ATE Promotes WiMAX RF Testing and Characterization

    ATE promotes WiMAX RF testing and characterization. WiMAX transceiver devices have proven to be beneficial to the development of the consumer electronics market, where they have found many uses, including connecting WiFi hotspots to the Internet. To ensure that the devices work as expected and get them to market quickly, device manufacturers need advanced multi-function test equipment and equally advanced test software. This is an essential step. Because an imperfect or even seriously flawed software can have a serious impact on the reputation of the manufacturer.

　　WiMAX Capabilities

　　WiMax (Worldwide Interoperability for Microwave Access) is the global microwave interconnection access. WiMAX is also called 802.16 wireless metropolitan area network or 802.16. WiMAX is an emerging broadband wireless access technology that can provide high-speed connection to the Internet, with a data transmission distance of up to 50km. WiMAX also has the advantages of QoS guarantee, high transmission rate, and rich and diverse services. WiMAX has a high technical starting point and adopts advanced technologies such as OFDM/OFDMA, AAS, MIMO, etc., which represent the future development direction of communication technology. With the development of technical standards, WiMAX gradually realizes the mobility of broadband services, while 3G realizes the broadband of mobile services. The integration of the two networks will become increasingly higher.

　　WiMAX is designed to operate in wireless bands that require a license or in public wireless bands. As long as the system company has a license for the wireless band and allows WiMAX to operate in the licensed band, WiMAX can use more bandwidth, more time periods and stronger power for transmission. Generally speaking, only wireless IS/7. companies will use WiMAX technology with licensed bandwidth. As for Wi-Fi, it is designed to work only between 2.4 GHz and 5 GHZ in the public band. The Federal Communications Commission (FCC) of the United States stipulates that the general transmission power of Wi-Fi must be between 1 milliwatt and 100 milliwatts. The general transmission power of WiMAX is about 100 kilowatts, so the power of WiFi is about one millionth of WiMAX. It is obvious that a WiMAX base station with a transmission power one million times that of a WiFi base station will have a greater transmission distance than a WiFi terminal.

　　WiMAX uses OFDM, a multiplexing method that divides bandwidth into multiple frequency subcarriers. In an OFDM system, the input data stream is divided into several parallel substreams with lower data rates, each of which is modulated and transmitted on a separate orthogonal subcarrier. In a 10MHz channel bandwidth, data rates of up to 63 Mbps are possible on the downlink between the base station and the mobile device, and up to 28Mbps on the uplink (Figure 1).

　　Figure 1. WiMAX modulation schemes include quadrature phase-shift keying and 16-point quadrature phase-shift amplitude modulation.

　　In the early days of mobile devices, in-phase (I) and quadrature (Q) information was transferred from the baseband processor to the RF portion of the device in an analog format. In today’s highly integrated devices, ADCs and DACs reside in the same package as the RF circuitry, forming the link between the RF device and the digital baseband processor or digital bus. Moving the ADCs and DACs from the baseband processor to the RF device makes it possible to manufacture the processor with the smallest lithographic features, which reduces bill of materials costs. Figure 2 depicts the layout of a typical RF MIMO transceiver with a digital interface and multiple RF ports.

　　Figure 2. Block diagram of a WiMAX 2x2 MIMO transceiver.

　　WiMAX Test System Requirements

　　To test WiMAX transceivers on a high-throughput manufacturing line, the automated test equipment (ATE) system requires the following key capabilities:

　　1 Digital supply and recording speed is the same as the device under test (DUT);

　　2. Use a clock with low phase noise to provide a reference for the synthesizer;

　　3. Auxiliary control circuit for clean power supply and relay control;

　　4. RF supply and recording;

　　5+ RF ports that can be easily calibrated to get accurate signal levels;

　　6 Methods for supplying and recording WiMAX modulated signals.

　　The ATE system also needs to have sufficient hardware and software resources to perform multi-site testing with a high degree of parallelism. With parallel testing, the total test time required for the system to test several devices should be close to the time required for a single-site system to test one device.

　　During test development, testers should arrange tester resources to minimize load board complexity. From the test engineer’s perspective, this automates the task of calibrating RF signal levels to the tester delivery plane. Device design should minimize the number of components on the final assembled product PCB, thereby reducing material costs. Similarly, ATE load boards should have as few components as possible. “Clean” load boards with minimal components require less time to design, layout, build, and debug, and they have proven to be more reliable in mass production.

　　To test MIMO devices, the tester needs to provide multiple receivers to record the device's transmit signals in parallel. It passes the recorded waveforms to a modulation analysis package that can connect to multiple input streams and analyze the combined information. The same process applies to the receive path, where multiple digital recording engines need to simultaneously record digital data from each device's receiver.

　　A 2×2 MIMO device has 2 input receive ports and 2 output transmit ports. To test such a device in a 4-site environment, the tester must provide 8 RF supply channels and 8 RF recording channels. To avoid the use of splitters or RF switches on the device interface board (DIB), the ATE needs to provide 16 RF ports.

　　A four-site application requires four high-purity reference clock inputs, one for each synthesizer of each DUT. It is critical that the phase noise of the clock inputs is low, as clock phase noise affects the performance of the device. A tester with a good clock source does not require a divider or crystal on the DIB. Crystals have good phase noise, but their frequency is not locked to the ATE, so they can cause digital synchronization problems. Therefore, if the tester does not require a crystal, the tester will get better test results.

　　Testing Challenges Facing WiMAX Devices

　　WiMAX devices must undergo a series of tests to ensure that they will work properly when used in radio equipment. This series of tests generally includes:

　　1. Continuity and leakage testing to ensure correct packaging and ESD protection;

　　2 Digital type tests (including some tests on scanning formats);

　　3. Traditional INL, DNL, ​​and THD performance measurements of converters;

　　4. Measure the power consumption of DUT in various working modes;

　　5 RF transmit and receive operations to test the specifications of sine wave signals and modulated signals.

　　The first step can be to test to determine if the device is working and whether further testing is needed, but this step may not be very time-efficient, depending on the yield and test method.

　　As devices become more complex, it becomes critical for designers to include "design for test" features. For example, several test modes for a test bus are designed into the DUT, which helps route signals to observation points that are not used during normal operation. This visibility helps test engineers accurately test a block or portion of the DUT.

　　Classic continuous wave (CW) testing of the RF transmit portion of a WiMAX transceiver includes output power, carrier, and sideband suppression measurements. Testers can also perform transmit testing to measure local oscillator (LO) suppression. This is not a regular transmit test, but they should know how much LO is leaking at the transmit pin location and therefore how much is radiated by the antenna. This level is very low, and the phase noise of the LO cannot be tested using classic CW methods.

　　On the receive side, gain, gain linearity, image rejection, and third order intercept (IP3) are all key CW tests. Received signal strength indicator (RSSI) is another test that should be considered. For RSSI, the device's own receive level indication provides a good basic test of receive functionality. RSSI testing usually requires reading register values, which can be a very convenient step, especially during wafer probing, when a fully loaded RF tester is often not available and a subset of tests are being performed.

　　RF modulation test

　　Modulation testing checks the device as it will be used in the final application. This provides an advantage - measuring the performance of the radio as a complete system.

　　A typical modulation test done on the transmitter is the error vector magnitude (EVM), also known as the receiver constellation error (RCE). EVM measures how far the constellation points are from the ideal value, and the lower the EVM, the better (Figure 3).

　　Figure 3. EVM calculations show the difference between the reference and the observed points on the constellation diagram, which is caused by phase error and amplitude error.

　　In an ideal situation, the constellation points of the modulated signal would be located at their ideal positions, but device imperfections caused by LO phase noise, nonlinearity, image rejection, and other issues can cause the constellation points to be located at non-ideal positions, thus limiting the data rate.

　　Channel mask testing is another common modulated emissions test where a bandwidth wider than the channel is recorded and the signal level outside the operating channel is measured to ensure it is low and within specification.

　　For the receive path, tests often measure EVM and bit error rate. BER is the ratio of error bits to correct bits, the lower the better. BER testing measures the modulated RF signal received by the DUT and counts the number of correctly received and incorrectly received bits. BER testing is generally time consuming because it takes a long time to test very low BER levels.

　　RF modulation testing can also be used for filter testing. A multi-tone signal containing one baseband component and three to six roll-off and stopband components can be used to quickly determine the 3 dB point and stopband performance of the device filter. This multi-tone signal method can be used for both receive and transmit filters, with the main advantage being that only one recording is required in the digital or video domain.

　　Modulation tests provide useful information about the performance of the DUT in a complete system. If the DUT fails these tests, they will most likely not perform satisfactorily. Unfortunately, it is difficult to pinpoint which block of the device is causing the problem in a production environment. To determine marginal RF performance, CW testing and modulation testing should be considered necessary.

　　Desktop equipment helps with characterization

　　The characterization of RF devices is done on a development bench, with some lab equipment designed to simulate the device's operating conditions in the end use and test the device to the relevant standards. This process is extensive and time-consuming, and requires a lot of benchtop equipment.

　　Because the same tools are used in the ATE world, lab and production personnel have the opportunity to work more closely together and use the same waveforms and analysis methods that adhere to industry and RF standards. Lab personnel will spend fewer hours in the lab, and production personnel will get faster answers to questions about device setup conditions, register values, and more.

　　Today's lab personnel and production personnel can share data more easily than their predecessors because most newer ATE systems are PC-based and run the Windows operating system. These systems can quickly run tests on many devices and quickly rerun tests with different power rails when needed, and the test results can be automatically exported to a spreadsheet such as an Excel workbook. Engineers can then plot the test results graphically, which allows for easy visual analysis and sharing with other team members and management.

　　Using the same analysis tools in the lab and in production greatly increases the chances of correlating benchtop and ATE measurements, but still requires skill to deal with factors such as different DUT sockets used in the two locations. In addition, the ATE board will most likely be much thicker than the lab evaluation board, and the power decoupling locations and RF signal delivery routes will also be different, requiring test engineering skills.

　　However, because the tools used for modulation work are the same, the integration team will do less work in the lab and more work on the tester, delivering engineering samples faster and in larger quantities. It also helps to have the same ATE and benchtop modulation debug displays and setup files. The bottom line is timely delivery of fully tested WiMAX devices that meet customer expectations.

    in conclusion

    In order to properly test WiMAX transceiver devices, this article describes the need for RF semiconductor testers to accurately and quickly perform tests and help complete device characterization. I believe this will be helpful to those who need WiMAX technology. For example, it provides a good way for manufacturers to test newly developed software.