so you cannot use high-impedance probes like those used with oscilloscopes. In addition, interconnect cables are susceptible to loss and delay, and unused device ports require proper terminations. You may also find that a mismatch between the test system and device under test can cause measurement impairment.
Devices without connectors must be tested in a test fixture, and you'll need to remove the effect of the test fixture for accurate device characterization. The data display used for RF signals can be much different from those used at lower frequencies (for example, Smith charts and constellation diagrams).
Testing begins when theory ends
RF and microwave test equipment is integral for verifying the physical layer characteristics of wireless devices. You will need to verify a device's power level, modulation quality, and its conformance with regulations governing interference. "Devices that have RF and microwave frequency components need to be tested at these frequencies," says Anritsu's Hakanson. "The most obvious application is testing radios and the components that go into radios, but as digital clock frequencies increase, the analog characteristics of digital hardware become more important."
In R&D, this is essential for effective system simulation, and in manufacturing, it is essential to ensure that components meet their published specifications. "RF testing also is needed to test the out-of-band performance of components that operate at lower frequencies," Agilent Technologies' Ballo said. "For example, it is common to test mobile telephone filters all the way out to 13 GHz, even though their intended band of operation is less than 2 GHz."
He added, "In the digital world, clock speeds have advanced to the gigahertz range, so it is very important to test PCBs at microwave frequencies to estimate the signal integrity quality that will result from using the PCB and its associated traces. Since digital signals have high harmonic content, it is common today to test PCB structures all the way to 50 GHz. State-of-the-art characterization is even done at 110 GHz and beyond."
Different tools are used for different types of testing. These include linear device characterization, nonlinear device and subsystem testing, signal substitution, stress testing, functional testing, and system verification. "VNA is the fastest and most accurate tool to test linear performance, such as S-parameters, and simple nonlinear performance like gain and phase compression," Ballo adds. "The VNA can sweep frequency and power of a sinusoid, and has the necessary receivers to test reflection and transmission responses of components. For more advanced nonlinear characterization, signal sources that can provide digitally modulated signals are used to simulate carriers with real-world modulation schemes. Spectrum or vector-signal analyzers are used to demodulate the carriers and provide advanced analysis of the modulation parameters."
Rising costs, new approaches
To highlight the impact RF/microwave measurements have on the overall cost of delivering a product to market, James Kimery, RF market development manager of National Instruments, related the following example. "A few years ago, the bill of materials of a typical "vanilla" cellphone totaled about $100. The test cost was a small part of the overall BOM, and the traditional methods were satisfactory. With the advent of single chip cellphone ICs where the complete cellphone (baseband + RF) are implemented on a single IC, cellphone BOMs now approach $20. The same RF tests have to be employed, yet the test equipment has not evolved. Therefore, percentage of the test costs compared to the overall cost of the phone have increased significantly. In some cases, the test cost now accounts for over 25% of the cellphone cost."
Cellphones now have multiband radios and in some cases, one cellphone will have multiple wireless standards such as UMTS+EDGE and UMTS+HSPA. Cellphone manufacturers must manage the addition of FM radios, Bluetooth, WLAN, and digital video. Each new wireless capability adds test cost. Traditionally, testing a new wireless standard amounted to adding a "box"—a stand-alone instrument to test the specific standard such as Bluetooth.
New test approaches are needed to meet these and other RF/microwave test challenges. Adding more boxes to test systems increases the size and cost of test while also increasing test times. "Modular instrumentation such as PXI combined with flexible software architecture enables test engineers to reuse the existing RF test instruments by simply adding standard specific software," Kimery said. "A further benefit can be realized if the upper layers of test software can simply reuse the underlying standard specific software underneath"
The benefits to Kimery's approach are smaller test systems, lower cost, and ultimately faster measurements. As engineers and test technicians examine this and other approaches, they must continually evaluate and weigh tradeoffs in test coverage, test time, and ultimately expense, to determine a test feasible strategy.
