RF modules pose tough test challenges

MUNICH, GERMANY—The proliferation of wireless communications devices is creating a voracious demand for the RF components and modules that make those devices work. The components must, of course, be small enough to fit comfortably within the mobile wireless consumer products they populate and not crowd out ancillary functions like MP3 players and digital cameras. And because consumer products increasingly support multiple communications standards, the miniature devices themselves must incorporate the functionality necessary to implement WiFi, WiMAX, Bluetooth, GPS, DVB-H, UWB, and the various multiband cellular technologies.

On the digital side, Moore’s Law has enabled vendors to make great strides in providing increased functionality within a single CMOS integrated circuit. Radios, however, require numerous other active and passive components as well as baseband digital chips, power amplifiers, switches, and low-noise amplifiers. Discrete passive components don’t follow Moore’s Law and are not amenable to continual miniaturization. Christian Block, VP and CTO of the SAW division at EPCOS (see “The Evolution of EPCOS") described the problem—and the solution—in a 2004 interview (Ref. 1). “Major progress can no longer be made by continuing to miniaturize discrete passive components alone,” he said. “So we are opting for passive integration based on LTCC technology.”

“LTCC” refers to low-temperature co-fired ceramic technology, which EPCOS uses to create multilayer ceramic substrates that embed passive components. Compared with the FR4 material or laminates, LTCC substrates provide lower loss and allow for the integration of many passive components in a compact space (Ref. 2).

Patric Heide prepares automated measurements of LTCC front-end modules for WiMax and WiFi applications. He said that the testing of such modules involves a small-signal test of the many embedded filter functions as well as the comprehensive large-signal characterization of the modules’ power amplifiers.  Photo by Carsten Lerp / NPN Worldwide.

The result of the high levels of integration, he continued, is that “we ship to our customers a fully tested RF system in a single package. In a mobile phone, you have the CMOS-based radio IC built in on one side, and on the other side you have the antenna. We provide a fully featured front-end module that goes between the two.”

An example is a compact all-in-one front-end module for Bluetooth and IEEE 802.11b/g/n WLAN applications, introduced in January. The module integrates a WLAN power amplifier, a WLAN/Bluetooth switch, and a receive balun as well as bias circuitry and ESD (electrostatic discharge) protection for all RF and DC ports, yet it offers an insertion height of only 1.4 mm and occupies a footprint of only 4.5x3.2 mm² on a circuit board. Designated the D6101, it also includes a coexistence filter that allows simultaneous operation of WLAN and Bluetooth applications with all worldwide cellular standards.

Jörg Schuler says that the drive toward miniaturization and high levels of integration results in a package with multiple RF ports that presents tough test challenges. Photo by Carsten Lerp / NPN Worldwide.

Early test systems employed by EPCOS, Dr. Schuler said, included a two-port VNA (vector network analyzer) and 12-port matrix. He added that EPCOS had early discussions with Rohde & Schwarz about test-equipment needs (see “University ties boost network-analyzer developments”). The discussions ultimately led to today’s use of an eight-port R&S ZVT VNA, which supports test coverage up to 20 GHz, combined with a 10-port matrix. The combination, he said, results in fewer switches connected to the unit under test, thereby minimizing loss, increasing dynamic range, and providing for a more accurate measurement.

Also important, according to Dr. Schuler, are fast calibration times, preferably with calibration able to be performed onsite. “What we use today is a 16-port calibration standard,” he said, “which offers the possibility to automatically calibrate 16 ports in one shot—just connect test equipment to the cal standard and start the autocalibration routine. It is very practical for mass production.” He added that automated calibration is particularly important at EPCOS’s manufacturing plant in Wuxi, China, and he explained that with color-coded cables and connectors, an operator can quickly make the necessary connections and push a button.

Figure 1.  Test for a combined WLAN and WiMAX front-end module extends from S-parameter measurements through harmonic measurements. The OFDM modulation scheme of WLAN and WiMAX devices requires the measurement of EVM (error-vector magnitude) as an indication of linearity.

Dr. Heide described specific testing challenges posed by highly integrated LTCC front-end modules that also include active components such as power amplifiers. He said that testing such modules involves small-signal measurement of the filter functions inside the ceramic itself as well as large-signal characterization of the amplifier functions also embedded within the module package. Specific tests vary with the application but typically include steps such as S-parameter measurement and harmonic measurements—the latter to ensure compliance with spurious emissions regulatory requirements.

Figure 1 shows test procedure required for WLAN and WiMAX modules. A key requirement for IEEE 802.11 and 802.16 devices, Dr. Heide said, is the linearity of the transmitter operating in OFDM (orthogonal frequency-division multiplexing) format. EVM (error-vector magnitude), he added, is a key indicator of linearity. To implement a comprehensive characterization of modules under development, the EPCOS engineering lab typically employs automated PC-based test stations that include an R&S ZVA24 VNA, a customized multiport test set, an R&S FSQ26 vector signal analyzer, and an R&S SMJ100 vector signal generator.

Dr. Heide explained that a time-consuming test procedure will not be adequate for mass production. “We cannot accept a test that takes half a minute or a minute. We need to complete the test in just a few seconds.”

Testing on a sample basis to speed production is not an option. “We do a full characterization of each and every product leaving our factory,” said Dr. Schuler. He added that EPCOS wanted to minimize the need for test-program adjustments when beginning production on a new module. “Our approach is always to be able to have the same test setup in the lab as we have in the test environment on the production line.”

Dr. Heide said that EPCOS has recently developed a combined module (Figure 2) that supports all worldwide WiFi and WiMAX standards (2.5-GHz WiFi, 2.5-GHz WiMAX, 3.5-GHz WiMAX, and 5.5-GHz WiFi) in a single unit. This tri-band module operates in four modes, he said, with WiFi and WiMAX sharing the 2.5-GHz band while the 3.5-GHz band supports WiMAX only and the 5.5-GHz band supports WiFi only.

Figure 2.  An LTCC tri-band front-end module, implementing 2.5- and 3.5-GHz WiMAX as well as 2.5- and 5.5-GHz WiFi, includes baluns and filters; it also includes power amplifiers and an SP4T switch mounted on the RF-tested LTCC substrate.

Patric Heide and Jörg Schuler discuss production-test concepts for the WiMax/WiFi front-end modules under development in the EPCOS Munich laboratory.  Photo by Carsten Lerp / NPN Worldwide.

Despite the emphasis on Rohde & Schwarz instruments, EPCOS engineers do have in their labs instrumentation from other suppliers of measurement equipment. Dr. Heide said that this additional equipment lets EPCOS engineers ensure full compatibility and reproducibility of test results for specific measurements performed by EPCOS customers or reference-design partners, for example. These measurements sometimes use very special digital modulation schemes generated using instruments from multiple vendors.

To fully characterize the FEMs at up to 20 GHz, EPCOS engineers employ a PC-based test station that includes an R&S ZVB20 vector network analyzer, a 10-port switch matrix, an FSQ26 vector signal analyzer, and an SMJ100 vector signal generator.  Photo by Carsten Lerp / NPN Worldwide.

Dr. Schuler added that through close cooperation with Rohde & Schwarz customer support, EPCOS also has the opportunity to work with products not yet on the market. “That gives us a chance to provide a lot of feedback, and it lets us make sure the instrumentation is mature when we deploy it in high-volume production.”