3G delays force flexibility

Despite recent announcements of higher speed networks by a few US wireless service providers and the launch of similar services by Japanese and Korean telecom companies, the timetable for the deployment of third-generation (3G) wireless networks is very much in question. Demand for applications such as wireless-Web access remains sluggish as service providers slowly gear up for higher data rates. Faced with high costs and customer indifference, leading European service providers have slowed their efforts to introduce new 3G services. Even 3G pioneer NTT Docomo in Japan sees slower acceptance of its I-mode service than originally anticipated.

If you're an engineer developing wireless systems, these delays present a conundrum: How do you continue to assemble the tools you need to test the growing number of 2.5G wireless products while simultaneously preparing to test 3G systems, and their many different modulation schemes, as soon as the market appears ready?

Doing both is no small task. For example, the European Telecommunications Standard Institute's (ETSI) Enhanced Data-Rates for GSM Evolution (EDGE) standard, widely viewed as a popular stepping stone to 3G systems, delivers higher data rates by introducing a second modulation format into the GSM system. While the distinctions between this second modulation scheme and the established GMSK (Gaussian minimum-shift-keying) modulation scheme used in existing GSM systems are subtle, they have a significant impact on the test equipment you'll need for power-versus-time and modulation-accuracy tests.

In addition, the wideband modulation techniques used in 3G RF protocols place new emphasis on adjacent-channel power-ratio (ACPR) tests. To help ensure interference-free communications on tightly spread channels such as those used in wideband CDMA (W-CDMA), you must use ACPR tests to compare the power within a specified channel to the power in adjacent or alternate channels. This demands a receiver that not only covers a wide band, but also offers excellent dynamic range.

One way to improve efficiency and cut costs is to reduce the number of instruments on the test bench. Matt Maxwell, product manager for Tektronix, says "One common theme we hear from engineers is that they're looking for a more universal test set that will allow them to do all 2.5G and 3G standards nearly interchangeably. With the delay in the rollout of 3G and money for projects drying up, they're looking for a more economical solution where the same rack of equipment will allow them to do cdma2000 one day and GSM/EDGE the next." While most design and test teams use multiple instruments, including a spectrum analyzer and vector signal analyzer, to characterize multichannel 3G signals, Tektronix and its partner Rohde & Schwarz (Munich, Germany; www.rohdeschwarz.com) have found a way to combine the functionality of a spectrum analyzer and vector signal analyzer in one box. With an integrated IQ bandwidth of 28 MHz, the FSQ spectrum analyzer (Figure 1) can capture and analyze the full breadth of a multicarrier W-CDMA signal for demodulation and analysis.

Figure 1 The FSQ spectrum analyzer, with its integrated wide-bandwidth (28-MHz) IQ demodulation capability, can analyze the full breadth of a multiple carrier W-CDMA signal. Courtesy of Tektronix.

To calculate demodulation tests quickly, the FSQ adds a faster post-processor than its predecessors. Initially, Tektronix is offering only 802.11a wireless LAN demodulation capability on the instrument. But the company plans to add general-purpose vector-signal analysis software for 3G applications by year's end.

Another way to improve your productivity is by simplifying the test-configuration process. Take RF signal generators, for example. These instruments have come a long way from the days when you set carrier frequencies with rotary dials or thumbwheel switches and then performed a laborious tuning process. The introductions of soft keys, LCDs, and microprocessor controls have given the test engineer tremendous ability to configure an instrument for specialized applications. Nevertheless, the emergence of 2.5G and 3G wireless systems, with their complex modes of modulation, have placed new demands on the user interface. Test engineers need an instrument that delivers the additional functionality required to support digital and vector modulation without becoming too cumbersome and difficult to use.

To simplify configuration of these increasingly complex 3G tests, IFR Systems has incorporated the first touch screen for RF signal generators in its new 3410 Series instruments (Figure 2). The simplified interface provides touch control, as well as equivalent keypad control, and eliminates the deeply nested menu structures that can slow down test configuration. Says David Asquith, senior product support engineer at IFR, "you just touch the area of the display you want to modify."

Figure 2 Featuring a touch-screen user interface, the 3410 Series digital RF signal generators support RF carriers up to 4 GHz and include both analog and broadband vector modulation in a compact 2U-high rack-mountable case. Courtesy of IFR.

An optional internal arbitrary waveform generator (ARB) can generate digitally modulated carriers with a RF modulation bandwidth of 45 MHz. The ARB features an interpolation-filter design, which simplifies the configuration process to a file-selection operation. Instead of using the complex reconstruction-filter selection processes found on many ARBs, the interpolation rate on the 3410 ARB is automatically set so that aliasing products of the wanted signal are shifted into the stop-band range of the anti-aliasing filter. "There's no filter or sampling rate to select," says Asquith. "You just download the file and touch the play icon."

Another issue to consider is memory support. The ARB for the 3410 can store 225 Msamples of data that can be partitioned into multiple small files or treated as one large file. "One of the things you want to avoid is downtime while you're reconfiguring your equipment," notes Keith Slinn, IFR's director of sources and analyzers. "Many RF phones support different formats, and you might be testing a phone to one standard and then need to change standards. By being able to download various files and switch between them almost instantaneously, you won't have your test equipment sitting idle."

Multiport VNAs have quickly become a staple in 2.5G and 3G wireless test. But if you've ever had to make manual coaxial calibrations for S-parameter measurements, you know how time-consuming and error-prone they can be. A four-port short-open-load-through (SOLT) calibration can require anywhere from 15 to 18 component connections in a process that is ripe for mistakes. "It's just so easy to mix up an open and a load or put a load where it doesn't belong," notes David Vondran, senior product marketing manager at Anritsu's Microwave Measurement Division.

To eliminate unreliable measurements caused by inaccurate manual calibrations, Anritsu has developed a new family of two–, three–, and four–port calibration modules that provide repeatable, high-quality coaxial calibrations to 40 GHz. Designed for the Scorpion MS462xx Series VNAs, the 36584 Series AutoCal modules (Figure 3) are controlled directly by the VNA's built-in serial port. Anritsu also offers a family of two-port automatic calibrators for both the four-port Scorpion and two-port Lightning 37000 Series VNAs.

Figure 3 By replacing tedious manual calibrations with a single cable connection, the 3658x Series AutoCal module (shown here with one of its manufacturer's Scorpion vector network analyzers) improves the accuracy and repeatability of two, three, and four-port S-parameter measurements. Courtesy of Anritsu.

Another way instrument manufacturers are addressing the high cost of testing new 2.5G and 3G wireless systems is by bridging the gap between R&D and manufacturing. Typically, the needs of these two groups differ dramatically.

In R&D, engineers want to optimize their test equipment setup, cabling, and DUT adjustments to achieve optimal results for individual tests such as adjacent-channel-leakage ratio (ACLR). To produce repeatable multichannel power amplifier (MCPA) test results, they want to control the DUT interface to provide calibrated signal stimuli and measurements at the DUT ports.

In the production environment, throughput and yield are the primary concerns. Engineers need test systems that will support multiple configurations, calibration techniques that will support multiple measurements, and automatic test procedures that are simple to run.

Holding the line on costs becomes even more complex when the manufacturer has to build systems to support multiple 2.5G or 3G standards. For example, base-station manufacturers often build expensive MCPA test sets to support specific test scenarios. But as soon as requirements change or the manufacturer moves to a new modulation format, they must scrap the platform.

With its GS-9200 MCPA test system (Figure 4), Agilent Technologies is trying to extend the lifetime of test assets within both R&D and manufacturing by offering a single system that both verifies R&D product designs and performs high-volume production test. The GS-9200 includes an RF source, network analyzer, signal analyzer, power meter and DC power source all tied together by what Agilent calls a "Precision System Interface" (PSI), which provides the signal routing for DUT measurements, system calibration, and system self-test. Through the PSI, you can perform multiple measurements without disconnecting the DUT or re-calibrating the system equipment.

Figure 4 New multicarrier power-amplifier (MCPA) test sets like the GS-9200 eliminate the need for expensive custom test systems for base-station power-amplifier tests by using a modular design. Courtesy of Agilent Technologies.

Of course, no single instrument or set of instruments can be a panacea in the roiling world of ever changing 3G test. Nevertheless, the signal sources, analyzers, and instrument combinations described here demonstrate that you have a selection of capable systems from which to choose. You can rest assured that their manufacturers will continue offering instruments and upgrades that keep pace with 3G technology and beyond.