Wireless communications technology has evolved from a simple analog product used primarily for business to today’s analog and digital wireless telecommunications systems operating in residential and business environments. They have transformed from the clunky, unwieldy contraptions to sleek, hand-held wonders. And along with the changing faces of the analog and digital wireless products has come the need for new, more sophisticated test equipment and standards.
Both technologies need specific equipment in place to operate properly. They must have a transmitter that accepts information from a source, prepares the information for transmission, transmits the information over a channel, and a receiver that reconstructs the information into a similar form as provided by the source.
For the analog system, the transmitter consists of functional blocks that accept the source information, generate a carrier signal, modulate the information on the carrier, and amplify the carrier for transmission. The analog receiver must identify and accept the carrier, extract the information from the carrier (demodulate), and present the result in a form that replicates the original input—be it audio, video, or data.
The digital transmitter contains circuits that accept and digitize the source information; encode the source to accomplish any data compression, compaction and encryption as well as the channel information; generate a carrier signal on the selected channel; modulate the carrier, and amplify the carrier for transmission. The digital receiver must identify and accept the channelized carrier; demodulate the carrier; decode the channelized information and the compression, compaction, or encryption, and reconstitute the information into its original format.
At first glance, the analog system seems much simpler than the digital system. This simplicity is apparent because the analog modulation techniques make few changes to the information as it is prepared for transmission. However, several circuits must be added to ameliorate interference and signal degradation common in analog systems.1
By outward appearances, you may not see much difference between analog and digital products such as a cellular phones, but they are as similar as an apple and an orange, said Steve Jackson, sales engineer at IFR Systems. Analog phones are based on proven technology. Even though the components that make up an analog phone are, for the most part, state of the art, they are stationary carrier devices operating the same as FM receivers/transmitters have for the last 50 years. Testing analog phones is mostly parametric to verify that the phone does not exceed FCC requirements.
The digital phones have a very simple hardware configuration, with software as the basis of the entire operation, continued Mr. Jackson. Most digital phones use spread- spectrum technology which spreads the signal over a wide frequency band. For example, spread-spectrum distributes baseband signal such as a voice channel of only a few kilohertz wide over a band of many megahertz. The voice signal is processed by a digital modulator and sent along with a wideband encoding signal.
Digital cell phones require different transmitter-quality measurements than analog phones, said Rob Van Brunt, product manager at Telecom Analysis Systems (TAS). For example, modulation-quality measurements on analog phones can involve verifying the FM performance of the hand set in terms of frequency deviation. A digital transmission scheme must be characterized using a different set of measurement techniques. For time-division multiple access (TDMA) phones, this measure of digital modulation accuracy is in terms of error vector magnitude. For code-division multiple access (CDMA) phones, the transmitter accuracy is quantified in terms of waveform quality.
On the receive side of wireless technology, the digital phone is more susceptible to the delay spreads introduced by multipath fading environments, continued Mr. Van Brunt. They must be tested with a different family of RF channel simulators than is used for analog hand sets.
Digital equipment generally requires higher-accuracy testers than analog, due to higher speeds of information processing and the volume of information, said Al Fisher, product marketing engineer at Anritsu America. One transmit frame of digital data can require hundreds of measurements and calculations in less than a second. This generally requires extensive use of digital signal processing.
On the other hand, analog equipment often uses analog filters that have settling times and require averaging of measurements over time. But, even analog can benefit from digital signal processing to decrease test times.
The test challenges for analog and digital systems include the capability to simultaneously handle high-speed serial digital data streams and small analog signals in a tightly integrated, low-noise-floor environment, said Jon Turino, marketing manager for Integrated Measurement Systems. Starting at the IC level, diagnosing problems to the appropriate digital or analog circuit in the design-test environment and using integrated interactive software can supply enough information to minimize the number of silicon iterations required to get the product to market on time with the minimum time to complete the validation and characterization tasks.
Digital Production-Test Issues
The complexity of the digital design demands a more rigorous and intricate test procedure than analog products. In the production environment, one of the biggest challenges is ensuring high-quality testing while keeping manufacturing costs low, said Allen Henley, product manager at Hewlett-Packard. Recently, this became more difficult because many manufacturers now provide multiband and multiformat radios. These new radios require many more tests as well as improved capabilities of the test equipment.
Wherever possible, production tests should be performed according to the requirements designed by the international radio standards committees, said Mr. Henley. The functions must closely simulate the conditions and measurements required to accurately characterize the mobile station in a real-world environment. The tests also should integrate easily into an automated manufacturing environment while providing accuracy, repeatability, and speed for testing.
The capability to measure critical performance areas, such as power-control response, receiver sensitivity under low power and impairment, and transmitter quality, is very important for production test, said Don Hershiser, market development manager at Tektronix. Testing to multiple standards and frequencies is important to some manufacturers with multimode and multiband phones.
For production test of digital phones, the important issues are calibration and test coverage vs test time, said TAS’s Mr. Van Brunt. Because cell phones are a high-volume consumer product, the time it takes to perform each test must be minimized. However, to increase yield confidence without sacrificing per unit test time, quality sampling of phones can be done with a more comprehensive test setup.
The production test of digital cellular phones can be broken down into parametric and protocol testing, said IFR’s Mr. Jackson. Parametric tests focus on the physical operation of the transmitter or receiver. This includes frequency error, transmitter power, digital modulation, and power-control capability. Typically, parametric testing involves very specific readings and tolerances that must be met to ensure proper operation.
The operational parameters are defined in the appropriate standards. However, in mature digital technologies such as the Global System for Mobile Communications (GSM), the end products, such as the receivers, perform uniformly once the functional design of the integrated circuits is proven. Failures normally are related to manufacturing flaws such as poor solder connections and broken PCBs. Once a failure is discovered, the assembly typically is discarded and no extraordinary attempts are taken to troubleshoot or repair the unit.
Protocol testing determines how the phone and software will interact with the variety of messaging, signaling, and call-processing functions required by the network, said Mr. Jackson. Various test cases must be established to prove the quality of the coding of the phone software.
Digital wireless phones, conforming to one or several of more than 10 global standards currently are in production, said Anritsu’s Mr. Fisher. To meet these multiple requirements, manufacturers need flexible test equipment and test setups. Additionally, the higher production volumes require more floor space and testing time. As a result, test stations must be made smaller and faster and offer multiformat capabilities to help meet these goals.
Design vs Production
Several differences exist between design test and production test of digital cell phones. The most important differences are speed, comprehensiveness, size, and flexibility.
Design engineers run many tests in loops for many hours and sometimes days, said Mr. Fisher. Design requires more extensive testing to prove the worthiness of the design over extended time, temperature, and various operating conditions.
In many cases, the device-under-test does not possess complete functionality and requires special test setups. While a single tester may perform this function, a combination of sources and analyzers is generally better suited for research and development.
The issues for design test are fundamentally different than those for production test, said Mr. Turino of IMS. In design test, the focus is on rapid information gathering for first silicon debug, circuit speed path analysis, and digital/analog circuit characterization and specification validation.
Design test engineers cannot afford the time it takes to use a production tester on their tasks or the time to develop custom test setups. They need a test station that performs first silicon debug and characterization tasks and validates the simulation models used so that future design simulations closely match the real world.
The key challenge in R&D is finding the test equipment with the capabilities to characterize measurements and protocols per the new digital standards, said HP’s Mr. Henley. To stay competitive, manufacturers often must design new mobile-station and base-station terminals before commercial test equipment is widely available. The commercial test equipment typically is highly specialized, designed for a particular radio standard. What the designer needs is a range of specialized, yet flexible, test equipment that can be programmed or configured for several radio types.
Design test and verification requires an increasing number of tests including protocol, interoperability, and hands-off checks not used in production but necessary to ensure fundamental correctness of a design, said Tektronix’s Mr. Hershiser. The needs are especially critical under marginal conditions and when equipment is designed for use in differing networks. Design test engineers also need flexible products to work with emerging standards such as the wireless CDMA.
Protocol testing determines the networkability of the digital phone, such as how the phone software reacts to the messaging, signaling, and call-processing functions required by the network, said Mr. Jackson. Test cases must be established to prove the quality of the coding of the phone software.
Production tests concentrate on the baseline functionality of the phone, said TAS’s Mr. Van Brunt. For CDMA, the functional test involves making a call between the phone and a base-station emulator. The phone’s sensitivity and performance also are evaluated during additive white Gaussian noise conditions. For transmit conditions, the quality of the handset transmit signal must be measured, including frequency and power accuracy.
Compliance Issues
Digital-phone testing issues depend upon whether the equipment will operate in the United States or the international market, said IFR’s Mr. Jackson. The U.S. digital-phone technologies are not as mature as the GSM standard. From a testing standpoint, this lack of maturity means that testing requirements are highly dynamic.
Another issue confronting U.S. digital-phone test vendors is the lack of agreement on digital technology and the associated new digital testing requirements, added Mr. Jackson. Presently, you must test the phones for network interoperability. For example, you need to know if the phone can roam into a system and maintain acceptable operation, such as a CDMA/AMPS phone roaming into an IS-136/AMPS network.
Today, many telephone carriers and telecommunications associations support independent testing for compliance before selling a wireless phone, said Anritsu’s Mr. Fisher. The standards process is very much self-regulating, with the marketplace as the prize for those that manufacture to the standard.
Reference
1. “Wireless Technology Backgrounder,” Hewlett-Packard, September 1997.
Wireless Test Products
Digital Mobile Radio Tester
Makes Measurements in <2 s
The MS8606A Digital Mobile Radio Transmitter Tester performs CDMA measurements in <2 s. It covers the 300-kHz to 3-GHz frequency range and combines a transmitter tester, spectrum analyzer, and power meter with software to perform the tests. An asynchronous mode helps make measurements via the antenna port. An external trigger mode and a short-code search mode conduct analysis in <4 s. The unit measures frequency, timing error, code-domain power, transmitter power, occupied bandwidth, and spurious emission. A menu-driven display allows you to control the instruments from the key pad. $26,500. Anritsu America, (800) 267-4878.Test Set Checks
Cellular/PCS Base Stations
The HP 8935 TDMA PCS/Cellular Base-Station Test-Set provides frequency coverage to 1 GHz and from 1.7 GHz to 2 GHz for PCS requirements. It makes TDMA measurements for carrier frequency error, true average power, and modulation quality. The equipment measures the radio-frequency performance characteristics of the base station that affect system coverage, capacity, and call quality. The internal spectrum analyzer and built-in RF source may be used for sweeping antennas, determining fault locations in cables, and searching for in-band interferences. $44,500. Hewlett-Packard, (800) 452-4844, ext. 5779.
CDMA Mobile Station Tests
Cellular and PCS Phones
The HP 8924E Mobile Station Service Test Set automates CDMA cellular and PCS phone testing. Applications include incoming inspection, failure and repair verification, and customer return as well as troubleshooting at repair stations on manufacturing lines. You can configure and run preselected automated tests by inserting the company’s AMPS/NAMPS/CDMA test-software card into the instrument. The test set measures average power and low-level power and performs confidence-limit testing. It also has analog test capabilities. $33,000. Hewlett-Packard, (800) 452-4844, ext. 5781.
Test Station Offers Integrated
Mixed-Signal Test Solutions
The Mixed-Signal Test Station (MSTS) extends the basic digital-test architecture to include dedicated wideband analog signal paths and integrated analog instrumentation. A digital test capability at speeds up to 200 MHz is provided for up to 448 digital I/O channels. Analog stimulus and measurement equipment integrated within the MSTS provides 1.4 GHz for up to 64 analog pins. Analog functions include audio and video test tools, signal generators, digitizers, and DMMs. From $705,000. Integrated Measurement Systems, (503) 626-7117.
Multipath Fading Emulator
Suited for Mobile System Tests
The MP2700 Multipath Fading Emulator provides RF channel simulation for third-generation mobile-phone systems. It has the high dynamic range and bandwidth required for testing UMTS, IMT-2000, or wideband CDMA (W-CDMA) equipment. A standard GPIB interface facilitates remote control via software, such as the company’s CATS-98A package. Although targeted toward CDMA and W-CDMA applications, the emulator and the software package can be used for other applications, such as TDMA and GSM system testing. Call company for price. NoiseCom, (201) 261-8797.
Automated Tester Characterizes
CDMA Mobile Phones
The CDMA Automatic Test System (CDMA-ATS™) now offers automatic performance characterization (APC) to find the performance limits of mobile phones. APC determines the lowest acceptable receive-signal level for phone operation. The system includes executive software to automate the testing process. It also tests cellular and PCS phones for adherence to CDMA performance standards. From $144,000. Telecom Analysis Systems, (732) 544-8700.
Wideband CDMA Test Solutions
Meet Signal and Analysis Needs
The FSE Series Spectrum Analyzers and the SMIQ Signal Generators meet wideband CDMA signal-generation and analysis needs. The FSE measures adjacent channel power (ACP) for W-CDMA equipment and has a >70-dB ACP dynamic range. It includes ACP measurement algorithms. The SMIQ generates 4.096-Mchip symbols when equipped with a modulation coder and data-generator options. The unit’s software offers several filter types, spreading codes, and modulation maps. SMIQ: starting at $27,950; FSE: Starting at $29,995. Tektronix, (800) 426-2200 press 3, code 1077.
Copyright 1998 Nelson Publishing Inc.
June 1998