Wireless local area network (WLAN) is the fastest growing field of mobile communications. In the past few years, the wireless network technology based on IEEE802.11b has achieved great success, and its driving force is the growth of affordable Internet access services in homes or offices. However, people continue to pursue higher throughput networks, and require that the quality of service is not affected. IEEE802.11a and IEEE802.11g are technologies that can meet these requirements very well. They can provide a higher data rate than 802.11b. The peak data rate reaches 54Mbit / s, while the peak data rate of 802.11b is only 11Mbit / s. The development of standards makes WLAN devices increasingly complex. If performance is to reach the expected level and dual-band or multi-format devices are popular in the market, these complex designs require more complete and thorough manufacturing test methods.
Figure 1 Gold RF-based manufacturing system
Figure 2 Integrated WLAN test device manufacturing system
The manufacture of WLAN access points and network interface cards is mainly done by contract manufacturers (CM) and original design manufacturers (ODM). Maintaining test costs at a low level is very important for these CMs and ODMs. Some key factors to reduce test costs include:
Most WLAN manufacturing tests use systems based on "golden radio" (golden radio), which consist of a known, excellent "golden" WLAN radio device, a spectrum analyzer, power meter, and power supply. In addition, in order for the DUT to get a proprietary test mode to process and control the golden radio frequency system, a set of test control software is also required.
Although golden RF-based systems are often regarded as affordable test systems, they also have some inherent problems that make them less suitable for dual-band / multi-format radio systems, that is, they are proprietary The error vector magnitude (EVM) cannot be measured.
The test system based on the golden radio frequency belongs to a proprietary test system, which is determined by its essential characteristics. It uses the same kind of chipset / device as the DUT. Both ODM and CM rely on chipset manufacturers to provide support to calibrate, operate and maintain these golden RF systems. The cost corresponding to the engineering effort required to support these systems increases as the output and variety of devices under test increase.
The test system based on the golden radio frequency cannot measure EVM: The 802.11 a / g radio frequency standard uses a wide variety of modulation methods, from the BPSK / QPSK format of the 802.11b system to 64QAM for high data rate tasks. For dual-band and multi-format radio frequency devices, to successfully interoperate with other radio frequencies, manufacturers must ensure that the transmitters are of high quality, which also means that their EVM must be tested. However, until recently, equipment for testing EVM was too expensive for the manufacturing environment.
The 802.11b standard, which uses a simple modulation method, does not depend strongly on the quality of the modulation. Data symbols can tolerate higher error rates without causing bit errors or packet errors. In low bit rate systems, the degradation of transmitter performance before reaching the limit of modulation accuracy test generally appears in the spectral mask.
For more advanced formats used in 802.11a / g (such as 64QAM), the quality of the modulation is more important. The modulation error will cause misjudgment of the data symbol, which will ultimately limit the transmission range and data throughput. Because of this, modulation accuracy testing is extremely critical for multi-format RF, and the 802.11a / g specification requires that the modulation quality of the transmitter be tested by measuring EVM. EVM measures the difference between the modulated carrier and the ideal reference signal and is a measure of the modulation accuracy obtained by the design and development of the WLAN chipset.
The difficult choice faced by cost-sensitive manufacturers is that until recently, the test equipment available on the market that can measure the EVM required (within a large bandwidth) is relatively expensive. Therefore, manufacturers have made efforts to adjust and expand the process adopted by the gold radio frequency system to measure the modulation quality. The golden RF-based system performs a test called transmit packet error rate (TxPER), which is similar to the EVM test goal (that is, to verify the modulation quality on the transmitter).
The TxPER test requires the DUT to transmit a predetermined stream of packets, and the receiver in the golden radio frequency is used to collect the data and compare it with the reference packet to verify whether the packet error rate (PER) has risen. The problem with this method is that it is proprietary to the manufacturer and it is difficult to reproduce on different golden radio frequencies. Changes in the performance of the golden radio frequency receiver will also bring different PER results. Even if there is no change in the transmitter performance, this will lead to "false disqualification" on the production line (that is, a good device is deemed to be unqualified), and will Cause fluctuations in product yield. These two situations are what every manufacturer wants to avoid.
An exciting development in test equipment will change all this. Obtaining an integrated test device with good cost-efficiency, flexibility and bandwidth that meets the needs of EVM measurement will make it possible to change the way WLAN devices are tested. Nowadays, integrated WLAN tester solutions have appeared on the market. These test devices can provide the accuracy, repeatability, flexibility and independence that the current golden radio frequency system lacks.
In this type of system, the functions of gold radio frequency, power meter, spectrum analyzer, and related cables and RF switches are integrated into one instrument, which greatly reduces the complexity and cost of the system. The entire test set includes a programmable vector signal generator and a broadband receiver, whose accuracy is similar to that of a power meter. The system only needs a PC to accommodate DUT control software and WLAN measurement software.
The calibration step is mainly to calibrate the power amplifier in the DUT by adjusting the output power to a specified absolute level or until the maximum allowable distortion occurs. In gold-based RF-based systems, the specific method for measuring this distortion is often to increase the power level of the DUT until certain characteristics of the transmitted signal, such as spectral side lobes, reach predetermined limits.
Since this test method generally has lower accuracy and predictability than direct measurement of EVM, the power level must often be reduced from the maximum possible level, so the measured system performance is lower than the possible magnitude. With the integrated test device, power, EVM and other IQ offsets and signal frequency error measurements can all be acquired in a single data capture, thus completing fast and accurate measurements of transmitter performance. This increase in accuracy and repeatability means that the entire range of DUT amplifiers can be used and the quality of the transmission is optimized. High accuracy and repeatability also reduce the measurement uncertainty, which means that the protective band at the measurement limit can be reduced, and the yield can be improved.
The integrated test device, which is a repairable test platform, will help manufacturers of WLAN modules to quickly increase production, increase manufacturing yield, and reduce the total cost of testing. Test systems that are independent of any manufacturer ’s chipset and have guaranteed basic requirements such as accuracy, bandwidth, and frequency range are ideal for WLAN manufacturers, and their architecture can provide good WLAN device testing. New methods, and can adapt to future technologies more easily. Because testing flexibility plays a key role in many manufacturing environments, this type of technology will be the solution for long-term applications.
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